Inside-mount blinds and shades are one of those upgrades that look “built-in” when they’re done right. They sit neatly inside the window frame, show off your trim, and generally feel more tailored than an outside mount. The catch? Inside mounts are less forgiving. If your measurements are even a little off, you can end up with a shade that rubs, gaps that bug you every time the sun hits, or hardware that simply won’t fit.
This guide walks you through measuring windows for inside-mount blinds and shades in a way that’s practical, repeatable, and friendly for real homes—where frames aren’t always perfectly square and drywall corners have opinions. If you’re measuring for a whole home, a single tricky bay window, or you’re comparing different shade types, you’ll find step-by-step instructions plus pro tips to avoid the most common mistakes.
And if you’re planning a more customized look—layering shades with drapery, solving privacy plus glare, or working with unique window shapes—it can help to consult custom window treatment specialists who deal with these details every day. Even if you measure yourself, understanding how pros think will make your results better.
Inside-mount basics: what “inside” really means (and why it matters)
An inside mount means the blind or shade fits within the window casing (the interior frame), rather than sitting on the wall above it. The headrail or cassette mounts to the top of the frame (or sometimes the sides), and the fabric/slats drop within that opening. The goal is a clean, flush look—like the window was designed with the treatment in mind.
Because everything has to fit inside the opening, you’re measuring the actual space available, not the trim you see around it. That’s why you’ll measure width at multiple points, height at multiple points, and depth to confirm the hardware can physically sit inside the frame without sticking out too far.
Inside mounts are popular for modern interiors, minimal looks, and homes with beautiful woodwork you don’t want to cover. They’re also great when you want to keep walkways clear—like near doors, tight hallways, or kitchen windows above sinks—because the treatment stays tucked in.
Tools and setup: what you need before you touch the tape
You don’t need a toolbox the size of a suitcase, but you do need a few reliable basics. The most important is a steel tape measure (not a soft sewing tape). Steel tapes stay straight, which helps you measure wide windows accurately without sagging.
Grab a pencil, a notepad (or your phone notes), and a step stool if the window is tall. If you have one, a small level or even a smartphone level app is handy for checking whether the sill or top frame is noticeably out of level—useful context when deciding how to handle a tricky opening.
Before measuring, clear the sill. Remove anything that blocks the tape from reaching the true edges: plants, decor, or existing blinds if they’re in the way. If you’re replacing an old inside-mount treatment, don’t assume the old size is correct—many older installs “worked” but weren’t ideal, and you’re measuring for a better fit now.
Know your product type first: blinds vs. shades change the rules
Measuring isn’t identical across every product. A wood blind with slats behaves differently than a roller shade with a tube, and a cellular shade with a fabric stack needs different clearances than a Roman shade with folds. Before you finalize numbers, identify what you’re ordering.
Here’s the big idea: width is usually measured tight to the inside edges, while height is often measured to the longest point because you can always adjust the drop a bit during installation. But the exact “deductions” (tiny reductions from your measurement) are sometimes handled by the manufacturer, sometimes by you, and sometimes not at all depending on the brand and style.
If you’re ordering off-the-shelf, follow the packaging instructions exactly. If you’re ordering custom, read the measuring guide for that specific product. This article teaches the method that works in most situations, and then helps you apply it intelligently so you don’t get surprised by depth requirements or fabric gaps.
Step-by-step: how to measure inside-mount width (the part that makes or breaks the fit)
Step 1: Measure width in three places
Measure the inside width of the window opening at three points: the top, the middle, and the bottom. Keep the tape measure level and pull it tight from inside edge to inside edge (not from trim to trim, and not including any rounded drywall corners outside the frame).
Write down all three measurements in inches (or millimeters if your supplier uses metric), and label them clearly: W-top, W-mid, W-bot. This takes an extra minute but saves you from ordering something that fits the top and jams at the bottom.
Even in newer homes, frames can be slightly tapered. In older homes, it’s common to see a difference of 1/4″ or more. Measuring only once is basically guessing.
Step 2: Use the smallest width for inside mounts
For inside mounts, you almost always use the smallest of the three width measurements. That’s the narrowest point the headrail and the bottom rail have to pass through, so it’s the measurement that prevents rubbing and installation headaches.
Example: if you measure 35 7/8″ (top), 35 3/4″ (middle), and 35 13/16″ (bottom), you’d use 35 3/4″ as your working width. It may feel counterintuitive to “give up” width, but it’s better than forcing a too-wide headrail into a tight spot.
If the difference between your widths is more than about 1/2″, pause and investigate. That can signal a seriously out-of-square opening, a bowed jamb, or thick paint buildup. You can still do an inside mount, but you may need to choose a style with more forgiving side clearance or consider an outside mount for a cleaner look.
Step 3: Decide how to handle deductions (don’t assume)
Some custom manufacturers automatically deduct a small amount from your width to ensure smooth operation (often 1/8″ to 1/4″ total). Others want your exact inside measurement and they’ll build to that. Off-the-shelf blinds usually come in set widths and may already be sized slightly smaller than the label indicates.
This is where people accidentally double-deduct: they subtract 1/4″ themselves and then the manufacturer subtracts again. The result is a treatment that’s noticeably too narrow with light gaps that feel like you’re wearing sunglasses with a crack in the lens.
Best practice: measure precisely, record the smallest width, and then follow the ordering instructions for your specific product. When in doubt, ask the supplier whether the width you provide is “finished size” or “opening size.”
Step-by-step: how to measure inside-mount height (and avoid the too-short surprise)
Step 1: Measure height in three places
Measure the inside height from the top inside edge of the window opening down to the sill at three points: left, middle, and right. Keep the tape measure straight and snug against the surface.
Record all three: H-left, H-mid, H-right. If you have a deep sill with a slope, measure to the point where the treatment will actually stop (usually the flat part of the sill). If you’re unsure, measure to the lowest point of the sill to avoid a shade that stops awkwardly above the bottom.
If the window has no sill (for example, a door window or a window that meets a flat return), measure to the bottom inside edge of the frame.
Step 2: Use the largest height for inside mounts
For inside mounts, you typically use the largest of the three height measurements. Why? A shade that’s slightly longer can be adjusted during install or will simply stack a touch more at the bottom when fully lowered. A shade that’s too short can’t be “stretched” to reach the sill.
Example: if you measure 59 7/8″ (left), 60″ (middle), and 59 3/4″ (right), you’d use 60″ as your working height. That ensures you’re covered even if the top or sill is slightly out of level.
For blinds with slats, the exact height is often less critical than width, but you still want the bottom rail to land where it looks intentional—especially on groups of windows in the same room.
Step 3: Consider “stack height” and clearance at the top
Some shades stack up more than others. Cellular shades and roller shades typically have compact stacks, while Roman shades can have a thicker fold stack that sits lower when raised. If your window has a beautiful view and you want maximum glass exposure when the shade is up, stack height matters.
Also consider obstructions like window cranks, locks, or alarm sensors. A shade might physically fit by measurement, but still bump into a crank handle unless you choose a style with a spacer bracket or a different mount position.
If you’re measuring a window with a protruding crank, note the crank’s position and how far it sticks out. You may need a shade with more projection or you may need to mount slightly forward using extension brackets (which can still be “inside mount” in many cases if the brackets are within the frame depth).
Depth check: making sure your treatment can actually sit inside the frame
Depth is the most overlooked measurement, and it’s the one that causes the most “Wait, what?” moments on install day. Inside mounts require enough depth for the headrail or cassette to fit without sticking out awkwardly or interfering with window operation.
Measure the depth from the front inside edge of the window opening (where the treatment will sit) back to the glass or any obstruction. Many products list two depth numbers: “minimum depth” (it will fit) and “flush mount depth” (it will sit fully recessed).
If you don’t have flush-mount depth, that’s okay—plenty of inside mounts project slightly. The key is that the window can still open, and the treatment looks intentional rather than cramped.
Minimum depth vs. flush depth: what it means visually
Minimum depth means the brackets and headrail can be installed securely, but part of the headrail may extend beyond the frame. This can still look great, especially with modern roller shades or sleek cellular shades. It’s also common on older windows with shallow frames.
Flush depth means the entire headrail/cassette sits inside the opening, giving that crisp, built-in look. If you’re after a clean line and you have deep frames, it’s worth aiming for flush mount.
If your depth is borderline, choose a style known for a slimmer headrail. Roller shades, some cellular shades, and aluminum mini blinds tend to be more depth-friendly than chunkier wood blinds or Roman shades.
Watch for hidden depth thieves
Depth isn’t always uniform. Window stops, mullions, decorative trim inside the frame, and even thick weatherstripping can reduce usable depth. Measure where the brackets will actually go—usually near the top corners.
Also check for window hardware: tilt latches, crank housings, and locks can stick into the space a headrail wants to occupy. If you’re doing an inside mount on a casement window, make sure the crank won’t collide with the shade’s bottom rail when it’s lowered.
If you find obstructions, note them on your worksheet. That little note can save you from ordering a style that technically “fits,” but doesn’t function well day-to-day.
Out-of-square windows: how to measure when nothing matches
Real windows are rarely perfect rectangles. Drywall can bulge, wood can warp, and older homes can settle. The good news is you can still get a clean inside mount—you just need to measure with the assumption that the opening is unique, not ideal.
If your width varies noticeably top-to-bottom, use the smallest width as discussed, and consider a style with better side coverage. For example, a cellular shade may show slightly less “gap drama” than a blind with rigid slats, depending on the light angle and room brightness.
If your height varies, using the largest height typically works, but you may see a slight angle at the bottom rail when the shade is lowered. That’s not always avoidable; it’s often a sign the sill is out of level. In those cases, decide what you care about more: a bottom rail that follows the sill or a bottom rail that looks level to the room. Some installers can shim brackets to balance appearance.
When to consider an outside mount instead
If the opening is dramatically out of square (for example, more than 3/4″ difference in width measurements), an outside mount can look cleaner because it ignores the imperfect opening and creates a new “rectangle” on the wall. It also improves light control by reducing side gaps.
Outside mounts are also helpful when you don’t have enough depth for the product you want, or when you need to cover unattractive trim. That said, if your goal is to keep the trim visible and your frame is only slightly imperfect, inside mount is still very doable.
Another option is a hybrid approach: inside mount for most windows, outside mount for the problem child window. Consistency matters, but function matters more—especially in bedrooms where light leakage can be a dealbreaker.
Pro tip: measure the opening, then measure the “visual” lines
Sometimes the tightest point isn’t the top/middle/bottom—it’s a paint ridge or a slightly bowed jamb. After you record your three widths, run the tape along the opening and look for spots where the tape catches or where the frame narrows.
This is especially common in older homes with many layers of paint. If you find a pinch point, treat it as your true minimum width.
If you’re ordering expensive custom treatments, it can be worth lightly sanding or scraping paint buildup before installation. That tiny bit of prep can turn a frustrating fit into a smooth one.
Measuring different window types: the steps are similar, but the details change
Once you know the core method (three widths, three heights, depth check), you can measure almost anything. The challenge is that different window styles come with different obstructions and “where does the treatment stop?” questions.
Below are common window types and what to watch for so your inside mount looks intentional and operates smoothly.
Casement windows (cranks, handles, and clearance)
Casement windows often have cranks that protrude into the space where a shade would hang. Measure depth at the top where the headrail mounts, and also check the crank area for clearance when the shade is fully lowered.
If the crank is a problem, you might choose a shade that sits closer to the glass (like a roller shade) or use spacer/extension brackets to move the shade slightly forward. The key is to keep the shade from rubbing the crank or forcing you to contort your hand every time you open the window.
Also consider top-down/bottom-up cellular shades for privacy while keeping the crank accessible. They can be a great compromise in kitchens and bathrooms.
Double-hung windows (locks and meeting rails)
Double-hung windows usually have a lock at the meeting rail. Most inside-mount treatments sit above that area, but if your frame is shallow, the headrail may end up close to the lock hardware.
Measure depth near the top corners and check for any protrusions. When the shade is lowered, make sure the bottom rail won’t hit the lock when you raise the lower sash.
If you frequently open the window, prioritize a style with easy lift (cordless or motorized) so you’re not tugging around hardware. Smooth operation matters more than you think once you live with it.
Sliding windows and doors (wide spans and deflection)
Sliding windows can be wide, and wide windows make tape measures sag. If your window is wider than your arm span, hook the tape securely on one side and use a helper to hold it level, or use a rigid measuring stick for part of the span.
For inside mounts on wide openings, pay attention to the headrail’s stiffness. Some products may require a center support bracket above a certain width. That bracket can affect how the shade stacks or how the blind tilts, so note it during planning.
For sliding doors, inside mounts are possible if you have a deep header, but many people choose outside mounts for better coverage. If you do inside mount, confirm the handle clearance so the treatment doesn’t interfere with door operation.
Bay windows and grouped windows (alignment and consistency)
Bay windows are gorgeous but measuring them is a bit like measuring three windows that want to behave as one. Treat each window as its own inside mount opening and measure separately—do not assume they’re identical.
Then think about alignment: do you want all bottom rails to line up perfectly when lowered? If so, you may choose a consistent finished height even if one window is slightly taller, but you’ll need to ensure the shorter opening can still accommodate that height without the shade bunching awkwardly.
In grouped windows (like two or three side-by-side), consistent style and similar headrail height create a cohesive look. If one opening is substantially different, consider whether a single continuous outside-mount treatment might look cleaner than multiple inside mounts.
Recording measurements like a pro (so you don’t mix up 12 windows)
Measuring is only half the job—organizing your measurements is the other half. If you’ve ever written “Kitchen window” on a scrap of paper and then realized you have two kitchen windows, you know what I mean.
Create a simple worksheet with columns for room, window name, width top/mid/bottom, height left/mid/right, depth, and notes (crank, lock, tile backsplash, etc.). Give each window a unique label like “LR-1,” “LR-2,” “BR-1,” and so on.
Also note whether you’re measuring in inches with fractions or decimals. Don’t mix systems. If you measure 35 3/4″ on one line and 35.75 on another, it’s easy to misread later when ordering.
Photos are your best friend
Take a quick photo of each window and name the photo to match your label (e.g., “BR-2”). If there’s an obstruction, photograph that too. When you’re ordering later, you’ll be amazed how helpful it is to see the exact frame and hardware.
If you’re coordinating colors across rooms, photos help you remember wall tones, trim color, and flooring—details that matter when choosing fabric opacity and shade color.
Photos also reduce the chance you order a blackout shade for the living room and a light-filtering shade for the bedroom because you swapped labels. It happens more than people admit.
Double-checking without re-measuring everything
If you have a lot of windows, you don’t necessarily want to measure everything twice from scratch. Instead, do a quick “sanity check” pass: confirm that similar windows have similar sizes, and that no measurement is wildly out of range.
For example, if most bedroom windows are around 35″ wide and one is recorded as 53″, that’s a flag to re-check. Same with heights—if everything is around 60″ and one is 80″, make sure you didn’t measure to the floor by accident.
This quick review catches the most expensive mistakes before you click “order.”
Light gaps, privacy, and the reality of inside mounts
Inside mounts almost always have small light gaps at the sides because the treatment has to move freely within the frame. The amount depends on the product type, the window’s squareness, and how the manufacturer handles deductions.
If you’re sensitive to light (hello, streetlights and early sunrises), plan for it. Blackout fabric helps, but side gaps still let in slivers of light. To reduce gaps, you can choose shades with side channels, layer with drapery, or consider an outside mount in bedrooms.
Privacy is similar: during the day, light-filtering shades can obscure views nicely, but at night with interior lights on, silhouettes can appear. If privacy is critical, consider room-darkening or blackout materials, or add a second layer like sheers or drapes.
Matching the shade to the room’s job
In a living room, you might want glare control without making the space feel like a cave. Light-filtering roller shades or woven shades can soften sunlight while keeping the room bright. In bedrooms, blackout cellular shades are popular for sleep and insulation.
Kitchens and bathrooms often need privacy plus moisture resistance. Faux wood blinds and certain synthetic roller materials handle humidity better than natural woven materials.
If you’re outfitting a workspace, consider screen shades to reduce glare while keeping your view. The right openness factor can make a home office feel calmer without turning it into a bunker.
Layering for a finished look
Inside-mount shades look especially polished when layered with side panels or drapery. The shade handles privacy and light control; the drapery adds softness, color, and a more “designed” feel.
Layering also gives you flexibility: keep the shade down for glare control, then close drapes at night for privacy and warmth. It’s a great approach if you want inside mounts but still want to minimize light gaps.
When layering, make sure your drapery hardware is mounted high and wide enough so it doesn’t crowd the inside-mount treatment. You want each layer to operate smoothly without bumping the other.
Special situations: when measuring gets a little weird
Some windows don’t play by the usual rules—think tile returns, deep recessed frames, or windows with decorative mullions. You can still measure them accurately, but you’ll want to slow down and think through how the treatment will sit and move.
Below are a few common “curveballs” and how to handle them.
Windows with tile or stone returns
In bathrooms and kitchens, the inside edge of the opening might be tile, not wood. Tile edges can be slightly uneven, and you don’t want to measure to a grout bump that makes your shade too small.
Measure carefully at top/middle/bottom and note any spots where the tile narrows the opening. If the tile edge is irregular, you may need to choose a slightly smaller width (or use an outside mount) to avoid chipping tile during installation.
Also consider how you’ll drill for brackets—tile requires the right bit and anchors. If you’re not comfortable drilling tile, it may be worth having an installer handle it.
French doors with glass inserts
Inside mounts on French door glass inserts can be tricky because the “frame” around the glass is often shallow. Measure the glass opening width and height, and then measure depth to see if a slim shade (like a mini blind or a thin cellular shade) can fit.
Many people use outside mounts on doors with hold-down brackets to prevent swinging. But if you have enough depth and want a cleaner, integrated look, an inside mount can work—just confirm the door handle clearance and how the shade will behave when the door opens and closes.
If the door sees a lot of use, consider cordless or motorized options to reduce wear and tear from frequent handling.
Arched or specialty-shaped windows
True arches and specialty shapes usually require specialty treatments, and “inside mount” might refer to mounting within the shape’s frame rather than a rectangle. If you’re measuring an arch with plans to cover it, you’ll likely need a template or professional measurement.
For many homes, a practical approach is to leave the arch uncovered and install inside-mount shades on the rectangular portion below. In that case, measure the rectangle as usual and ensure the headrail doesn’t interfere with the arch trim.
If you’re determined to cover the specialty shape, consult the manufacturer’s measuring instructions carefully—this is one of those areas where generic advice can lead to expensive mistakes.
Ordering tips: turning measurements into the right product
Once you’ve measured, the next challenge is ordering correctly. Different retailers and manufacturers use different terminology, and it’s easy to assume “width” means the same thing everywhere.
Look for whether the listed width is “headrail width” or “fabric width.” On roller shades, the fabric is often slightly narrower than the overall bracket-to-bracket width. That’s normal, but it affects light gaps. If side gaps are a big concern, choose a system designed to minimize them or plan to layer.
Also confirm lift type (cordless, corded, motorized) because it can affect headrail size and minimum depth. Motorized shades often need a bit more room for the battery tube or motor housing.
Consistency across rooms
If you’re doing multiple rooms, decide on a consistent approach for light control: light-filtering in common areas, blackout in bedrooms, moisture-resistant in bathrooms. Keeping the same style across a floor can look cohesive even if opacity varies by room.
Color consistency matters too. Whites vary—some are bright white, some are warm, some are gray. If you can, order swatches and view them in the room at different times of day.
If you’re matching to trim, hold the swatch directly against the trim paint. Lighting can trick your eye, especially in north-facing rooms.
When it’s worth getting help
If you’re measuring a whole home, have lots of out-of-square openings, or you’re investing in premium materials, professional measuring can be worth it. It’s not just about the tape measure—it’s about knowing which compromises look best when the window isn’t perfect.
It’s also worth getting help if you’re planning layered treatments, motorization, or you have tall, hard-to-reach windows. A small measuring error on a large shade can be an expensive lesson.
For homeowners in Southern California looking for tailored options, services like Long Beach shutters and shades can be a helpful reference point for what’s possible when you want a more custom fit and finish.
Measuring for commercial spaces: same method, higher stakes
If you’re measuring for an office, storefront, clinic, or any professional environment, inside mounts can look especially sharp—clean lines, less visual clutter, and a more intentional design. The measuring method is the same, but the expectations are higher because the space needs to function consistently day after day.
Commercial windows are often larger, and you may have more variables: tinted glass, security film, HVAC drafts, and sunlight that shifts across workstations. Accurate measurement helps ensure smooth operation and a uniform look across multiple windows.
If you’re outfitting a workplace, it’s worth exploring solutions designed specifically as window coverings for businesses, since durability, safety compliance, and glare control can matter more than they do at home.
Uniformity across multiple windows
In a commercial setting, small differences stand out more because you often have rows of windows side-by-side. Measure each opening individually (don’t assume they’re the same), but plan for consistent product choices and consistent mounting height.
If the building has slight variations in window openings, you might choose a standardized finished size where possible, but only if the smallest opening can accommodate it. Otherwise, you’ll need custom sizing per window to keep everything operating smoothly.
Also consider whether you need chain controls, cordless, or motorized systems—especially for tall windows where accessibility and safety are important.
Glare control and screen selection
For offices, a screen shade can be a game-changer. The openness factor determines how much light and view you keep while cutting glare. A lower openness reduces glare more but darkens the room; a higher openness preserves view but may not tame harsh sun enough.
Measure carefully and consider how the sun hits the space during working hours. A west-facing window that looks fine in the morning can become blinding at 4 p.m.
If you’re unsure, it can help to test with a temporary film or shade to understand the light before committing to a full install.
Common measuring mistakes (and how to avoid them)
Most measurement problems come from a few predictable missteps. The good news is that once you know them, they’re easy to avoid.
Here are the big ones I see people run into when measuring for inside mounts.
Mistake 1: Measuring trim instead of the opening
Inside mounts are about the inside edges of the window frame. If you measure the outer trim, you’ll order something too wide. Always place the tape measure inside the opening and measure edge-to-edge where the brackets will sit.
If the opening has rounded drywall corners, measure to the point where the treatment will actually contact—usually the straight part of the jamb, not the curve.
When in doubt, measure slightly deeper into the frame where the edges are more defined, but confirm that the treatment can still mount securely there.
Mistake 2: Forgetting depth until after ordering
It’s easy to focus on width and height and assume depth will be “fine.” Then the shade arrives and the headrail sticks out awkwardly or hits the window handle.
Always record depth during your measuring session. Even if you end up choosing a different product, you’ll be glad you have the number.
If your depth is shallow, you can still do inside mounts—just choose a slim system or accept a slight projection as part of the look.
Mistake 3: Mixing up which number to use
Smallest width, largest height is the rule of thumb for inside mounts. People often do the opposite because it feels logical to “maximize coverage.” But inside mounts need clearance to operate.
Write your numbers clearly and circle the one you plan to use. If you’re ordering online, double-check that you’re entering the correct “order width” and “order height” fields.
And if you’re ordering multiple windows, label everything. A single swapped measurement can throw off the whole project.
A quick measurement walkthrough you can copy for every window
If you want a repeatable routine, use this checklist window-by-window. It keeps you from skipping a step when you’re on window number nine and your coffee wore off an hour ago.
1) Label the window (e.g., “LR-1”).
2) Measure width top/mid/bottom; record all three.
3) Circle the smallest width.
4) Measure height left/mid/right; record all three.
5) Circle the largest height.
6) Measure depth at top corners; record minimum usable depth.
7) Note obstructions (crank, lock, alarm sensor) and sill type (flat/slope).
8) Take a photo of the window and any obstruction.
Do that consistently, and you’ll have a clean set of measurements you can trust—whether you’re ordering one shade or outfitting the whole house.
Final confidence check: does your plan match your goal?
Before you order, take a moment to connect your measurements to what you actually want the room to feel like. If you want a crisp, recessed look, confirm you have flush-mount depth (or that you’re okay with a slight projection). If you want maximum darkness, plan for side-gap solutions like channels or layering.
Also consider how you’ll use the window. Do you open it daily? Is there a plant on the sill you don’t want to move? Is the window behind a couch where reaching a cord would be annoying? These lifestyle details influence whether you choose cordless, motorized, top-down/bottom-up, or a simpler option.
Measuring is the foundation, but the best inside-mount setup is the one that fits your space and your habits—so it looks great on day one and still feels great months later.