Measuring for blinds sounds like one of those “quick weekend tasks” until you’re standing there with a tape measure, realizing your window isn’t perfectly square and your trim isn’t as even as you thought. The good news: you don’t need to be a pro to get pro-level results. You just need a clear plan, the right measuring habits, and a solid understanding of the two main installation styles—inside mount and outside mount.
This guide walks you through the full process, step by step, with plenty of practical tips for real-life windows (the slightly crooked ones, the deep ones, the shallow ones, the ones with beautiful trim you don’t want to cover). If you’re shopping for custom blinds, accurate measurements are the difference between a smooth install and a frustrating redo.
We’ll cover what tools you need, how to choose inside vs. outside mount, how to measure each style correctly, and how to avoid the most common mistakes that lead to gaps, rubbing, or blinds that simply don’t fit. Along the way, you’ll also learn how different blind types and window shapes affect your measuring decisions.
Before you measure: the “why” behind inside vs. outside mount
Inside mount means your blinds sit inside the window frame. The headrail mounts to the top (or sides) of the window recess, and the blind drops neatly within that opening. It’s the look people often picture when they think “clean and tailored,” especially when you want to show off nice trim or keep the window area feeling crisp.
Outside mount means your blinds mount on the wall or trim above the window and overlap the opening. This is a go-to choice when you want better light blocking, when your window frame isn’t deep enough for an inside mount, or when the window itself is a bit… quirky (uneven, shallow, or missing a proper recess).
Neither is “better” universally. The right choice depends on your window depth, your privacy and light-control goals, and the style you’re after. The rest of this article will make that decision much easier.
Tools and prep that make measuring painless
What you’ll want on hand
A metal tape measure is non-negotiable. Cloth measuring tapes can sag and give you inaccurate numbers, especially across wider windows. You’ll also want a pencil, paper (or a notes app), and a step stool if your windows are tall.
If you have one, a small level can help you understand whether the top of your frame is truly level—useful when you’re deciding between inside and outside mount. A flashlight can also help you spot obstructions inside the frame (like a crank mechanism or a protruding latch).
Finally, clean the area around the window. Dust, cobwebs, and old hardware can get in the way, and you’ll measure more confidently when you can clearly see where the blind will sit.
How to record measurements so you don’t mix anything up
Write down each window name first (e.g., “Kitchen over sink,” “Primary bedroom left,” “Living room bay center”). Then create three lines for width and three lines for height. Even if you’re measuring a simple rectangle, you’ll take multiple measurements to account for slight variation.
Always record measurements in inches (unless your provider specifies otherwise) and round to the nearest 1/8 inch. It’s better to be precise now than to guess later. If you’re using a phone, consider taking a photo of the window and marking the numbers right on the image.
One more thing: don’t assume two windows that “look the same” actually measure the same. In many homes, especially older ones, small differences add up. Measure every opening individually.
Choosing inside mount: when it works beautifully
Frame depth: the first gatekeeper
Inside mount only works if you have enough depth in the window recess to fit the headrail (and sometimes the blind stack). Different blind styles require different depths, so check the product specs. As a general idea, many blinds need at least 2 inches for a comfortable inside mount, but some need more for a fully recessed look.
Depth isn’t just “is there a recess?”—it’s whether that recess is unobstructed. Window cranks, alarm sensors, protruding trim, or even a deep lip at the front of the frame can interfere. Measure depth at the top of the window where the headrail will sit.
If you’re borderline on depth, you might still do an inside mount but accept that the headrail will project a bit. That can be totally fine visually, but it’s good to decide intentionally rather than be surprised later.
When inside mount gives you the best look
Inside mount is ideal when you want a streamlined style that doesn’t cover your trim. It’s also great when you plan to layer curtains over blinds, because the blinds stay tucked in and the curtains can frame the window without feeling crowded.
It’s also a nice choice for smaller rooms where you don’t want the window treatment to visually “grow” the window area. Because the blind sits within the frame, it keeps the lines neat and contained.
That said, inside mount can leave small light gaps at the sides due to how blinds operate. If you’re very sensitive to light (think: bedroom, nursery, media room), you may prefer outside mount for better coverage.
Choosing outside mount: when it solves real problems
Shallow frames, uneven openings, and extra light control
Outside mount is your friend when the window recess is too shallow for the headrail. It’s also the easiest solution when the opening isn’t perfectly square. Instead of fighting the frame, you create your own “new” boundary by mounting on the wall or trim.
Another big reason people choose outside mount is light control. Because the blind overlaps the opening, you reduce side gaps and top gaps. This can make a noticeable difference in bedrooms, street-facing rooms, or anywhere you want more privacy.
Outside mount can also visually enlarge a window. If you mount wider and a bit higher than the frame, the window can look taller and more substantial—an interior designer trick that’s simple but effective.
Trim and wall space: the practical checks
Before committing to outside mount, make sure you have enough flat surface to mount the brackets securely. If you’re mounting to trim, confirm the trim is wide enough for the hardware. If you’re mounting to drywall above the window, you may need anchors or to hit studs, depending on the blind’s weight.
Also consider what’s around the window: do you have a nearby cabinet, a corner wall, or a door that swings open? Outside-mounted blinds need clearance so they don’t interfere with adjacent features.
Finally, think about the stack. When raised, the blind will sit above the window opening. If you’re mounting high, check that it won’t block artwork, bump into crown molding, or look awkward against architectural details.
How to measure for inside mount blinds (the exact method)
Measuring width: three spots, smallest number wins
For inside mount, measure the width of the window opening in three places: at the top, middle, and bottom. Place the tape measure inside the frame, from inside edge to inside edge. Keep the tape level—if it angles, you’ll accidentally measure longer than the true opening.
Write down all three widths, then circle the smallest one. That smallest number is the one you typically use for ordering, because it ensures the blind will fit even if the frame narrows slightly in one area.
Don’t subtract anything unless your supplier specifically tells you to. Many custom blind manufacturers build in the appropriate deductions so the blind can operate smoothly within the frame.
Measuring height: three spots again, longest number matters
Measure the height in three places: left, center, and right. Measure from the top inside edge of the frame down to the sill (or to where you want the blind to end if there’s no sill). Keep the tape straight and snug.
For inside mount, you’ll usually use the longest height measurement. That ensures the blind is long enough to cover the full opening even if one side is slightly taller.
If you have a deep sill or an obstruction (like a handle), note it. Sometimes you’ll want the blind to stop above the obstruction rather than brushing against it every time you raise or lower.
Measuring depth: don’t skip this step
Depth is the measurement that decides whether inside mount is truly viable. Measure the depth at the top of the window recess from the front edge (where the blind would sit) to the glass or the back wall of the recess.
Do this in a couple spots, because trim can be uneven. If the depth varies, record the shallowest depth. That’s the one that matters for bracket placement and headrail fit.
If your depth is less than the minimum required, switch to outside mount rather than forcing it. A too-tight inside mount can lead to blinds that scrape, bind, or sit crooked.
How to measure for outside mount blinds (the exact method)
Choosing overlap: the secret to better coverage
Outside mount measurements start with deciding how much you want the blind to overlap the window opening. A common guideline is 2–3 inches of overlap on each side for better light control and privacy, but you can adjust based on your space.
More overlap generally means less light seepage, but it also means a larger visual footprint. If you have limited wall space or nearby obstacles, you might need to reduce overlap on one side. That’s okay—just plan it consciously.
Also think about top overlap. Mounting the blind a few inches above the opening can help cover the top gap and can make the window look taller. Just make sure the raised blind won’t cover something you need (like a transom detail).
Measuring width: window opening plus planned overlap
Measure the width of the window opening from outside edge to outside edge (or from where you want the coverage to begin and end). Then add your overlap amount to both sides. For example, if the opening is 36 inches and you want 3 inches of overlap on each side, your ordered width would be 42 inches.
When measuring, check at the top, middle, and bottom of the opening. If the opening varies, use the widest measurement as your base so you don’t accidentally under-cover one section.
Unlike inside mount, you’re not trying to fit within a tight box—you’re trying to create enough coverage. So err on the side of slightly wider if your space allows.
Measuring height: decide where it starts and where it ends
For outside mount, measure from where you plan to mount the headrail (usually above the top of the opening) down to where you want the blind to end. Many people choose to extend a bit below the sill to prevent light from sneaking under the bottom.
If there’s a radiator, a desk, or a countertop below the window, you may want the blind to stop just above it. In kitchens, for example, stopping above a sink faucet can prevent constant contact and wear.
Record your final height clearly, including whether you’re measuring to the sill, below the sill, or to a specific stopping point. This is one of the most common places people “assume” instead of decide.
Inside vs. outside mount: quick decision scenarios that come up a lot
Bedrooms: privacy, darkness, and early mornings
If you’re trying to make a bedroom darker, outside mount often wins because it reduces side light gaps. Pairing outside mount with a light-filtering or room-darkening option can make mornings noticeably calmer, especially if you’re sensitive to sunrise light.
Inside mount can still work in bedrooms if you’re okay with a little light at the edges, or if you’re layering drapery over the blinds. In that case, the curtains can help cover the gaps while the inside-mounted blind keeps the look clean.
Also consider window depth in bedrooms. Many newer homes have deep frames that are perfect for inside mount, while older homes may have shallow frames that make outside mount the more straightforward path.
Kitchens and bathrooms: moisture, obstacles, and cleaning
Kitchens often have obstacles—faucets, window cranks, or backsplashes that push you toward inside mount (to keep things tight) or toward a carefully planned outside mount height (to avoid contact). The key is to measure with those obstacles in mind, not as an afterthought.
Bathrooms bring privacy to the top of the list. Depending on the window placement, outside mount can provide better coverage. But if the window is near a shower area, you’ll also want materials that handle humidity well.
In both spaces, think about cleaning access. Inside mount keeps the blind tucked in, which can be easier to wipe down around. Outside mount can cover more area, which is great for privacy, but it may also collect more dust on the headrail if it sits proud of the wall.
Living rooms: style, trim, and “designer” proportions
Living rooms often have the prettiest trim—and that’s where inside mount shines. If your casing is a feature, inside mount lets it stay visible and keeps the window looking architectural.
Outside mount can still be a great design move in living rooms when you want the window to feel larger. A wider, higher mount can make ceilings feel taller and the room feel more open.
When you’re deciding, stand back and look at the wall space around the window. If you have room to expand, outside mount can be a subtle upgrade. If the window is tight to a corner or adjacent wall, inside mount may look more intentional.
Tricky windows: how to measure when nothing is standard
Bay and bow windows: treat each section like its own project
Bay and bow windows look amazing, but they demand careful measuring. Each window panel often has slightly different dimensions, and the angles between them can affect how blinds operate and stack.
Measure each section individually—width, height, and depth—and label them clearly (left, center, right). If you’re doing inside mount, pay extra attention to depth because bay windows sometimes have unique framing that changes from panel to panel.
Also consider clearance for the blind when raised. In a bay, the stacked blinds can visually “crowd” the top area, so planning mounting height and blind style matters more than you might expect.
Sliding doors and large openings: managing weight and coverage
Large openings require extra care because small measurement errors become very noticeable. Use a sturdy tape measure, measure multiple times, and consider having a second person hold the tape for accuracy.
For wide spans, think about whether one large blind makes sense or whether multiple blinds side-by-side will operate better. The mounting surface also matters: you’ll want secure attachment points to handle the size and movement.
If you’re measuring for an outside mount on a large opening, plan overlap thoughtfully so you don’t block door handles or interfere with the door’s sliding path.
Windows that aren’t square: how to avoid rubbing and crooked lines
Many window openings are slightly out of square, especially in older homes. That’s why measuring in three spots is so important. For inside mount, always use the smallest width; it’s your insurance policy against binding.
If the difference between measurements is significant (say, more than 1/4 inch), consider outside mount. It can mask the unevenness and give you a straighter visual line.
You can also note the out-of-square condition when ordering or consulting with a pro. Sometimes a small shim during installation can correct a slight tilt, but it’s better to know what you’re dealing with upfront.
Common measuring mistakes (and how to dodge them)
Mistaking “opening size” for “order size”
One of the biggest mix-ups is assuming the measurement you take is always the measurement you order. For inside mount, many manufacturers make small deductions automatically. For outside mount, you usually add overlap yourself.
Read the measuring instructions for the specific product you’re buying. If you’re working with a provider, ask whether they want exact opening size or final order size. This one detail prevents a lot of headaches.
If you’re unsure, write down both: the raw opening measurement and your planned order measurement. Clarity beats guesswork every time.
Measuring to the wrong points on the frame
For inside mount, you must measure inside edge to inside edge, not from trim to trim. For outside mount, you’re measuring coverage area, which may include trim and wall space.
It sounds obvious, but it’s easy to slip into “close enough” measuring when you’re moving quickly. Slow down and place the tape exactly where the blind will sit.
If your frame has decorative molding inside the recess, measure behind it (if that’s where the blind will mount) or plan an outside mount. Decorative profiles can steal usable depth and width.
Forgetting about handles, locks, and cranks
Window hardware is often the reason a blind doesn’t sit flush or doesn’t drop cleanly. Before you finalize measurements, open and close the window and note what sticks out into the space where the blind will hang.
If you’re doing inside mount, check whether the blind will clear a crank handle. If you’re doing outside mount, check whether the blind’s bottom rail will hit a latch when lowered.
When in doubt, take a couple photos and measurements of the obstruction itself (how far it protrudes). That info helps you choose a mount style and a blind type that won’t fight your window.
How blind type affects measuring decisions
Horizontal blinds: clean lines, but side gaps are normal
Horizontal blinds (like wood or faux wood) are popular because they’re timeless and easy to adjust for light. For inside mount, they look especially tailored. But it’s normal to have small light gaps at the sides due to the way the slats and ladder system work.
If side light is a deal-breaker, consider outside mount or layering with curtains. Measuring for outside mount with generous overlap can make a big difference in perceived privacy.
Also note that thicker slats can mean a larger stack when raised. That can influence how high you mount outside-mounted blinds or whether you want a fully recessed inside mount.
Vertical blinds and panels: think about clearance and travel
Vertical blinds and panel systems are often used for sliding doors and wide windows. Measuring width is only part of the story—you also need to consider stack direction (left, right, split) and whether the stack will block a walkway.
Outside mount is common for these larger openings because it provides better coverage and can hide uneven edges. But you’ll want to measure height carefully so the bottom doesn’t drag on the floor.
If you have baseboards or a thick trim piece, note it. The track placement and the bottom clearance can change how the whole system hangs.
Layering with curtains: plan the whole wall, not just the glass
If you’re adding curtains, decide whether the blinds are the “workhorse” for privacy and light, while curtains are decorative, or whether both need to function daily. This affects whether you prioritize inside mount (to keep blinds discreet) or outside mount (to maximize coverage).
Measure with your curtain rod placement in mind. If the rod brackets will sit near the same area as the blind brackets, you’ll want to avoid conflicts. Sometimes a slightly higher blind mount solves everything.
And if you’re going for that airy, wide curtain look, you may intentionally choose a wider outside-mounted blind so the curtains can sit farther out without revealing gaps.
Double-checking your numbers like a pro
The “measure twice” routine that actually works
Measure once, write it down, then measure again from scratch. Don’t just glance at the tape and say “yep, same.” Reset the tape, re-seat it against the edges, and confirm your numbers.
If your second measurement differs, measure a third time. Use the result that repeats. This is especially important on wide windows where the tape can sag or shift.
It also helps to measure at a similar time of day if sunlight glare makes it hard to see the tape markings. Small visibility issues can create small errors—and small errors matter with inside mounts.
When to bring in help (and why it’s not “cheating”)
If your windows are unusually large, high, or oddly shaped, having a second person makes measuring safer and more accurate. One person can hold the tape firmly while the other reads and records.
It’s also smart to consult a specialist if you’re investing in multiple rooms and want everything to look consistent. Measuring is the foundation—once the blinds are made, there’s not much wiggle room.
Many homeowners like to get guidance from a local window treatment business because they’ve seen every weird window scenario imaginable and can spot issues you might not notice until installation day.
Making the process easier when you’re ordering custom
What to ask before you place the order
Before you hit “buy,” confirm what measurements the provider expects for inside vs. outside mount. Ask whether they take deductions automatically for inside mounts, and if so, how much. Also ask about minimum depth requirements and whether the listed depth is for “minimum” vs. “fully recessed.”
Ask about returns or remakes if measurements are off. Some custom products have limited return options because they’re made to order, but many companies have policies that help if something arrives incorrect or if guidance was unclear.
And if you’re ordering multiple windows, ask how to keep things visually consistent—like matching headrail heights across a room even if window heights vary slightly.
Why local expertise can save time (and sometimes money)
Custom ordering can feel overwhelming because there are so many choices: mount type, slat size, color, control type, and more. Having someone walk you through the options can prevent mismatches—like choosing an inside mount when your depth can’t support it.
If you’re exploring custom window coverings in Jersey, you’ll often find that local providers understand common regional home styles (and their quirks), from older frames that aren’t square to newer builds with deep, clean recesses.
Even if you measure yourself, it can be helpful to have a pro sanity-check your approach—especially for big investments like a full-home refresh.
Real-world measuring examples you can copy
Example 1: Inside mount on a standard bedroom window
Let’s say you measure width at the top (35 7/8″), middle (35 3/4″), and bottom (35 7/8″). Your smallest width is 35 3/4″—that’s the number you’ll use for inside mount ordering in most cases.
Then you measure height on the left (58 1/8″), center (58″), and right (58 1/4″). Your longest height is 58 1/4″—that’s typically what you’ll use so the blind fully covers the opening.
Finally, you measure depth at the top and find 2 1/4″ of clear depth. If the product requires 2″ minimum, you’re good to go.
Example 2: Outside mount for better light blocking in a nursery
Your window opening measures 34″ wide. You decide on 3″ overlap on each side for better darkness, so your ordered width becomes 40″.
For height, your opening is 52″ tall. You plan to mount 3″ above the opening and extend 2″ below the sill, giving you 57″ total height (52 + 3 + 2). That extra coverage helps reduce top and bottom light leaks.
Before finalizing, you check that you have enough flat wall space above the window for brackets and that the blind won’t interfere with nearby shelving.
Example 3: Inside mount on a slightly out-of-square kitchen window
You measure widths and get: top 29 7/8″, middle 29 3/4″, bottom 29 5/8″. That bottom number is the smallest, so it becomes your inside mount width reference.
You then measure heights: left 41 7/8″, center 42″, right 42 1/8″. You’d likely use the longest height, 42 1/8″.
Because the width varies by 1/4″ from top to bottom, you note that the opening is out of square. That’s not a deal-breaker, but you’ll want to be careful during installation to keep the headrail level and ensure smooth operation.
Keeping your project on track from measuring day to install day
Labeling and organizing multiple windows
If you’re measuring more than a couple windows, organization becomes half the battle. Create a simple table with columns for room, window label, mount type, width (three measurements), height (three measurements), depth, and notes (obstructions, trim, special requests).
When you’re ordering, double-check that the window label on your order matches your notes. It’s surprisingly easy to swap “Living room left” and “Living room right” when you’re moving quickly.
And if you’re mixing mount types in the same room, note the reason. That way, if you revisit the plan later, you’ll remember why one window is outside mount (shallow depth) while the others are inside mount.
What to do if you’re between inside and outside mount
If you’re torn, use these tie-breakers: choose inside mount if you have enough depth and you care most about a clean, built-in look. Choose outside mount if you care most about light blocking, your frame is shallow, or your opening is noticeably uneven.
You can also do a quick mock-up with painter’s tape: outline the blind’s footprint on the wall for an outside mount and step back. If it looks too bulky, inside mount might be the better aesthetic choice. If it looks balanced and improves proportions, outside mount can be a design win.
When you’re ready to explore options, checking out providers like Bumble Bee Blinds can help you compare styles and get clarity on depth requirements and measuring specifics for the exact products you’re considering.
A final checklist you can use at the window
Inside mount checklist
Confirm you have enough unobstructed depth at the top of the recess. Measure width in three places and use the smallest. Measure height in three places and use the longest. Note any obstructions like cranks, handles, or sensors.
Make sure you’re measuring inside edge to inside edge, not trim to trim. If you find large variations between measurements, consider whether outside mount would look and function better.
Before you order, verify whether the manufacturer makes automatic deductions for inside mount—this is key to getting the right fit.
Outside mount checklist
Decide overlap on each side (commonly 2–3 inches) and confirm you have the wall/trim space to support it. Measure the opening and add overlap for width. Decide your mounting height above the opening and where you want the blind to end below.
Check for nearby obstacles: cabinets, doors, corners, or molding. Confirm the raised blind won’t block something important or look awkward.
Write down your final “coverage plan” clearly so your order reflects your intent, not just the raw opening size.
With careful measuring and a little planning, you’ll end up with blinds that fit beautifully, operate smoothly, and look like they were always meant to be there.