How to Measure Windows for Blinds (Inside Mount vs Outside Mount)

Measuring windows for blinds sounds like it should be a quick “grab a tape measure and go” kind of task. But once you start looking at real-life windows—trim that isn’t perfectly square, drywall that bows a little, a deep sill here, a shallow frame there—it gets clear why so many people end up with blinds that scrape, gap, or just don’t sit right.

This guide walks you through measuring for the two most common installation styles: inside mount (blinds sit within the window opening) and outside mount (blinds sit on the wall or trim around the opening). You’ll learn what to measure, how to measure it, how to avoid common mistakes, and how to think through tricky windows like bay windows, sliding doors, and older homes where “standard” is more of a suggestion than a reality.

If you’re the DIY type, you’ll be able to measure confidently by the end. And if you’d rather have a pro double-check things, you’ll also know what a good measurement process looks like—so you can feel confident you’re getting a clean, tailored result.

Start with the big decision: inside mount or outside mount?

Before you measure anything, decide how you want the blinds to sit. Inside mount gives that built-in, streamlined look because the blind headrail fits inside the window frame. It’s often the go-to choice for modern homes or any room where you want a tidy, minimal profile.

Outside mount is more flexible. It’s installed above or around the window opening, usually on the wall or on the trim. It’s a great fix for shallow windows, uneven frames, or when you want maximum light control and privacy—because you can overlap the window opening and reduce those light gaps on the sides.

One quick way to choose: if your window frame is deep enough and fairly square, inside mount is usually easiest to make look polished. If the frame is shallow, the opening is uneven, or you’re trying to cover an awkward window, outside mount is often the better option.

Tools and setup that make measuring way easier

You don’t need a toolbox the size of a garage, but you do need the right basics. A steel tape measure (not a cloth one) is the big one—steel gives you accuracy and won’t sag. A pencil and paper (or a notes app) helps you record measurements clearly. If your windows are tall, a step stool makes the whole process safer and more accurate.

It also helps to measure in good light. If your window area is dim, turn on lights or measure during the day. You’ll be more likely to see if your frame is slightly bowed or if the trim is uneven—details that matter when you’re trying to get a clean fit.

Finally, decide how you’ll label things. A simple system saves headaches: “Living room – front window – inside mount” and then list width and height. It sounds obvious, but when you measure multiple windows, it’s easy to mix up which notes belong to which room.

Inside mount measuring: what “fits” really means

Inside mount blinds sit within the window opening. That means the width measurement must fit between the left and right inside edges of the frame (or jamb). The height measurement runs from the top inside edge down to the sill (or the bottom inside edge, depending on your window style and the blind type).

Inside mount looks crisp, but it’s less forgiving. If you measure too wide, the headrail may not fit. If the opening isn’t square, the blind can rub or hang unevenly. The goal is to capture the “tightest” width and the “tallest” height so the blind fits and covers properly.

Also, inside mount isn’t only about the opening size—it’s about depth. Many blinds need a minimum depth to sit flush or partially recessed. If your frame is shallow, the blind may protrude more than you expect, which can affect how it looks next to trim, handles, or window cranks.

How to measure inside mount width (the part most people get wrong)

Measure the width in three places: at the top, middle, and bottom of the window opening. Place the tape measure inside the frame, touching the inside left edge and stretching to the inside right edge. Write down all three numbers.

Then use the smallest of the three widths. This matters because if the window opening is slightly narrower at one point (very common), the blind must fit that narrowest spot. Using the smallest width helps prevent the headrail from binding or not fitting at all.

If the measurements vary a lot (say, more than 1/4 inch), that’s a sign the opening is out of square. Inside mount can still work, but you’ll want to be extra careful about the style you choose and how much light gap you can tolerate on the sides.

How to measure inside mount height (and why you still do three measurements)

Measure height in three places too: left, center, and right. Measure from the top inside edge of the opening down to the sill (or to the bottom inside edge if there’s no sill). Record all three measurements.

For height, you typically use the largest measurement. That ensures the blind is long enough to cover the full opening even if one side is slightly taller. A blind that’s a bit longer is usually fine (most styles have a bottom rail that can sit slightly lower), but a blind that’s too short can look awkward and let in unwanted light.

If your window has a deep sill or an uneven one, note that too. Some blinds sit differently depending on how the sill is shaped, and that can affect how “finished” the bottom edge looks when the blind is fully lowered.

Depth check: the inside mount detail that affects the final look

Depth is the distance from the front edge of the window opening back to the glass (or back to any obstruction). Measure depth at the top of the window frame where the headrail will sit. If you have window cranks, locks, or handles, check whether they’ll interfere with the blind when it’s lowered.

Different blind types have different depth needs. Some can be installed with minimal depth but will protrude. Others need more depth to sit flush. If you’re aiming for a fully recessed look (where the blind sits neatly inside the frame), depth matters even more.

When depth is borderline, outside mount can be a smarter choice. Or you can choose a slimmer blind style. The key is to decide before ordering so you don’t end up with a blind that technically fits but looks bulky.

Outside mount measuring: when you want more coverage and fewer light gaps

Outside mount blinds are installed on the wall or the trim around the window. They’re a great solution for windows with shallow frames, irregular openings, or when you want to cover the whole window area more generously.

Outside mount is also helpful for room-darkening setups. Even the best blinds can let light leak around the edges; outside mount lets you overlap the window opening so you get better light control and privacy.

The trick with outside mount is deciding how much overlap you want on each side and above/below. Too little overlap and you don’t get the benefits. Too much and the blind can look oversized or interfere with nearby trim, cabinets, or doorways.

How to measure outside mount width (and choose overlap)

Start by measuring the width of the area you want to cover. Often that’s the window opening plus extra on both sides. A common rule of thumb is adding 2–3 inches of overlap per side (so 4–6 inches total), but you can adjust based on your space.

Measure from where you want the blind to start on the left to where you want it to end on the right. If you’re mounting on trim, you might measure edge to edge of the trim. If you’re mounting above the trim on the wall, you’ll measure a wider span.

Also think about what’s near the window. If there’s a corner wall close by, a cabinet, or a light switch, you may need to reduce overlap on one side. It’s okay to be asymmetrical if it’s the only way to make it work—just plan it intentionally.

How to measure outside mount height (and where the top should go)

Measure the height from where the top of the headrail will be mounted down to where you want the blind to end. Many people mount the headrail a few inches above the window opening to make the window look taller and to cover the top edge more effectively.

For the bottom, decide whether you want the blind to end at the sill, below the sill, or down to the floor (common for patio doors). If you want better light control, letting the blind extend a bit below the window opening helps.

Outside mount height is also where you can “cheat” a little to improve proportions. Mounting higher can make a small window feel bigger. Just make sure you’re mounting into something solid—studs, wood trim, or appropriate anchors—so the blind is secure.

Outside mount and trim: making sure it looks intentional

If your window has thick decorative trim, outside mount can look really clean when aligned with that trim. In many homes, mounting the blind on the trim creates a natural frame and keeps the hardware visually tidy.

If you’re mounting on drywall above the trim, consider how the headrail will look from the side. Some blinds have a valance or fascia that helps disguise the headrail. Without that, you may see the top hardware more than you expect.

Take a step back and picture the finished look. Outside mount is more forgiving with measurement, but it’s more visible—so placement and symmetry matter for that “custom” feel.

Real-world window quirks (and how to measure around them)

Most measuring instructions assume your window is perfectly square and your walls are perfectly flat. In real homes—especially older ones—windows can be slightly trapezoid-shaped, sills can slope, and trim can be uneven. That doesn’t mean you can’t get a great result; it just means you need to measure like a realist.

When you notice inconsistencies, don’t ignore them. Write them down. Take photos. Note which side is tighter or taller. If you’re ordering online or working with an installer, those notes can help avoid surprises.

If you’re ever unsure, measuring twice (or three times) is faster than returning a custom product. A few extra minutes now can save weeks later.

Out-of-square window openings: spotting them early

If your inside mount top width is 35 1/2 inches, middle is 35 3/8, and bottom is 35 1/4, that window is narrowing toward the bottom. That’s common. The fix is usually using the smallest width so the blind fits, then accepting that you may see a slightly larger light gap on one side at certain points.

You can also check squareness by measuring diagonally corner to corner inside the frame. If the diagonals differ, the opening isn’t square. This doesn’t automatically rule out inside mount, but it tells you to be cautious with tight-fitting styles.

Outside mount sidesteps a lot of these issues because it doesn’t rely on the opening being perfectly consistent. If a window is really out of square, outside mount can be the simplest path to a polished look.

Windows with cranks, handles, and locks

Casement windows often have cranks that stick out. Even if you have enough depth for inside mount, the blind might hit the crank when lowered. In that case, you might need a spacer bracket, a different blind type, or an outside mount.

Measure how far the crank protrudes from the window surface and compare it to how the blind will hang. If you’re using a blind with a bottom rail that sits close to the window, it may bump into hardware.

Sometimes the easiest fix is choosing a style that stands slightly off the glass or mounting outside the frame. Planning for these obstacles upfront prevents daily annoyance later.

Deep sills and decorative ledges

Deep sills can be lovely, but they change how blinds look and function. Inside mount blinds may sit back farther, which can create a shadow line around the opening. That can look great, but it’s worth anticipating.

If you plan to place plants or décor on the sill, think about clearance. Some blinds stack at the top and may reduce the usable space near the window when raised. Others have a bulkier headrail that can cast a shadow over the sill area.

When measuring, note the depth at the top where the headrail mounts, not just the sill depth. The headrail depth is what determines whether inside mount is comfortable and how far the blind will protrude.

Measuring for different blind types: small differences that matter

Not all blinds behave the same way. A wood blind has slats and a headrail that can be bulkier than a slimmer aluminum blind. Cellular shades stack differently than roller shades. Even within “blinds,” the lift system and valance can change how much space you need.

That’s why it’s smart to decide the blind type before you finalize measurements. Some manufacturers also have specific measuring instructions—like whether they deduct a little for inside mount width automatically or expect you to provide the exact opening size.

If you’re mixing styles across rooms, keep notes for each window. The kitchen might need moisture-resistant materials, while a bedroom might prioritize room darkening and privacy.

Wood and faux wood blinds: account for headrail bulk

Wood and faux wood blinds often have a thicker headrail, especially with larger slats. That can make depth more important for inside mount. If your window frame is shallow, the blind may protrude beyond the trim line.

They also tend to have a valance, which can look great but needs space. If you’re doing inside mount and want the valance to sit neatly, check that you have enough depth for the full assembly.

Measure carefully and consider whether a partial recess is acceptable. In many homes, a slight projection is totally fine—it can even add visual presence—but it’s better if it’s a choice, not a surprise.

Roller shades and solar shades: simpler measurements, different priorities

Roller and solar shades are often more forgiving because they’re a single panel rather than slats. Inside mount still requires accurate width, but you may have slightly different light-gap behavior at the edges depending on the bracket style.

Solar shades are popular when you want glare control without losing the view. If that’s your goal, outside mount can help reduce side light gaps that cause glare streaks in the morning or late afternoon.

With roller shades, think about where the roll will sit and whether it interferes with window hardware. Depth and bracket placement matter, even if the measuring feels simpler.

Cellular shades: stack height and sill clearance

Cellular shades are great for insulation and softness. When you measure height, consider where the bottom rail will land and whether you want it to rest on the sill or float slightly above it.

Also consider stack height—the amount of shade that remains visible when fully raised. Larger windows and thicker fabrics can create a bigger stack, which can block part of the glass even when the shade is up.

If you’re measuring for a room where you want maximum view (like a living room with a great landscape), note that stack height and mount placement can affect the final feel.

Room-by-room measuring tips (because each space has different needs)

Measuring isn’t just about numbers; it’s about how you use the room. The same window size might call for different mounting choices depending on whether it’s in a bedroom, kitchen, or home office.

Think about privacy lines, glare, and how often the blinds will be raised and lowered. A guest bedroom might be lowered most of the time, while a kitchen window might be adjusted constantly throughout the day.

Also think about aesthetics across the home. Consistent mount styles can look cohesive, but mixing inside and outside mounts can be the smartest choice if different windows have different constraints.

Bedrooms: prioritize light control and privacy

In bedrooms, outside mount is often the secret weapon for better darkness because you can overlap the window opening and reduce side gaps. That’s especially helpful if you’re sensitive to streetlights or early morning sun.

If you prefer inside mount for the clean look, consider pairing it with curtains for extra light blocking. In that case, measure for inside mount blinds and then plan curtain rod placement so both layers work together.

When measuring bedroom windows, also consider whether the bed is near the window. You don’t want cords or controls awkwardly placed where they’re hard to reach.

Kitchens and bathrooms: moisture, clearance, and easy cleaning

In kitchens, you may have a sink or countertop right under the window. Inside mount can keep blinds from bumping into faucets or items on the counter. Measure carefully from the top of the opening to the sill, and note any obstructions.

Bathrooms often need privacy while still letting in light. Outside mount can help cover more area, especially if the window is high up or oddly sized. Just make sure the mounting surface is solid and the blind material can handle humidity.

In both rooms, think about how often you’ll wipe things down. A style that’s easy to clean can make daily life smoother, and that may influence which mount you choose.

Living rooms and street-facing windows: the “finished” look matters

Street-facing windows are where small measuring and mounting choices show up the most. Inside mount can look tailored and high-end, especially when the frame is deep enough for a clean recess.

Outside mount can look equally polished when aligned with trim and installed at a consistent height across multiple windows. If you have a row of windows, measure and plan so the headrails line up visually, even if the window openings vary slightly.

When measuring multiple living room windows, record each window separately. Even “identical” windows in the same room can be off by a fraction of an inch.

Special window types: measuring beyond the standard rectangle

Some windows don’t fit the typical “one opening, one blind” plan. Sliding doors, bay windows, and arched windows all require a bit more strategy.

The good news is that the same core principles apply: decide your mount style, measure in multiple places, and plan for clearance. The difference is you’ll often be measuring multiple sections and thinking about how the blinds operate day to day.

If a window type feels complicated, it’s not a sign you can’t DIY—it’s just a sign to slow down and map it out carefully.

Sliding glass doors: width, height, and handle clearance

For sliding doors, outside mount is common because it offers better coverage and can help with privacy. Measure the full width you want to cover, and consider how far you want the blind to extend past the glass on each side.

Handle clearance is a big deal. If the door handle sticks out, inside mount blinds or shades can rub or not sit flat. Measure how far the handle protrudes and choose a mounting approach that keeps the blind from catching.

Also decide whether the blind will stack to one side or split in the middle (depending on the product). That affects how you’ll use the door daily.

Bay and bow windows: measure each section like its own window

Bay windows are usually multiple windows set at angles. Measure each window section individually—don’t assume they match. Inside mount is popular here because it keeps each section neat and avoids awkward overlaps.

Depth and angle matter. Some bay windows have limited depth near the corners, which can affect bracket placement. Measure depth at the top where the headrail will sit for each section.

If you’re planning multiple blinds in a bay, think about how they’ll look when raised. Consistent headrail height helps the whole setup feel intentional and balanced.

Arched or specialty windows: when blinds may not be the whole answer

For arched windows, many people cover the rectangular lower portion with a blind and leave the arch uncovered, or use a specialty shade. If you’re only covering the rectangular portion, measure that opening as usual.

If you’re doing an outside mount that covers part of the arch, be careful: you might block more light than intended. Measure and mark where the blind will sit, then step back and check the sightlines.

Specialty windows are also where professional guidance can be helpful, especially if you want a seamless look across different shapes.

Common measuring mistakes (and how to dodge them)

Most mistakes happen because people measure once, assume the window is square, or don’t decide inside vs outside mount before writing down numbers. The fix is simple: measure in multiple places and label everything clearly.

Another big mistake is rounding too aggressively. If your measurement is 35 3/8 inches, write 35 3/8 inches—not “about 35.5.” Small differences matter when you’re dealing with a snug inside mount.

Finally, make sure you’re measuring the right surface. For inside mount, you measure inside the frame, not trim-to-trim. For outside mount, you measure the area you want to cover, not just the glass.

Mixing up width and height (it happens more than you think)

When you’re measuring multiple windows, it’s easy to flip numbers. A simple habit helps: always write “W x H” and then the numbers, like “35 1/4 x 58 1/2.”

If you’re using a notes app, keep each window as its own bullet point. If you’re using paper, draw a quick sketch of the window and write the width across and the height down.

Before ordering, do a quick sanity check: does the height seem taller than the width? For most windows, yes. That quick glance can catch a swapped measurement.

Not measuring at multiple points

Measuring only once is tempting, but it’s the fastest way to miss a narrow spot. Inside mount especially needs top, middle, bottom width and left, center, right height.

Even outside mount benefits from checking multiple points if your trim is uneven. If the trim is wider at the top than the bottom, you may want to align the blind to the widest point for a balanced look.

Multiple measurements don’t take much longer, and they give you confidence that the blinds will fit and look right.

Ignoring obstructions like alarms, sensors, and molding

Window sensors, security stickers, and decorative molding can interfere with where brackets sit. Before finalizing measurements for outside mount, look at the wall space above the window and on the sides.

If something is in the way, you can often shift the blind slightly higher or use a different bracket position—but you need to know before ordering, not after.

For inside mount, check for anything that protrudes into the opening, like part of the latch mechanism. That can reduce the usable width or depth.

When it’s worth getting help (and what to ask for)

Even if you’re comfortable measuring, it can be worth having a second set of eyes—especially for large orders, tricky windows, or if you’re investing in higher-end materials. A quick professional measurement can prevent costly mistakes.

If you’re looking for local expertise, many homeowners prefer working with specialists who understand the quirks of homes in their area. For example, if you want someone nearby who can help you think through mount choices, light control, and style, you can check out window decor designers in South Dakota to get oriented and see what’s available.

When you talk to a pro, ask how they handle out-of-square windows and what they recommend for your goals (privacy, glare reduction, insulation, kid/pet safety). A good measurement process should include multiple points and a conversation about how you want the blinds to function day to day.

Planning your order: turning measurements into the right product

Once you have measurements, the next step is choosing the blind style and confirming how the manufacturer wants measurements submitted. Some brands ask for exact opening size and make deductions automatically for inside mount. Others want the finished blind size you want delivered. This is a big deal—so always check the measuring instructions for the specific product.

If you’re exploring options and want to see how different blind styles can fit your rooms, it can help to browse a curated selection and compare features like light control, moisture resistance, and design. If you’re in the Sioux Falls area and want ideas, you can look at styles that transform your space with custom blinds and use that as a reference point while you match measurements to the right type.

Also consider color and finish. The same blind in a bright white vs a warm off-white can change the whole feel of a room. If you can, get samples and hold them up near the trim and wall color at different times of day.

Smart-home upgrades: measuring with motorization in mind

Motorized blinds are no longer a rare luxury—they’re becoming a practical choice for tall windows, hard-to-reach spots, and anyone who wants consistent light control throughout the day. If you’re even considering motorization, it’s smart to plan for it while you measure.

Motorized systems may have slightly different headrail sizes, and they can require a bit more clearance depending on the battery pack or motor placement. That doesn’t mean the measuring process is totally different, but it does mean depth checks and obstruction checks matter even more.

If you want to explore what’s possible, especially for big living room windows, stairwell windows, or a whole-home setup, take a look at automatic window coverings to get a sense of features and design considerations that can influence mount choice and placement.

Hard-to-reach windows: the best reason to plan ahead

If a window sits above a staircase, behind a tub, or high in a vaulted room, you don’t want to be fighting with a manual lift every day. Measuring accurately is step one, but planning control access is step two.

For outside mount on tall windows, consider mounting height carefully so the blind covers well without making the headrail look too high or disconnected from the window. For inside mount, confirm you have enough depth and that the blind won’t rub as it raises and lowers.

In these spaces, it’s also worth thinking about serviceability. If batteries need replacing, how will you access them? That practical question can affect where you mount and which style you choose.

Consistency across multiple windows: making motorized sets look cohesive

If you’re doing a row of windows (like in a living room), consistency is everything. Even small differences in mounting height can stand out. Measure each window individually, but plan a single “mount line” so the tops align visually.

Outside mount makes alignment easier because you can choose the same headrail height across windows even if the openings vary slightly. Inside mount can still look cohesive, but only if the frames are consistent and the recess depth is similar.

When you map it out, sketch the wall and mark where each headrail will sit. It’s a simple step that makes the final result look much more intentional.

A simple measuring checklist you can follow window by window

If you want a quick process to follow without second-guessing yourself, use this checklist for every window. It’s repetitive on purpose—consistency is what prevents mistakes.

For inside mount: measure width at top/middle/bottom (use smallest), measure height at left/center/right (use largest), measure depth, and note obstructions. Label the window clearly.

For outside mount: decide overlap, measure total width to cover, measure total height to cover, choose mounting location (wall vs trim), and check for nearby obstacles. Label the window clearly.

If you do those steps carefully, you’ll be in great shape—whether you’re ordering one blind for a bathroom window or outfitting an entire home.