Measuring windows sounds like one of those “I’ll do it later” tasks—right up until you’re staring at a sample book of fabrics and realizing that one wrong number can mean a shade that scrapes the trim, leaves a light gap, or won’t mount the way you pictured. The good news: you don’t need special tools or a contractor’s brain to measure correctly. You just need a clear plan, a steel tape measure, and a few practical tricks.
This guide walks through everything you need to measure for custom shades, with a big focus on the two most common installation styles: inside mount (fitted within the window opening) and outside mount (mounted above or around the opening). Along the way, you’ll also learn how to spot tricky window situations, how to avoid common measuring mistakes, and how to communicate your numbers so your custom shade order matches your real-life windows.
If you’re shopping for window treatments in NW San Antonio, accurate measurements are especially helpful because many homes in the area mix modern large-pane windows with older frames, deep sills, and decorative trim. Those details can be gorgeous—but they can also change which mount style makes the most sense.
Before you measure: pick the mount style with your room in mind
Inside mount and outside mount both look great, but they solve different problems. Inside mount is the “tailored” look: the shade sits neatly inside the window frame, showing off your trim and keeping the profile clean. Outside mount is the “coverage” look: it overlaps the window opening, which helps with light control, privacy, and sometimes makes a small window feel bigger.
Don’t worry if you’re not sure yet—sometimes the window itself makes the decision for you. For example, if your window frame isn’t deep enough for an inside mount, outside mount may be the only option. On the flip side, if you have beautiful wood casing you want to show off, inside mount may be worth prioritizing.
It’s also smart to think about how you use the space. Bedrooms often benefit from outside mount (especially for blackout shades) because it can reduce light gaps. Kitchens and bathrooms sometimes lean inside mount because it keeps fabric away from splashes and steam and avoids bumping into nearby cabinets or mirrors.
Tools and setup that make measuring painless
What you’ll need (and what you can skip)
A steel tape measure is non-negotiable. Cloth sewing tapes stretch and can throw off your numbers. A pencil and notepad (or your phone’s notes app) helps you keep measurements organized, especially if you’re measuring multiple windows in one go.
A small step stool is handy for tall windows, and a flashlight can help you see inside the top of the frame—useful when you’re checking for depth and obstructions. You can skip fancy laser measures for most homes; they’re great tools, but a standard tape measure is usually more reliable when you need to hook onto edges.
If you’re measuring for motorized shades, keep a note of where outlets are located or where you’d prefer a charging port to be accessible. It’s not part of the width/height math, but it matters for daily usability.
How to label windows so you don’t mix numbers
Before you measure, give each window a simple name: “Living Room—Front Left,” “Primary Bedroom—North,” “Kitchen Sink,” and so on. If you have identical windows, still label them separately. You’d be surprised how often “identical” turns into “almost identical” when you get the tape out.
Write measurements in a consistent format, like “W x H,” and always note the mount type you’re planning: “Inside mount” or “Outside mount.” If you’re undecided, mark it as “TBD” and jot down frame depth and trim width so you can decide later without re-measuring.
A quick photo of each window with its label can save you later, especially if you’re coordinating colors and opacity levels room by room.
Inside mount measuring: the clean, built-in look
Inside mount shades fit within the window opening. Because the shade is “contained” by the frame, small variations in width and height matter more. The key idea: you measure the opening itself, not the glass. And you measure in multiple spots because window frames are rarely perfectly square.
Also, inside mount requires enough depth in the window frame to fit the headrail (the top mechanism). Different shade styles have different depth requirements, so treat depth as a real measurement—not a guess.
Step 1: Measure width in three places
Measure the inside width of the window opening at the top, middle, and bottom. Place the tape measure tight against the inside edges of the frame (not the trim). Write down all three numbers.
For inside mount, you typically use the smallest of the three widths. That’s because the shade must fit through the narrowest point of the opening. If you size to the largest width, the shade may not fit or may rub when you raise and lower it.
If the difference between your largest and smallest width is more than about 1/4 inch, that’s a sign your window opening is out of square. Inside mount can still work, but you may want to consider a style that’s forgiving (like cellular shades) or shift to outside mount for a cleaner visual line.
Step 2: Measure height in three places
Measure the inside height at the left, center, and right. Again, record all three. Depending on the shade type, you’ll often use the largest height so the shade fully covers the window when lowered.
Be clear about where you’re measuring from. For most inside mounts, you measure from the top inside edge down to the sill (or to the bottom inside edge if there isn’t a sill). If there’s a deep sill, decide whether you want the shade to stop at the sill or extend slightly beyond it—then measure accordingly.
Some windows have a sloped sill. In that case, measure to the point where you want the shade to stop visually, not necessarily to the deepest part of the slope. A shade that hits the slope awkwardly can look “off” even if the number is technically correct.
Step 3: Check depth and look for obstructions
Depth is the inside distance from the front of the window frame to the glass (or to any obstruction). Measure it at the top of the frame where the headrail will sit. If you have a crank handle, lock, alarm sensor, or decorative latch, note how far it sticks out.
Inside mount can be “flush” (headrail fully hidden in the frame) or “partially recessed” (headrail sticks out a bit). Both are normal. What matters is knowing which you’re getting so you’re not surprised by the final look.
Obstructions are common on older windows and some sliding windows. If a handle or lock protrudes, you may need a different shade style, a spacer, or an outside mount. It’s much easier to plan for that now than after the shade arrives.
Outside mount measuring: maximum coverage and flexibility
Outside mount shades install above or around the window opening, overlapping the frame. This style is incredibly forgiving for out-of-square windows and is often the go-to for blackout or privacy-focused rooms because it can reduce light seepage around the edges.
Outside mount is also a smart choice when your window frame is shallow, when you want to visually enlarge a small window, or when you want to hide less-than-perfect trim. The main trick is deciding how much overlap you want on each side and above/below.
Step 1: Decide your overlap (and write it down)
A common overlap is 1.5 to 3 inches on each side, plus a few inches above the window to mount the headrail and cover the top edge. For blackout shades, more overlap is usually better, within reason and space constraints.
Look around the window: Is there crown molding, a nearby corner, a cabinet, a vent, or a light switch? Outside mount needs clear wall space for brackets and for the shade stack (the part that gathers when raised).
Write your overlap plan down before measuring. It keeps your math consistent across multiple windows and helps you avoid ordering one shade that overlaps 2 inches and another that overlaps 4 inches unless you truly intend that difference.
Step 2: Measure width based on coverage, not the opening
For outside mount, measure the width of the area you want the shade to cover. That usually means the window opening width plus your chosen overlap on the left and right.
For example, if the opening is 36 inches wide and you want 2 inches of overlap on each side, your target shade width is about 40 inches. (Always follow the ordering rules for your specific shade brand, but the concept stays the same: outside mount width is about coverage.)
If you’re mounting on trim instead of drywall, measure the trim width too. Sometimes it’s cleaner to align the shade edges with the outer edge of the trim for a symmetrical look.
Step 3: Measure height with stacking and sightlines in mind
Measure from where the top of the shade will be mounted down to where you want the shade to end. If you want it to cover the entire opening and extend a bit below the sill, include that in your height.
Think about what’s below the window: a sofa back, a radiator, a desk, a kitchen counter. You might want the shade to stop just above an obstacle, or you might want it to extend past it for better coverage. Outside mount gives you the freedom to choose.
Also consider the “stack height” when the shade is raised. Some shade types stack taller than others, which can partially block the top of the window. If you love a bright room, mount the shade higher so the stack sits above the glass line when raised.
Inside vs. outside mount: choosing based on real-life scenarios
Bedrooms that need darkness without fuss
If your top priority is a darker room, outside mount often wins. Even the best-fitting inside mount shade can have small light gaps along the sides due to the hardware clearance that lets the shade move freely.
Outside mount can overlap the opening to reduce that glow. Pair that with a blackout fabric and you’ll get a noticeably stronger result. If you’re sensitive to morning light, this is one of those “measure twice, sleep better” situations.
If you’re committed to inside mount in a bedroom for the sleek look, consider adding side channels or layering with drapery panels. The measurement approach stays the same—you’re just planning the system as a whole.
Living rooms where you want the trim to shine
Inside mount is great when you have beautiful casing or a clean modern frame you want to keep visible. It can look intentional and architectural, especially with woven shades or light-filtering roller shades.
But living rooms also tend to have larger windows, and larger openings sometimes show more variation (a slightly bowed frame, a subtle tilt). That’s why measuring in three spots is so important. If your width varies a lot, outside mount may look straighter even if the window isn’t.
Another living-room factor is glare. If the room faces strong afternoon sun, you might choose a solar shade for glare control and UV protection. Those can work in either mount style, but outside mount can provide a bit more edge coverage.
Kitchens and bathrooms with tight clearances
In kitchens, you might have cabinets close to the window edge or a backsplash that leaves limited wall space. Inside mount can keep the shade neatly contained so it doesn’t interfere with cabinet doors or decor.
Bathrooms often have frosted glass, but privacy still matters at night. Moisture-resistant materials and easy-to-clean surfaces are a plus. Measuring is the same, but it’s worth noting any handles, locks, or tile trim that could affect mounting.
If the window is above a sink, outside mount might cause the shade to hang closer to splashes. Inside mount can keep it tucked in, especially if you frequently open the window for ventilation.
Common measuring mistakes (and how to avoid them)
Rounding too aggressively
Custom shades are built to your measurements, so precision matters. Avoid rounding to the nearest inch. Use the smallest increment your tape measure allows—usually 1/8 inch—and write it exactly.
If you’re ordering through a provider that asks for measurements in 1/8-inch increments, stick to that. If they ask for 1/4-inch increments, follow their instructions, but don’t round early in your notes. Keep the exact number and then convert at ordering time.
When in doubt, consistency is your friend. Measure the same way every time: tape hooked on the left, pulled taut to the right, read at eye level.
Measuring the glass instead of the opening
It’s tempting to measure the visible glass because it’s easy to see. But for shades, you need the window opening (inside mount) or the coverage area (outside mount). Glass measurements can lead to shades that are too small, leaving unwanted gaps.
If you’re unsure what counts as the “opening,” look for the flat surfaces where the shade brackets would actually sit. That’s your reference point.
For windows with deep trim, double-check that you’re not accidentally measuring from trim edge to trim edge when you intended an inside mount within the frame.
Ignoring depth until it’s too late
Depth is the silent dealbreaker. You can have perfect width and height numbers, but if the headrail doesn’t fit, you’ll be forced into a different mount or a different product.
Measure depth at the top where the headrail mounts, not just at the sill. Some frames taper or have decorative profiles that reduce usable depth at the top.
If you’re tight on depth, ask about shallow-mount bracket options or shade styles designed for slim profiles. Roller shades, for example, often need less depth than some layered or cellular systems.
Special window types that need a slightly different approach
Bay windows and grouped windows
Bay windows look amazing with shades, but measuring is more about planning than math. Decide whether each window gets its own shade (common) or whether you’re using a continuous treatment (less common for shades, more common for drapery).
For inside mount in a bay, measure each window separately. Even small differences between panels are normal. Also consider how the shades will clear each other at the corners if the windows meet at angles.
For outside mount, check wall space between windows. You may not have enough flat area for brackets unless you mount on trim or use specific hardware solutions.
Sliding doors and large openings
For sliding glass doors, you can use vertical shades, panel tracks, or large roller shades depending on the look you want. The measuring concept still applies: inside mount uses the opening; outside mount uses coverage.
Pay attention to handles and door movement. You don’t want a shade that interferes with the door handle or rubs when the door slides.
If you’re also thinking about shading the outdoor area beyond the glass, it’s worth knowing that solutions like patio awnings can complement indoor shades by cutting heat and glare before sunlight even hits the glass. That can make indoor shade choices easier because you’re not fighting the full force of the sun.
Arched, angled, and specialty shapes
Specialty windows often require templates or professional measuring, especially for true arches or steep angles. If you’re pairing a standard rectangular shade with an arched top window, you may decide to cover only the rectangular portion for privacy and leave the arch uncovered for natural light.
For angled windows, measure the shortest and longest points carefully and note the angle direction (left high/right low). Many manufacturers have specific forms for this because “width and height” alone aren’t enough.
If you’re not sure whether your window counts as “specialty,” a good rule is: if a rectangle doesn’t describe it, ask before ordering. It’s far easier than trying to retrofit a standard shade onto a non-standard shape.
How to measure for popular custom shade styles
Roller shades and solar shades
Roller shades are clean and modern, and solar shades are especially popular in sunny rooms because they reduce glare while preserving some view. Measurement-wise, they’re straightforward, but edge gaps can be more noticeable because the fabric is a flat plane.
For inside mount roller shades, use the smallest width measurement. Expect small light gaps at the sides due to bracket clearance—this is normal. If you want more coverage, consider outside mount or a slightly larger overlap.
For outside mount, decide your overlap intentionally. A couple inches on each side can make a big difference in glare control, especially when the sun is coming in at an angle.
Cellular (honeycomb) shades
Cellular shades are great for insulation and energy efficiency. They’re also forgiving on slightly out-of-square windows because the fabric compresses and expands subtly as it moves.
Inside mount cellular shades often look very “built-in,” especially in deeper frames. Measure carefully in three spots, and pay attention to depth if you’re choosing a double-cell style (which can be thicker).
Outside mount cellular shades can be excellent for bedrooms because they help with light control and temperature. If your goal is comfort, this is one of the most practical shade styles to consider.
Roman shades and woven wood shades
Romans and woven woods bring texture and warmth. They can also have a larger “stack” when raised, meaning they take up more space at the top of the window.
For inside mount, check depth and consider whether you’re okay with a partially recessed look. Some woven wood shades naturally project outward a bit due to their structure.
For outside mount, mount height matters. If you mount higher, you can keep more glass visible when the shade is raised—helpful if you’re choosing a thicker material.
Getting your measurements ready for ordering (and avoiding surprises)
Write measurements the way the workroom expects
Different manufacturers have different conventions: some want “exact opening size,” others want “finished shade size,” and many will make small deductions for inside mount automatically. Read the ordering instructions carefully so you don’t accidentally double-deduct.
A safe habit is to keep two columns in your notes: “Raw measurements” (what you measured) and “Order size” (what you submit). That way, if you need to reorder one shade later, you still have the original data.
If you’re working with a designer or installer, share the raw measurements and your mount preference. They can help translate that into the correct order size for the specific product line.
Decide on control side and clearance
For corded or continuous-loop shades, you’ll often choose which side the control sits on. Think about what’s easiest to reach and what avoids tangling with furniture, plants, or door handles.
For motorized shades, decide where you want the motor and how you’ll charge it (if it’s rechargeable). This doesn’t change your width/height, but it can influence bracket placement and accessibility.
If you have kids or pets, cordless or motorized options are usually the simplest day-to-day. It’s one of those choices that feels small until you live with it.
Plan for layering if you want a designer look
Shades can stand alone, but layering shades with side panels or drapery can add softness and improve light control. If you’re layering, decide whether the shade should be inside mount (clean base layer) with drapery outside, or whether everything should sit outside the opening.
Layering affects measurement because you need to ensure there’s enough space for both systems—especially if you’re using outside mount and the drapery rod needs to sit above the shade headrail.
If you’re aiming for a cohesive home-wide plan—light-filtering in living areas, blackout in bedrooms, moisture-resistant in baths—this is also the moment to think about consistency in color and texture across rooms.
Local climate and lifestyle notes that influence shade choices
Heat, glare, and energy use
In warm, sunny climates, your shade choice can affect comfort more than you’d expect. Solar shades and light-filtering rollers can cut glare without making rooms feel cave-like, while cellular shades can help with insulation and temperature swings.
Outside mount can help reduce edge light and heat gain by covering more of the frame area. If you’ve ever felt heat radiating near a window in the afternoon, that extra coverage can matter.
It’s also worth thinking about which direction your windows face. West-facing windows often need more aggressive glare control than north-facing ones, and that can influence whether you prioritize overlap (outside mount) or a minimal profile (inside mount).
Privacy and nighttime lighting
During the day, many light-filtering fabrics feel private enough. At night, when interior lights are on, silhouettes can show through more than people expect. Bedrooms and street-facing rooms often benefit from room-darkening or blackout materials.
If privacy is a big concern, outside mount can reduce side gaps. Another trick is choosing a shade style with a more structured edge (or adding channels) so the coverage stays consistent.
When you’re measuring, think about where sightlines actually are—neighbors’ windows, sidewalks, or a second-story view into your space. That helps you pick the right opacity and mount style for each room.
When it’s worth getting a second set of eyes
If you’re measuring a straightforward set of rectangular windows, you can absolutely do it yourself. But if you have very large windows, specialty shapes, or you’re investing in premium materials, it can be worth having a professional verify measurements—especially for inside mounts where small errors show up fast.
Another time to ask for help is when you’re mixing mount styles across the home. For example, you might want inside mount in the living room for a built-in look, but outside mount in bedrooms for better darkness. That’s a great strategy, but it helps to confirm how each will look with your trim and wall spacing.
If you’re exploring custom shades in Texas, bring your measurements, photos, and a few notes about what bothers you in each room (glare, heat, privacy, early-morning light). The more context you provide, the easier it is to match the right product and mount style to your space.
A quick measurement checklist you can copy into your notes app
Inside mount checklist
Record window label/name.
Measure width at top/middle/bottom; note the smallest.
Measure height at left/center/right; note the largest (unless your product instructions say otherwise).
Measure depth at top; note any obstructions (handles, locks, sensors).
Confirm whether you want flush or partially recessed appearance.
Outside mount checklist
Record window label/name.
Decide overlap (left/right/above/below) and write it down.
Measure width of desired coverage area (opening + overlap).
Measure height from mounting point to desired bottom point (include extra below sill if desired).
Check for nearby obstacles (molding, corners, cabinets) and note stack clearance.
Measure with confidence, order with clarity
Once you’ve measured a couple windows using the three-point method (top/middle/bottom for width, left/center/right for height), the process starts to feel surprisingly simple. The biggest wins usually come from slowing down, writing everything clearly, and making mount decisions based on how you actually live in the space.
Inside mount gives you that crisp, fitted look. Outside mount gives you flexibility and coverage. Neither is “better” across the board—each is just better for certain rooms, certain frames, and certain goals.
If you take your time with the tape measure now, you’ll feel it later every time you raise the shade and it glides smoothly, sits straight, and looks like it was made for that exact window—because it was.
