To measure a window for blinds, decide first whether the blinds will sit inside the window frame or outside it. Then measure the width in three places (top, middle, bottom) and the height in three places (left, middle, right), and record every number to the nearest 1/8 inch. For an inside mount, you use the narrowest width and the longest height. For an outside mount, you measure the area you want covered and add overlap. Below you’ll find the full process, the exceptions by blind type, the mistakes that cause most wrong orders, and a short section on measuring walls for wallpaper, since the same habits apply.
TL;DR
- Choose inside mount (inside the frame) or outside mount (on the wall or trim) before you pick up the tape.
- Take three width readings and three height readings, to the nearest 1/8 inch, because windows are rarely perfectly square.
- Inside mount: narrowest width, longest height, and no deductions of your own. Outside mount: the area to cover plus overlap.
- Use a steel tape, never a cloth one, and never round your numbers.
- Check depth, handles, cranks and trim before you order.
Table of Contents
- What you need before you start
- Step 1: Choose inside mount or outside mount
- Step 2: Measure the width
- Step 3: Measure the height
- Step 4: Don’t make your own deductions
- Measurement rules by blind type
- Step 5: Check for obstructions
- Step 6: Double-check, then order
- Common mistakes that lead to wrong orders
- Our perspective: measure once, carefully, and record it
- A tool that makes measuring easier
- Measuring walls for wallpaper
- FAQ
- Sources
What you need before you start
You don’t need much, but the right tools matter more than most people expect.
- A steel measuring tape. A steel tape is more accurate than a cloth tape, which can stretch and sag over a wide window.
- A pencil and a worksheet. Record each measurement as width × height so you never confuse the two when you order. Write down the room and window name too, such as “Kitchen, left window”.
- A step ladder for tall windows, so you can hold the tape flat and read it at eye level.
- A second pair of hands for wide windows, if you have one.
If you prefer a digital readout to squinting at tick marks, our digital measurement tape is one option. Check the product page for its specifications and accuracy details before you rely on it for a custom order.
Step 1: Choose inside mount or outside mount
This choice changes every number that follows, so make it first.
Inside mount means the blind sits within the window recess. It looks clean and tailored and shows off your trim. It needs enough depth in the frame and a reasonably square opening.
Outside mount means the blind is fixed to the wall or the trim above the window. It hides uneven frames, covers a shallow recess, and can make a window look larger. It also blocks more light around the edges when it overlaps the opening generously.
A quick way to decide:
| Situation | Better choice |
|---|---|
| Deep, clean recess and attractive trim | Inside mount |
| Shallow recess or obstructions inside the frame | Outside mount |
| Window frame is clearly out of square | Outside mount |
| You want maximum light blocking | Outside mount with extra overlap |
| You want a minimal, built-in look | Inside mount |
Before committing to an inside mount, confirm the window is deep enough. Most shades need roughly 2 to 4 inches of clear depth, and the exact figure depends on the product, so check the depth listed for the blind you want.
Step 2: Measure the width
Stand square to the window and hold the tape flat and level.
Inside mount width
Measure inside the frame from side to side at the top, the middle and the bottom. Use the narrowest of the three as your width. The narrowest point is the one the blind has to clear, so it sets the limit.
Here is an example. You measure 35 1/4 inches at the top, 35 1/8 inches in the middle and 35 inches at the bottom. You order 35 inches. If you want to double-check, the gap between your widest and narrowest reading tells you how far out of square the frame is.
Outside mount width
Measure the area you want to cover. Include the trim if you want the blind to sit over it. For overlap, retailers give slightly different guidance. One suggests at least 2 1/2 inches on each side, while another suggests adding about 3 inches to the width and height. More overlap blocks more light and hides more of the gap. Either way, the ordering instructions for your specific blind come first.
Step 3: Measure the height
Measure from the top of the opening to the sill at the left, the middle and the right.
Inside mount height
Record the longest of the three so the blind covers the whole opening. Your sill may slope slightly or your head rail may sit a bit off, and the longest reading makes sure no gap shows at the bottom.
Outside mount height
Decide where the blind will start and stop. Start above the window, high enough to mount the headrail, and stop below the sill or apron if you want that covered. For horizontal blinds, one home improvement guide suggests including the trim plus 3 inches above and 1 inch below. For the width of an outside mount on horizontal blinds, it suggests measuring trim to trim and adding 1 1/2 inches per side.
Where should the blind end? If you want a full-length look to the floor, measure to the floor. If a radiator or furniture sits under the window, stop just above it.
Step 4: Don’t make your own deductions
This is the mistake that surprises people most. For custom inside-mount blinds, provide the exact opening measurements without making deductions yourself. The manufacturer subtracts the small amount needed for the blind to fit and operate. If you subtract a fraction too, the blind ends up too small, and light leaks around the edges.
The same principle appears across the industry. Customers submit exact inside-mount measurements and the factory applies any deductions. Ready-made blinds sold in standard sizes are different. Those are typically already cut slightly narrower than their labeled size, so read the packaging, and if you’re unsure, check the product details.
Measurement rules by blind type
Most horizontal blinds follow the three-point rule above. A few styles take shortcuts or different readings.
| Blind type | What to record (inside mount) |
|---|---|
| Faux wood, wood, aluminum (Venetian) | Narrowest of three widths, longest of three heights |
| Cellular and pleated shades | Narrowest width, longest height, plus a depth check |
| Roller shades | Width at the top is often enough; confirm with the product’s guide |
| Sheer shades and vertical blinds | Width at the top; shortest height for sheer shades and vertical blinds |
| Shutters | Shortest height; follow the maker’s instructions closely |
The details of the last three rows come from one retailer’s guide, which says to record the width at the top only for roller shades, sheer shades and vertical blinds, and the shortest height for sheer shades, vertical blinds and shutters. Because rules differ by product, always follow the guide for the exact blind you’re ordering.
Measuring for Venetian blinds
Venetian blinds are horizontal slat blinds, and for an inside mount you use the smallest of the top, middle and bottom widths. Height follows the usual longest-of-three rule. Because slats stack at the top when raised, leave enough clear depth and height for that stack if you want the view unobstructed.
Step 5: Check for obstructions
A perfect measurement means nothing if the blind can’t hang or move. Walk the window and look for:
- Window handles and cranks. Confirm there is clearance for handles and cranks when the blind is lowered and raised.
- Doorknobs and deadbolts on doors and French doors, which can catch the blind.
- Molding and trim that sticks into the recess or sits where an outside-mount bracket would go.
- Mounting surface. Inside-mount brackets need solid material to screw into, and the surface requirements differ by product.
- Locks, alarm sensors and vents inside the frame.
If something gets in the way, you have three choices: switch to an outside mount, order a blind with a different headrail or mounting option, or move the obstruction if that’s practical.
Step 6: Double-check, then order
Measure each window twice, ideally on different days or after a short break, so a misread number is more likely to show up. Compare your two sets. If they match, you’re done. If they differ, measure a third time and look for what changed.
Measure every window separately, even if they look identical. Houses settle, and windows in the same row can differ by a quarter inch or more. Never assume they’re the same.
Common mistakes that lead to wrong orders
- Measuring the old blind instead of the window. The existing shade is not a reliable guide, because it may have been the wrong size or installed differently.
- Assuming the window is square. That’s why you take three readings per direction.
- Using a cloth tape. Cloth tapes are not recommended for this job.
- Rounding. Round to neither the nearest inch nor even the nearest quarter. Stay with 1/8 inch.
- Deducting a fraction yourself on an inside mount. The factory does that.
- Forgetting handles, cranks and knobs. See the obstruction checklist above.
- Mixing up width and height when you write the order. Always width first.
- Skipping the depth check on an inside mount.
Our perspective: measure once, carefully, and record it
We sell gadgets and accessories, not blinds, but measuring is something our customers ask about, because a digital measuring tape is one of the practical tools in our range. Here is what we have seen help most.
First, treat measuring as a small project with a worksheet, not a quick job with a scrap of paper. A neat list of window names and numbers takes five extra minutes and saves a return that takes weeks.
Second, take photos. A picture of each window with the tape in place, and one of any obstruction, gives you something to check when you order and something to show if you ever need to ask the maker a question.
Third, don’t let a tool replace your judgment. Whether you use a steel tape or a digital one, it won’t tell you the frame is crooked or the handle will hit the slats. Look at the window as well as measuring it.
Finally, we stand behind what we sell with free worldwide shipping and a 30-day return policy, because we know buying the right size of anything is easier when a mistake isn’t expensive. For blinds that are custom-made, read the maker’s return terms before you order, since made-to-measure products often have different rules.
— mick
A tool that makes measuring easier
Accurate measurements start with a tool you trust and can read at a glance. Our digital measurement tape is built for exactly this kind of job, with the reading shown on a display rather than a tick mark. It is one of the many practical gadgets in our store, alongside smart home and everyday tech.
Orders ship free worldwide, payment is protected by SSL encryption, and you have 30 days to return something that isn’t right.
Measuring walls for wallpaper
Many readers who search for window measuring also have wallpaper on their list. The habits carry over: measure carefully, write it down, and add a margin.
- Measure wall by wall. Record the height and width of every wall you plan to cover.
- Decide about doors and windows. They are optional deductions. If you subtract them, be careful, since cuts around openings waste paper. If you skip the deduction, you simply have more spare.
- Add waste. A 10% allowance covers trimming and fitting.
- Account for the pattern repeat. It is listed in the product details, and a larger repeat means more paper lost when you match strips.
- Allow for uneven walls. Walls may not be straight or flat, so add a mounting margin.
- Buy everything at once. Check that the rolls share identical lot numbers and pattern-match numbers so the color doesn’t vary between strips.
A worked example: a room has two walls that are 12 feet wide and two that are 10 feet wide, with 8-foot ceilings. The total wall area is 44 feet of perimeter × 8 feet, which is 352 square feet. Add 10% and you get about 387 square feet before you consider pattern repeat and the coverage of your chosen roll. Always use the coverage stated on the roll you buy, and let the seller’s calculator confirm your total.
FAQ
Should I measure for blinds inside or outside the frame?
Choose inside mount if your frame is deep enough and reasonably square, because it gives the cleanest look. Choose outside mount if the recess is shallow, there are obstructions inside the frame, or you want better light blocking. Check the minimum depth for your product before you decide.
Do I subtract anything from my measurements for inside-mount blinds?
No. For custom blinds, give the exact opening measurements, narrowest width and longest height. The manufacturer makes the small adjustments needed for the blind to fit and operate. Subtracting yourself usually makes the blind too small.
How accurate do my measurements need to be?
Measure to the nearest 1/8 inch and never round up or down to the whole inch. Take three readings for width and three for height, since windows are rarely perfectly square. Then repeat the process to confirm.
How much should an outside-mount blind overlap the window?
Guidance varies by retailer and product. Common recommendations run from about 1 1/2 to 3 inches per side. A wider overlap blocks more light and hides the gap around the frame, so follow the ordering guide for your exact blind.
Can I use a cloth tape measure?
It is better to use a steel tape. A cloth tape can stretch or sag over a wide window and give you an inaccurate reading. For custom blinds, a small error can mean a blind that doesn’t fit.
Sources
- How to Measure and Install Blinds (home improvement guide, 2026)
- How to Measure for Blinds and Shades (home improvement guide, 2025)
- How to Measure Windows for Blinds and Shades (manufacturer support page)
- Measuring guides for blinds, shades and shutters
- How to measure for blinds and shades by blind type
- Measuring for window treatments: inside mount and outside mount
- Wallpaper calculator with pattern repeat
- How to install wallpaper: a step-by-step guide
- Step-by-step guide to wallpaper installation (PDF)
