Measure three times, write it down, and let the calculator do the rounding. This page has the free worksheet from the video, a calculator that turns your readings into the size to order, and the rules behind it.
The rules, in one box
- Use a steel tape and measure to the nearest 1/8 inch.
- Inside mount width: measure top, middle and bottom. Order the narrowest.
- Inside mount height: measure left, center and right. Order the longest.
- Reading lands on a 1/16 mark? Round the width down and the height up to the nearest 1/8.
- Enter exact sizes. Don't take deductions: the factory does that for inside mounts.
- Outside mount: add at least 2 1/2 inches on each side of the window.
Watch the 2-minute video
- Inside or outside mount
- What you need
- Measure the width
- Measure the height
- Depth, cranks and latches
- Outside mount
- Ordering
What you need
- A steel tape measure. A cloth tape stretches and sags, which is enough to make an inside-mount blind too wide.
- A pencil and the worksheet. Write every number down the moment you read it.
- A step stool for the top of tall windows.
Step 1: Choose inside or outside mount
Decide this first, because each one is measured differently.
| Inside mount | Outside mount | |
|---|---|---|
| Where it sits | Inside the window frame | On the wall or trim, covering the opening |
| Look | Clean, built-in | Makes the window look bigger |
| Light gaps | Small gaps at the sides | Fewer gaps, better for blackout |
| You need | Enough frame depth, and no crank, latch or lock in the way | Flat wall or trim above and beside the window |
Go with an inside mount if the frame is deep enough and nothing sticks out into the blind's path. If not, go outside.
Step 2: Measure the depth
Measure from the front edge of the frame back to the glass (or to anything that sticks out, like a crank or a sash lock) at three points. Use the smallest number.
Every blind lists a minimum mounting depth in its product specifications, and it varies a lot from product to product. Check your number against that product's spec before you order an inside mount.
Step 3: Measure the width (inside mount)
Measure inside the frame, wall to wall, at the top, middle and bottom. Windows are rarely perfectly square, so the three numbers often differ. Order the narrowest: it's the only width that fits all the way down.
Example: readings of 35 1/2, 35 7/16 and 35 1/2. The narrowest is 35 7/16, which lands on a 1/16 mark, so round down to 35 3/8.
Step 4: Measure the height (inside mount)
Measure from the top of the opening down to the sill on the left, center and right. Order the longest, so the blind covers the whole window.
Example: 59 1/8, 59 and 59 3/16. The longest is 59 3/16, on a 1/16 mark, so round up to 59 1/4.
These follow their own guides. Blinds.com, for example, asks for the shortest height on these products. Check the measuring note on the product page.
Step 5: Outside mount
For an outside mount you're not measuring the opening. You're measuring the area you want covered.
- Width: measure the opening (or the outside of the trim) and add at least 2 1/2 inches on each side. That's at least 5 inches in total.
- Height: measure from where the headrail will sit, a few inches above the frame, down to where you want the blind to stop. Check the product's guide for bracket space.
- Outside-mount blinds are made to the exact size you enter. No deductions.
Order-size calculator
Enter your readings in inches. Pick the fraction from the list (your tape's smallest marks are sixteenths). The calculator picks the right reading and does the rounding.
Step 6: Order without the classic mistakes
- Width first, then height. Swapping them is one of the most common ordering errors.
- Pick inside or outside mount on the order form, to match how you measured.
- Enter your exact numbers. Don't subtract anything for an inside mount.
- Check your depth against the product's minimum mounting depth.
- Order free samples first if you're unsure about color or light blocking.
Why other guides say something different
Measuring advice online doesn't always agree. The most common differences:
- "Deduct 1/4 inch yourself." Most retailers make the inside-mount deduction at the factory. If you deduct too, the blind ends up too narrow, with gaps at the sides.
- "Round everything down." Some guides say this. The rule on this page (width down, height up) is the one Blinds.com and SelectBlinds use.
- Outside-mount overlap. Recommendations range from about 1 1/2 to 3 inches per side, depending on the brand.
The measuring instructions on the order form of the blind you're buying always win. Read them before you check out.
Tricky windows
- Bay and corner windows: measure each window separately and decide how the blinds meet at the corners. The retailer's bay-window guide is worth following exactly, or book a professional measure.
- French doors: door-mounted blinds go on each door; check that the handle and the door's swing are clear.
- Very wide windows: check the product's maximum width. You may need two or three blinds side by side.
Common questions
Do I measure the glass or the frame?
The frame. For an inside mount, measure the opening where the blind will sit, from one side of the frame to the other.
Should I round up or down?
Measure to the nearest 1/8 inch. If a reading lands on a 1/16 mark, round the width down and the height up.
Do I take off a little so the blind fits?
No. For inside mounts the factory makes the deduction. Enter the exact size you measured.
How much should an outside-mount blind overlap?
At least 2 1/2 inches on each side, per Blinds.com. Some brands suggest a different amount, so check the product's guide.
How deep does the window frame need to be?
It depends on the blind. Every product lists a minimum mounting depth; compare it with the smallest of your three depth readings.
What if my window isn't square?
That's why you measure three times. Ordering the narrowest width and the longest height covers small differences. If the readings differ a lot, an outside mount hides it better.