πͺ Blinds Size Calculator
Get exact order measurements for inside or outside mount blinds.
Order This Size
How to Use This Calculator
Measuring for blinds takes two minutes with a steel tape measure. Enter the window size, pick your mount type, and the calculator applies the deduction and overlap rules that manufacturers expect.
Measure the window width in three places (top, middle, bottom) and the height in three places (left, center, right). Use the smallest number for each. For this calculator, enter those smallest numbers.
Choose inside mount or outside mount. For inside mount, you measured the exact inside of the frame; for outside mount, you measured the frame including trim.
For outside mount, set how much extra coverage you want beyond the frame on each side, above the top, and below the sill. The defaults (3 in sides, 4 in top, 2 in bottom) work for most windows.
Order the size the calculator shows. For inside mount the factory takes a 3/8 in deduction from your width, which is already applied.
If you entered a frame depth under 2 inches for an inside mount, heed the warning and switch to outside mount.
Inside Mount vs Outside Mount Rules
An inside mount sits within the frame for a clean, built-in look. The blind must clear the frame, so factories subtract 3/8 inch from the width you order: that is why you enter the exact inside measurement and never pre-deduct. Height needs no deduction because the blind stacks at the top of the frame. An outside mount hangs on the wall or trim around the window, so it needs to be wider and taller than the window itself. The side overlap blocks the light gaps that inside mounts always leave, which is why outside mount is the right call for bedrooms and media rooms where darkness matters.
Worked Example
A 36x48 in window with an inside mount. Order width = 36 - 3/8 = 35 5/8 in. Order height = 48 in exactly, no deduction. So you order a 35 5/8 x 48 in blind. The factory builds it to 35 5/8 in wide, which slides into the 36 in frame with just enough clearance to raise and lower smoothly. If the same window used an outside mount with the default 3 in side coverage, 4 in above, and 2 in below, you would order (36 + 6) x (48 + 6) = 42 x 54 in.
Ordering Tips That Prevent Returns
Order width first, height second: that is the industry convention, and mixing them up is the most common ordering mistake. Round to the nearest 1/8 inch, since most manufacturers build to that tolerance. Double check whether your retailer wants the exact frame measurement (they take the deduction) or the final blind size (rare, usually stated clearly). For windows with cranks or handles, an inside mount blind may hit the hardware, so check clearance or go outside mount. And measure every window individually even if they look identical: in most houses, supposedly identical windows differ by a quarter inch or more, which is enough to ruin an inside mount fit.
Frequently Asked Questions
Inside mount vs outside mount: which is better?
Inside mount looks cleaner and more tailored because the blind sits inside the window frame, but it needs at least 2 inches of frame depth for the brackets and shows a small light gap at the sides. Outside mount is better for shallow frames, gives better light blockage and privacy since the blind overlaps the frame, and can make a small window look larger. Pick inside mount for a crisp built-in look on deep frames; pick outside mount for light control or tricky windows.
How much do you deduct for inside mount blinds?
Manufacturers deduct between 1/4 and 1/2 inch from your width measurement so the blind fits inside the frame and operates freely. This calculator uses the industry standard 3/8 inch deduction. Do not subtract anything yourself: order the exact inside width of the frame and let the factory take the deduction, because double deductions leave an ugly light gap. For height, order the exact inside measurement with no deduction.
What if my window is not square?
Most windows are slightly out of square, so measure the width in three places: top, middle, and bottom of the frame. Use the SMALLEST of the three widths for an inside mount, otherwise the blind can jam against the narrowest point. Do the same for height, measuring left, center, and right, and use the smallest height. Record all six numbers so you can spot a window that is badly out of square.
How deep must a window be for inside mount?
You need at least 2 inches of flat frame depth to mount standard blind brackets inside the window. For a fully recessed look where the blind valance sits flush with the wall, most blinds need 3 to 3.5 inches of depth. Measure from the front edge of the frame back to the glass. If you have less than 2 inches, choose an outside mount instead: trying to force an inside mount into a shallow frame ends with brackets that do not hold.
Should blinds be wider than the window?
For an outside mount, yes: add 2 to 3 inches of coverage on each side beyond the frame so the blind blocks light and looks intentional rather than undersized. A common default is 3 inches per side, which also hides the brackets. For an inside mount, no: the blind must fit within the frame, so you order the exact inside width and the factory applies its standard deduction.