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How to Measure for Blinds

Calculator + chartEnter your measurements below and it works the answer out.

By Srinivas Gaddam, integration architect · Reviewed by OneCalculatorHub editors, figures checked against the cited standards · Updated 29 August 2026

Inside mount: measure the opening in three places each way and order the narrowest width and the tallest height. Outside mount: add 3 inches each side and above. Enter your numbers and the calculator gives the size to order.

Your window measurements

Minimum frame depth for an inside mount

Blind typeFlush mount (fully inside)Minimum mount
Roller shade2½ in1½ in
Cellular / honeycomb2 in1 in
Faux wood 2 in slats3 in1½ in
Real wood 2 in slats3 in1½ in
Vertical blind3½ in2 in
Roman shade3 in1½ in

Less than the minimum: mount outside. Between minimum and flush: the blind works but sticks out past the frame.

How this was worked out

Inside mount. The blind must clear the frame at its narrowest point, so the order width is the smallest of the three widths, and the order height is the largest of the three heights so it reaches the sill everywhere. Give the exact measurement; the factory takes its own ¼ to ½ inch deduction for clearance — do not deduct yourself, or the blind will have gaps on both sides.

Outside mount. The blind should cover the opening and overlap the wall to block light: 1½ to 3 inches past each side (the calculator uses 3, so 6 inches added to the widest width) and 3 inches above the frame, plus the height of the opening. Order the width as the exact overall size — there is no factory deduction on outside mounts.

Depth check. An inside mount needs a frame deep enough for the headrail; the table gives the minimum for each type. Measure from the glass (or the window's inner stop) to the front edge of the frame.

Sources. There is no published standard for blind measuring or for factory deductions — every figure here is a manufacturer convention taken from measuring guides, and they differ between brands, which is why this page gives ranges rather than single numbers. The ¼–½ in inside-mount deduction, the 1½–3 in outside-mount overlap and the minimum mount depths are the common ranges across US makers. Confirm the deduction on the order form of the brand you buy from before you cut anything — the manufacturer’s own sheet, not this page, is what governs a made-to-measure order. For window sizes generally see the Window and Door Manufacturers Association. Last reviewed 29 August 2026.

Common questions

Do you measure inside or outside the window frame for blinds?

Inside the frame for an inside mount (the blind sits in the recess), outside on the trim or wall for an outside mount. Inside looks cleaner; outside blocks more light and hides an out-of-square window.

Should I deduct for an inside mount?

No. Send the exact narrowest width; the manufacturer deducts ¼–½ inch so the blind clears the frame. If you deduct as well, the blind ends up with light gaps.

How much wider should outside mount blinds be?

3 inches wider on each side of the opening (6 inches total) is the usual figure; 1½ inches is the minimum. Add 3 inches above for the headrail.

Why measure width in three places?

Window frames are rarely square. An inside blind must fit the narrowest point or it will not go in; an outside blind must cover the widest point.

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