Blind Size Calculator

This free blind size calculator works out the exact width and drop to order for a roller, Roman, Venetian or vertical blind, fitted inside the window recess or outside it over the frame, for homeowners and renters in the UK, Ireland and Europe. Enter three width measurements and the drop, and it applies the right deduction or overlap, and can tell you how much to trim from a ready made blind.

Quick answer: A blind size calculator takes the smallest of three recess widths and subtracts about 1 cm for an inside fit, so the blind does not catch. For an outside fit it adds 5 to 7.5 cm overlap each side and about 10 cm to the drop. A recess measuring 91.5 cm at its narrowest needs a 90.5 cm blind.

What Is a Blind Size Calculator?

A blind size calculator is a tool that turns window measurements into the blind width and drop to order, allowing for how the blind will be fitted.

The formula in plain words: for an inside recess fit, the blind width equals the smallest of your width measurements minus a small deduction, usually 1 cm, and the drop equals the shortest recess drop. For an outside recess fit, the blind width equals the widest measurement plus an overlap on both sides, and the drop equals your drop plus an extra allowance so the blind covers above and below the opening. If you enter a ready made blind width, the tool subtracts the width to order to show how much to trim.

How Do You Use the Blind Size Calculator?

1. Choose the Fitting: inside the recess, or outside the recess over the frame.
2. Measure the recess with a steel tape and enter the Width at the top, Width in the middle and Width at the bottom.
3. Measure the drop on the left, middle and right, and enter the shortest as Drop (shortest of left, middle and right).
4. For an inside fit, check the Recess width deduction. Set it to 0 if the blind maker says they deduct for you.
5. For an outside fit, set the Overlap each side and Extra drop.
6. To trim a ready made blind, enter its width in Ready made blind width to compare (optional).
7. Read the blind size to order, then open “Show the formula and working” to see the measurements used.

How Much Smaller Should a Blind Be Than the Recess?

An inside recess blind should be about 1 cm narrower than the narrowest point of the recess, so the brackets fit and the blind runs up and down without rubbing. Most made to measure suppliers ask for the exact recess size and make this deduction themselves, so read the order form carefully and never deduct twice. Ready made blinds have no deduction built in, so for those apply it yourself. On a roller blind, the fabric itself is about 3 to 4 cm narrower than the overall width ordered, because the control and end caps take up space, which leaves a small light gap at each side.

How Much Should a Blind Overlap the Window Outside the Recess?

An outside recess blind should overlap the opening by at least 5 cm on each side, and 7.5 cm or more gives better light control. Add around 10 cm to the drop so the blind covers the top of the frame and finishes just below the sill. For a bedroom where you want as little light as possible, an overlap of 15 cm each side and above and below the window blocks more light. Outside fitting suits shallow recesses, windows with handles that stick out, and frames that are badly out of square.

Should You Fit a Blind Inside or Outside the Recess?

Fit a blind inside the recess when the recess is deep enough for the blind and its brackets, and fit it outside when the recess is shallow or the window handles would stop the blind dropping. Inside fitting looks neater and leaves the sill free, while outside fitting blocks more light and makes a small window look larger. Check the minimum recess depth for the blind type before you order: it can range from a few centimetres for a slim Venetian to over 20 cm for some cassette roller systems. If you are painting the window reveal first, the paint calculator sizes the tin.

Inside and Outside Fit Reference Table

For an inside fit, use the narrowest of your three widths; for an outside fit, use the widest. Drops for inside fits are the shortest recess drop, and outside fits usually add about 10 cm.

Worked Example: A Kitchen Roller Blind

Example: a roller blind is going inside a kitchen window recess. The width measures 92 cm at the top, 91.5 cm in the middle and 91.8 cm at the bottom, and the shortest drop is 120 cm. Keep the 1 cm deduction when buying a trim to fit blind 120 cm wide.

• Smallest width: 91.5 cm.
• Blind width: 91.5 cm minus 1 cm = 90.5 cm.
• Blind drop: 120 cm.
• Trim to fit: 120 cm ready made minus 90.5 cm = 29.5 cm to cut off.

If the same blind were fitted outside the recess, the tool would use the widest width of 92 cm and suggest 102 cm wide by 130 cm drop. The fabric of the inside fit roller would measure roughly 86.5 to 87.5 cm across.

Are Blind Cords Safe for Children?

Looped blind cords and chains are a strangulation risk for young children, so every internal blind should have a child safety device and the cords should be kept short and out of reach. Internal blinds are covered by the safety standard BS EN 13120, and council trading standards advice on blind cords recommends cutting loops, fitting a cord tidy or tension device, and never placing a cot, bed or high chair near a window. A cordless or motorised blind removes the risk altogether in children’s rooms.

Read more about the different blind types in the window blind article on Wikipedia, and find more planning tools in our home calculators.

Blind Size Calculator FAQs

Checked October 2026. Sources: Dunelm, How to Measure for Blinds (1 cm recess deduction, 5 cm overlap, 10 cm extra drop, 15 cm for blackout), Blinds Direct, How to Measure for Blinds (three measurements, 7.5 cm overlap, fabric 3 to 4 cm narrower, recess depths), South Tyneside Council, Internal blind cords safety information.