Every blind ordered online is cut to the numbers you type in, and made-to-measure goods generally can’t be returned because you measured wrong. So this page is deliberately pedantic. Ten careful minutes with a metal tape is the difference between a blind that looks built-in and an expensive strip of fabric that fits nowhere.

The two ways to fit a blind

Recess fit (inside the window opening). Neat, keeps sills usable, shows off the window. But any blind in a recess has small light gaps at the edges, and the recess must be deep enough for the blind’s mechanism.

Exact/outside fit (on the wall, covering the opening). Blocks more light, hides out-of-square openings, works when the recess is too shallow. The blind sits proud of the wall and the sill is covered when it’s down.

Decide this first — it changes what you measure and what the retailer does with your numbers.

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Rules that apply to every measurement

  • Use a metal tape measure. Fabric tapes stretch and flex.
  • Record millimetres, and write each number down as you go.
  • Measure every window separately, even in a row of “identical” windows — they rarely are.
  • Label the numbers (“kitchen left, width, recess”) — a week later, unlabelled numbers are worthless.
  • Width is always quoted first, then drop. Transposing them is a classic irreversible mistake.

Our printable measuring worksheet has a block for each window with space for all of these numbers — print it, or fill it in on your phone as you go.

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Measuring for a recess fit

  1. Width: measure the recess at the top, middle and bottom. Record the smallest.
  2. Drop: measure at the left, centre and right. For most blinds use the smallest so the blind clears the sill.
  3. Depth: measure from the glass (or the nearest obstruction — handles count) to the front face of the wall. Compare with the minimum depth on the product page of the blind you’re buying; that figure varies by blind type and brand.
  4. Submit the numbers as recess size — the maker applies their own deductions so the blind runs freely. Don’t deduct anything yourself unless the retailer’s instructions explicitly tell you to; a double-deducted blind arrives narrow with wide light gaps.
Diagram of a window recess that is slightly narrower at the bottom than at the top. Three width arrows run across the inside of the recess at the top, middle and bottom, and three drop arrows run from the top of the recess down to the sill on the left, in the centre and on the right.
Recess fit: measure the width at the top, middle and bottom, and the drop on the left, in the centre and on the right, all inside the opening. Order the smallest width (and, for most blinds, the smallest drop).

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Measuring for an exact/outside fit

  1. Decide coverage: overlap of roughly 50–75 mm per side is a common starting point (more for bedroom light control), plus enough height above the opening for the brackets to fix into solid wall.
  2. Measure the total finished width and drop you want the blind to be, and submit them as exact/blind size — the maker cuts to those numbers with no further deduction.
  3. Check what the blind will hit: handles, taps, radiator pipes, a protruding sill. Hold a stiff tape or a batten where the blind will hang and look from the side.
Diagram of a blind hung on the wall outside a window opening. The blind is wider and taller than the opening, overlapping it on each side, with its brackets fixed into the wall above. Arrows show the blind's full width and drop.
Exact fit: the blind covers the opening plus an overlap on each side (roughly 50–75 mm is a common starting point), with the brackets fixed into solid wall above. Order the finished blind width and drop.

The one rule that overrides this page: every retailer publishes its own measuring instructions, and some differ on deductions, especially for romans and verticals. Read the instructions for the specific product before ordering. Where they conflict with anything here, the retailer’s rules win — they’re the ones cutting the fabric.

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Type-specific quirks

Each blind type has its own wrinkles — stack sizes, slat projections, cassette depths. They’re covered in the individual guides: roller, venetian, vertical, roman and day & night. Bay windows are their own discipline: see blinds for bay windows. Roof windows such as VELUX are the exception to all of this: you order by the size code on the window, not by measuring. See blinds for roof windows and skylights.

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If you’d rather not carry the risk

Before ordering, check any retailer’s measuring-error cover: eligibility, the claim deadline, return and replacement charges, and whether you would pay for fitting again. A remake promise does not automatically pay for a fitter’s second visit.

If your windows are awkward — bays, out-of-square Victorian openings, very wide spans — a measure-and-fit service moves that risk onto a professional, and the cost guide shows what that insurance costs.

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Frequently asked questions

Do I measure in millimetres or centimetres?

Measure in millimetres. Retail order forms accept mm or cm, but measuring in mm removes rounding ambiguity — a blind maker cuts to the millimetre, and 'about 1.2 m' is how wrong blinds get ordered.

What if my recess isn't square?

Almost none are, which is why you measure width and drop in three places. For a recess fit use the smallest width so the blind doesn't jam. If measurements differ by more than about 10 mm, consider fitting outside the recess where the wonkiness won't show.

Who is responsible if a made-to-measure blind doesn't fit?

If the blind was made correctly to your order, a measuring mistake is usually your responsibility. Check any optional measuring-error cover, its conditions and whether you would pay for refitting. Your rights if goods are faulty or not as described still apply without buying cover. With a measure-and-fit service, ask who pays for remaking and refitting if the provider measures incorrectly.

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A free tool to use with this guide

  • Measuring worksheet Record width and drop in three places, recess depth and notes for each window. Download a text record of your measurements or print the sheet.