Made-to-measure blinds are cut to the size you give, so the measurement is the whole job. Get it right and the blind drops in perfectly; get it wrong and there is usually no return on a bespoke product. The good news is that it is simple if you take your time. Use a steel tape, not a fabric one, work in millimetres, and measure each dimension in more than one place - frames are rarely perfectly square.

First decide: inside the recess or over it

There are two ways to fit a blind, and they are measured differently, so decide before you start.

A recess (inside) fit sits within the window opening, flush and tidy, with the sill left clear. It is the neatest look and the one most people choose, but it needs enough depth in the recess for the blind to sit without catching the handle.

A face (outside) fit mounts on the wall or frame above the opening, with the blind hanging over it. Choose this where the recess is too shallow, where you want to cover an awkward or out-of-square opening, or where you want maximum darkness, because a blind that overlaps the reveal leaks less light around the edges.

Measuring a recess fit

  1. Measure the width in three places - top, middle and bottom of the opening - and write down the smallest of the three. A recess often narrows slightly, and a blind cut to the widest point will not fit.
  2. Measure the drop (height) from the top of the recess to the sill, again in a couple of places, and take the measurement that lets the blind cover the glass.
  3. Do not take anything off for clearance yourself. With a recess fit the retailer normally deducts a small tolerance so the blind runs freely - give them the exact opening size and let them make the deduction, but check whose job that is when you order, because a double deduction leaves a gappy blind.

Measuring a face fit

  1. Measure the width of the opening, then add an overlap of around 40 to 75 mm on each side so the blind screens the gaps at the edges of the glass.
  2. Measure the drop from where the brackets will sit (a little above the opening) to where you want the blind to finish - level with the sill, or below it if you are covering the whole reveal.
  3. Here you give the finished size you want the blind to be, because there is no recess to deduct for.

Notes by blind type

Most blinds measure as above, but a few have quirks worth knowing. A roller loses a little fabric width to its brackets and end caps, so the cloth is always a touch narrower than the cut width - normal, and the reason a recess roller leaves a small light gap at the sides. A vertical blind is often face-fitted on wide windows and patio doors so the louvres can stack clear of the glass. A perfect-fit blind is the exception: you do not measure the recess at all, you measure the glass within the frame, because it clips into the window bead. If in doubt for a particular product, follow the retailer's own measuring guide for that style.

The rules that save a remake

  • Measure twice, write it down, and measure width and drop the way the retailer asks (width first, then drop - getting them the wrong way round is the commonest mistake).
  • Use millimetres and a rigid tape.
  • Never reuse an old blind's size - measure the opening fresh.
  • If a window is badly out of square or you are spending a lot, order a single blind or a sample first and check before committing to the rest.