How to measure for blinds, properly
Measure the recess width in three places and take the smallest, measure the drop in three places and take the largest, and decide between recess fit and exact fit before you do either. On older Teesside windows almost nothing is square, which is why a made-to-measure blind is measured on site rather than guessed.
Last updated: August 2026.
A blind quoted from your own measurements is your responsibility if it arrives wrong, which is why supply-and-fit quotes include a survey. Even so, it helps to understand what the measurer is doing, and to sanity-check figures before you commit.
First decide: inside or outside the opening
Recess fit means the blind sits inside the window opening. It looks neat, keeps the sill usable and is the default in modern houses. It needs enough depth for the headrail, typically 40 to 70mm depending on the type, and it always leaves a small light gap at the sides.
Exact fit means the blind is mounted on the wall above the opening and overlaps it. It is the answer when the recess is too shallow, when handles get in the way, when you want to hide an ugly frame, and when you want darkness, because the overlap kills the edge leak.
The three-point rule
Older housing is not square. In a Victorian terrace in Norton, a 1930s semi in Acklam or a converted flat on the Hartlepool Headland, the top and bottom of the same opening can differ by 10mm or more.
- Width: measure at the top, middle and bottom of the recess. Use the smallest figure, so the blind clears all the way down.
- Drop: measure at the left, centre and right. Use the largest figure, so the blind covers the whole opening.
- Exact fit: add 5 to 10cm each side and above the opening, and decide where the bottom finishes: at the sill, or below it.
Use a steel tape, not a fabric one, and record millimetres. Never deduct anything for clearance yourself: the manufacturer applies the deduction for the specific system, and deducting twice makes a blind that is 30mm too narrow.
Obstructions the tape does not show
Walk the window before you measure. Tilt-and-turn and top-hung handles need the blind packing off the frame or a perfect-fit solution. Trickle vents in the head of a uPVC frame have to stay clear. Deep tiled reveals in a bathroom, coving that eats the top corner, a radiator sitting proud under the sill, a window seat, a projecting cill nosing: each of these changes the fitting type, and each is a reason a site survey beats a phone quote.
Blind-specific quirks
- Rollers: the fabric is narrower than the bracket-to-bracket width. That is normal, not an error.
- Venetians and wooden blinds: the stack of slats sits 5 to 10cm deep at the top when raised, more on a long drop.
- Roman blinds: the folded stack is 15 to 25cm. On a short window, mount above the opening.
- Verticals: decide which way they draw, and remember the stack sits over the glass at that end.
- Bays: every face is measured separately, plus the angles, which is a job for the surveyor.
The five errors that ruin a blind
- Measuring the glass instead of the recess.
- Taking one width in the middle of a tapered opening.
- Deducting clearance that the manufacturer also deducts.
- Ignoring a handle, vent or radiator that sits in the blind's path.
- Recording centimetres when the order form expects millimetres.
What a proper survey covers
An hour in an average house: every opening measured at three points, fitting type agreed window by window, obstructions noted, fabric samples held against the light in the actual room, child-safety devices specified, and the whole lot written up so you can see the figures. Then the blinds are made to those numbers, and fitting is one visit with nothing improvised.