How to Measure Windows for Blinds

Almost every ill-fitting blind traces back to the same moment: a rushed measurement. Because made-to-measure blinds are cut to your figures and usually can’t be returned, five careful minutes with a tape measure is the most valuable part of the whole project — more valuable, in practice, than the style you pick or the price you pay. Get the numbers right and a blind drops in and simply works for a decade. Get them wrong and you’re either sending it back or living with a headrail that doesn’t sit flush. Here is how to measure properly, with the exact arithmetic a retailer’s order form will apply behind the scenes.

Gather the right tools first

Reach for a steel tape measure, not a cloth or plastic one. Cloth tapes stretch and sag under their own weight across a wide window, and even a couple of millimetres of error can leave a blind that binds in the recess or lets light spill down one side. A rigid steel blade sits flat against the frame and gives you a repeatable reading every time. You’ll also want a pencil and paper or a notes app (write the figures down the moment you take them, not from memory afterwards), a step stool for anything above head height, and ideally a second pair of hands for windows wider than about five feet, where one person holding the tape at both ends inevitably introduces sag. Record every measurement in the same unit — pick either inches or centimetres for the whole project and stick with it — and write to the nearest eighth of an inch or single millimetre. Rounding to the nearest inch feels harmless until you’re a quarter-inch out on a recess that has almost no spare clearance.

Decide the mount before you measure anything

You cannot measure sensibly until you know whether the blind will sit inside the window recess or outside it on the wall or frame, because the two are measured in opposite directions. An inside mount is measured to the exact opening so the factory can trim a small clearance from your figure; an outside mount is measured to the same opening, but then has coverage added on top. Confuse the two and you don’t just get a mildly wrong size — you get a blind sized for the wrong job entirely. If you’re undecided, our full comparison of inside vs outside mounts walks through recess depth, light control, and looks before you pick up the tape.

Measuring for an inside mount

Windows are rarely perfectly square — a house settles, a frame is fitted slightly out of true, plaster is never perfectly even — so you take several readings and let the smallest win:

• Width: measure across the recess at the top, the middle, and the bottom. Use the narrowest of the three — a blind sized to the widest point will jam against the tighter section as it travels.
• Height (drop): measure from the top of the recess down to the sill on the left, the centre, and the right. For an inside mount most people use the longest of the three so the blind still reaches the sill even where the recess runs slightly deeper.
• Depth: measure how deep the recess is, front to back. Every blind has a minimum depth for a flush inside fit; if yours is too shallow, the headrail will protrude past the trim or you’ll need to switch to an outside mount instead.

Say your three width readings come back 36 in at the sill, 36¼ in in the middle, and 36½ in at the top. You’d record 36 in as your opening width — the narrowest reading — and feed that straight into the Blind & Shade Size Calculator. For an inside mount at that width, the calculator deducts its standard 0.25 in bracket-clearance allowance and returns an order width of 35.75 in, with the drop left unchanged. That deduction isn’t arbitrary padding: it’s the gap the factory needs so the finished blind can be fitted and operated without its edges scraping the reveal.

Measuring for an outside mount

Here you are deciding how much of the wall the blind should cover, not fitting it inside a fixed opening, so the arithmetic runs the other way. Measure the opening as you would for an inside mount, then add overlap: the calculator’s own defaults add 1.5 in on each side and 2 in above the opening (with nothing added below, since the blind usually finishes level with the sill). On that same 36 in by 48 in opening, an outside mount comes out to an order size of 39 in wide by 50 in high — three inches added to the width (1.5 in on each side) and two inches added to the height. That overlap is what closes the light gaps an inside mount leaves down each edge. Before you commit to an outside mount, check there’s a flat, solid surface above and beside the opening to screw the brackets into, and enough clearance for the stacked blind to clear the glass fully when it’s raised.

Measuring depth, sills, and obstructions

Depth trips up more inside-mount orders than any other single figure. Measure from the front face of the recess back to the glass (or to whatever the blind will clear), and compare it against the minimum-depth figure the product listing gives for your chosen headrail. A blind that needs 2 in of depth won’t sit flush in a 1.5 in recess — it will protrude, catch on curtains, or simply refuse to tilt fully closed. While you’re at it, check for anything that eats into the usable opening: a tilt-and-turn window handle that swings into the recess when the sash opens, a radiator valve or pipe run tight under the sill, deep tile reveals in a bathroom, or a picture rail crossing the top of the opening. Any of these can force you toward a shallower headrail or an outside mount even when the raw opening looks generous on paper.

Measuring more than one window in a room

Don’t assume a matching pair of windows in the same room share a size — measure each one individually, even when they look identical from across the room. Older houses in particular can have openings that differ by half an inch or more between two windows fitted by the same original carpenter decades apart. Label each set of figures by room and by window (“kitchen, sink window”, “kitchen, back door window”) rather than trusting yourself to remember which figure belonged to which opening once you’re three rooms further into the job. If a window is unusually shaped — a bay, an arch, or a tilt-and-turn casement with a handle that intrudes into the reveal — those need a slightly different approach, which we cover in our guide to measuring bay, arched, and tilt-turn windows.

Common mistakes that ruin an otherwise good measurement

A handful of errors account for most sizing disasters, and every one of them is avoidable:

• Measuring only once. Always take the width in three places and the height in three; an out-of-square window is the norm, not the exception, and a single reading hides that entirely.
• Using the widest reading instead of the narrowest for an inside mount. If your three width readings were 36, 36¼, and 36½ in, using the widest 36½ in by mistake gives an order width of 36.25 in — half an inch wider than the 35.75 in the true narrowest point actually allows for. Half an inch is more than enough to make a blind bind partway down its travel.
• Mixing units. A width recorded in inches and a drop recorded in centimetres on the same order form is a recipe for a costly, oversized or undersized mistake — pick one unit for the whole project.
• Deducting the clearance twice. Some retailers want your exact raw opening and deduct the clearance themselves on their end; others expect you to submit the already-deducted finished size. Check which your retailer wants, and never run a figure through a deduction twice. Feeding an already-deducted 35.75 in back into the inside-mount calculator as if it were still the raw opening returns 35.5 in — a further quarter-inch narrower than necessary, and now you’ve manufactured a light gap that didn’t need to exist.
• Ignoring handles and obstructions. Tilt-and-turn handles, cranks, deep tile reveals, and radiator pipework all affect whether an inside mount will actually clear once the blind is fitted, not just whether the raw opening measures wide enough.

What a mismeasurement actually costs you

It’s worth being concrete about the stakes. A made-to-measure blind is cut to the millimetre and, on most retailers’ terms, can’t be returned once it’s been manufactured to your figures — you measured it, you own the size. An order that’s too wide for an inside mount either won’t fit at all or will bind and wear at the point where it rubs; one that’s too narrow leaves a visible light gap down the side that no amount of adjustment will close. Either way, the fix is a second blind at a second cost, plus the wait for it to arrive. That asymmetry — five minutes of careful measuring against weeks of delay and a wasted purchase — is exactly why this step deserves more care than it usually gets.

Write it down and double-check before you order

Note each window’s width, drop, depth, and chosen mount on paper or your phone, label it clearly by room, and then measure a second time before you actually place the order. Treating the numbers as final only after that second pass is the single habit that separates blinds that drop in perfectly from the ones that go straight back in the box. Once your measurements are locked in, the size calculator turns them into an order size in seconds — enter the raw opening once, choose inside or outside, and it applies the correct deduction or overlap for you, so there’s no manual arithmetic left to get wrong.