How to Measure Your Windows for Custom Blinds Like a Pro

Decide recess or face fix before you touch the tape

This is the decision everything else depends on, and doing it second is why measurements go wrong.

A recess fit sits the blind inside the window opening. It looks neater, it keeps the sill usable, and it suits deep reveals in older houses. A face fit mounts the blind on the wall outside the opening. It looks bulkier, but it covers more, blocks far more edge light, and clears window handles.

The rule of thumb: if you want it to look tidy, recess fit. If you want it to actually block light, face fit. Every blackout blind that 'doesn't work' is a recess-fitted one.

Rule one: measure in three places, every time

Window openings are not rectangles. In a house built before 1960 they're often out by 10mm or more, and plastered reveals in newer builds are no better.

For width, measure across the top, the middle and the bottom of the recess. For a recess fit, use the smallest of the three. For drop, measure down the left side, the centre and the right side, and use the longest.

Smallest width, longest drop. If you remember one thing from this article, make it that.

Rule two: use a steel tape, and use millimetres

Not a fabric tape, which stretches. Not a laser on its own for width in a recess, because it needs a flat surface to read against. A steel tape, and record everything in millimetres — not centimetres, and definitely not inches.

Most Irish suppliers work in millimetres. Converting a rounded centimetre figure introduces a 5mm error before you've started.

Rule three: do not make any deductions yourself

This is the single most expensive mistake, and it happens constantly. Suppliers apply their own clearance deduction — usually about 10mm on a roller — to the recess width you give them.

If you helpfully subtract 10mm first, the supplier subtracts another 10mm, and your finished blind is 20mm narrow with daylight down both sides. Give the exact opening size and let them do their job. If you're worried, write 'exact recess size, no deductions made' on the order.

Rule four: for a face fix, be generous

A face-fixed blind should overhang the opening by at least 50mm each side, and by 75–100mm each side if it's blackout. Above the opening, allow 100mm for a roller and 150–200mm for a roman, because a roman blind stacks to roughly a fifth of its drop and will eat the top of your window if you mount it tight.

More overhang is almost never wrong. Less overhang is a light leak you'll look at every morning.

Rule five: check what's in the way

Before you order, look for the things that catch fitters out: window handles that will foul closed slats, trickle vents at the top of the frame, an alarm sensor in the corner of the reveal, tiled sills that extend into the opening, and radiator shelves.

Open and close the window fully with your hand where the blind will sit. If your knuckles hit the frame, so will the blind.

Rule six: bay windows are never one window

A bay is two, three or five separate windows that happen to meet at angles. Measure each pane individually — including its own three width readings and three drop readings — and note the angles between them.

Any quote that treats a bay as a single measurement is a quote from someone who hasn't looked at it properly. This is the most common reason bay blinds arrive unusable.

Write it down properly

Record for every window: room name, window position, recess or face fix, width in mm, drop in mm, control side (left or right), and any obstructions. Photograph each window with the tape in place.

Send that to whoever is quoting. It removes half the back-and-forth and makes comparing quotes on identical specifications possible — which is where the real savings are.