How to Measure Windows for Custom Blinds (and the Mistakes That Ruin the Fit)

Friday, August 21, 2026 — Wholesale Blind Factory

Measure the window before you fall in love with the blind. Nine out of ten fit problems we see at Wholesale Blind Factory trace back to a measurement taken in the wrong spot, with the wrong tool, or with the mount decision made after the fact. The good news: measuring for custom blinds takes ten minutes, a steel tape, and a little discipline. Here's how to do it right — and the mistakes that cause light gaps, blinds that won't fit the recess, and returns nobody enjoys.

Decide inside mount or outside mount first

Everything downstream depends on this choice, so make it before you write down a single number. An inside mount sits within the window recess for a clean, built-in look — it needs enough depth and a reasonably square opening. An outside mount fixes to the wall or trim above the window and is the forgiving option: it hides an out-of-square frame, adds better light blockage, and works on shallow recesses. If your recess is shallower than the blind's minimum mounting depth, or the window sits flush with the wall, outside mount is the answer.

The right way to measure an inside mount

Use a steel tape measure, never a cloth one — cloth tapes stretch and flex, and custom blinds are made to the millimetre.
Measure the width at the top, middle, and bottom of the recess. Record the narrowest of the three.
Measure the height at the left, centre, and right. Record the tallest for most products, so the blind reaches the sill.
Measure the recess depth from the glass (or the nearest obstruction, like a crank handle) to the front edge of the frame.
Write measurements down as width first, then height — every manufacturer reads them in that order, and reversing them is the classic ruinous mix-up.
Do not deduct anything yourself. The factory takes its own clearance allowances; a DIY deduction on top of ours produces a blind that's narrow twice.

The right way to measure an outside mount

Measure the exact area you want covered, then add overlap so the blind actually blocks light instead of framing it: a good rule is at least 4 cm (about 1.5 inches) past the opening on each side, and 8 cm or more above so the headrail sits clear of the glass. If there's a protruding sill, measure to the sill; if not, add overlap below the opening too. For blackout performance in a bedroom, be generous — the wider the overlap, the smaller the halo of light.

The mistakes we see most

Measuring one spot only. Windows — especially in older Lower Mainland homes — are rarely square. A single width measurement hides a recess that tapers a centimetre from top to bottom.
Assuming all windows in a room are identical. They look identical; they're often not. Measure every window and label each one ("kitchen left", "kitchen right") as you go.
Ignoring obstructions. Crank handles, alarm sensors, tile ledges, and door handles all live exactly where a blind wants to hang. Note them, and note their distance from the frame — sometimes a shallower product or an outside mount solves it.
Rounding to the nearest half inch. Custom means custom. Record to the millimetre or to the nearest 1/8", and resist the urge to tidy the numbers.
Deciding the mount after measuring. An inside-mount measurement is useless for an outside-mount order, and vice versa. Mount first, numbers second.
Measuring an old blind instead of the window. The old blind carries its own deductions and its own mistakes. Always measure the opening itself.

What depth do you actually need?

Every product has a minimum mounting depth — the shallowest recess it can sit in — and a larger depth for a fully recessed (flush) look. As a general guide, faux wood blinds and california shutters want the deepest recesses, cellular shades and roller shades the shallowest. If your recess is borderline, the blind can still inside-mount but will stand slightly proud of the frame — fine functionally, just worth knowing before install day. When in doubt, note your depth and ask; that's a thirty-second conversation with the factory.

When to hand it to us instead

If any of this reads like more than you signed up for — three-storey windows, bay windows, tile returns, a wall of glass in a strata unit — book the free in-home consultation. A measure by our own installer comes with something a tape measure can't give you: the fit is guaranteed, because the person who measured is the person accountable for the install. That's the quiet advantage of buying factory-direct from the people who make the product. Bring us your numbers or let us take them — either way, the blinds are made here in Coquitlam to the exact millimetre of your windows.