First decision: inside the recess or outside it?
Before you touch a tape measure, decide where the blind will sit. There are two options, and they need completely different measurements.
Recess fit means the blind sits inside the window opening, tucked up against the glass. It looks neat, keeps the windowsill usable, and is the most common choice for standard windows. When you give a recess measurement, the blind maker deducts a small allowance so the blind operates without scraping the walls — so you always supply the full recess size, never a size you've reduced yourself.
Exact fit (also called outside-recess fit) means the blind hangs on the wall above the window and overlaps the opening on all sides. It's the right choice when a recess is too shallow for the headrail, when the plasterwork inside the recess is rough, or when you want better light blocking, because the overlap covers the gaps a recess fit leaves. Here the blind is made to precisely the size you give, with no deduction.
Use a metal tape measure — nothing else
A fabric dressmaking tape or a school ruler will let you down. Fabric tapes stretch and sag over wider spans, and even a small error matters when a blind is manufactured to the millimetre. A rigid metal tape measure holds a straight line across the full width of the window, which is exactly what you need.
Measure in millimetres if you can. Centimetres invite rounding, and rounding is where mistakes creep in. Write each figure down as you go rather than trusting your memory between windows — after the third window of the afternoon, they all start to blur together.
Measure the width and drop in three places
Here's the part most people skip. For a recess fit:
- Width — measure across the recess at the top, the middle and the bottom. Use the smallest of the three figures. The blind has to clear the narrowest point, or it will jam.
- Drop — measure from the top of the recess to the sill at the left, the centre and the right. Again, take the smallest measurement.
For an exact (outside-recess) fit, measure the width you want the blind to be, allowing a sensible overlap beyond the opening on each side and above it, plus however far below the sill you'd like the blind to finish. A generous overlap makes a visible difference to light control, which matters for bedrooms — our blackout blinds guide explains why.
The mistakes that catch people out
Almost every mis-measured blind we hear about comes down to one of these:
- Measuring the old blind. The blind that's coming down was itself made with deductions, may have been wrong to begin with, and tells you nothing about the actual opening. Always measure the window, never the blind.
- Assuming the window is square. Very few are — especially in older properties, where settlement over decades leaves recesses noticeably wider at one end than the other. That's precisely why you measure in three places.
- Ignoring obstructions. Window handles that stick out, tiled splashbacks that reduce the recess, dado rails, telephone points and curtain fittings can all stop a blind sitting flat or operating freely. Check what the blind will have to clear before you settle on a fitting position.
- Forgetting recess depth. The headrail and brackets need enough depth to mount inside the recess. If the recess is shallow, an exact fit on the wall is usually the better answer.
When no drilling is needed at all: Perfect Fit
If your windows or doors are uPVC, there's a third route that side-steps most of this. Perfect Fit blinds sit in a slim frame that clips into the rubber gasket around the glass — no drilling, no screws, no damage to the frame, and no recess measurements to agonise over. The measuring is done from the glass and the surrounding bead instead, and it needs to be precise, which is one of the reasons we always measure Perfect Fit ourselves at the home visit.
Perfect Fit works beautifully on tilt-and-turn windows and uPVC doors, where a conventional blind would swing about or foul the handle, and it's a favourite for conservatories. Pleated fabrics are a popular pairing there, helping to take the edge off summer heat.
Which fitting position suits which blind?
There's no single rule, but this is how the common types are usually fitted:
| Blind type | Typical fitting position |
|---|---|
| Roller blinds | Recess where depth allows; exact fit for better light blocking |
| Vertical blinds | Either; usually exact fit over patio doors and wide openings |
| Venetian blinds | Recess, sitting close to the glass for a tailored look |
| Roman blinds | Often exact fit, so the stacked folds clear the glass |
| Day & night blinds | Recess or exact, like a standard roller |
| Perfect Fit blinds | Clipped to the uPVC frame itself — no drilling |
| Skylight blinds | Mounted to the roof window sash, sized to the exact model |
Or skip the tape measure altogether
Here's the honest truth: a made-to-measure blind is cut, welded and finished to the sizes on the order, and once it's made, it can't be returned because the numbers were wrong. That risk sits with whoever did the measuring.
Which is why we'd rather do it for you. Our free home consultation covers everywhere within twenty miles of Mansfield: we bring the sample books, measure every window with the proper kit, advise on recess versus exact fit for each one, and take full responsibility for the sizes. If a blind we measured didn't fit, that would be our problem to put right — not yours. Book a visit or call 01623 325 054 and let the tape measure be someone else's worry.
Frequently asked questions
Should I measure my windows in centimetres or millimetres?
Millimetres, ideally. Blinds are manufactured to fine tolerances, and measuring in millimetres removes the temptation to round figures up or down. If you're supplying a recess measurement, give the full size of the opening and let the maker apply the operating allowance — never deduct anything yourself, and always say whether your figures are recess or exact sizes.
What if my window recess is too shallow for a blind?
It's a common issue, particularly where windows have chunky handles or deep sills. The usual answer is an exact fit, with the blind mounted on the wall above the opening so recess depth no longer matters. On uPVC windows, Perfect Fit blinds are another neat solution, clipping to the frame itself without needing any recess depth at all.
Do bay windows need to be measured differently?
Yes. A bay is treated as a series of individual windows, each measured separately, and the angles between the sections affect how the headrails meet at the corners. It's one of the trickiest measuring jobs there is, and honestly one we'd recommend leaving to us — bay measuring is bread and butter at our free home visits.
What happens if a blind is measured wrong?
If you measured it yourself, unfortunately the cost usually falls on you, because made-to-measure blinds are manufactured to your figures and can't be resold. That's the biggest single argument for using our free measuring service: when we take the sizes, the responsibility for them is entirely ours, and our fitting work is backed by a 12-month guarantee.
How much should a blind overlap the window outside the recess?
There's no law about it, but a useful rule of thumb is a comfortable overlap on each side and above the opening, and a little below the sill. The more the fabric overlaps the wall, the less light sneaks around the edges — which is why bedroom blinds fitted outside the recess often achieve a darker room than recess-fitted ones.