Blind Sizing Questions Answered for Your Home

A blind that is a few millimetres out can spoil an otherwise tidy window. Too narrow, and light leaks around the edges. Too wide, and it may catch on handles, tiles or nearby furniture. These blind sizing questions cover what you need to know before ordering made-to-measure blinds, whether you are fitting a bedroom blackout roller blind, refreshing vertical slats or replacing a full blind in a rental property.

The key is to decide how the blind will be fitted before picking up the tape measure. Your measurements should describe the available space accurately, not the size you think the finished blind ought to be.

Start with the fitting type

There are two main ways to fit a blind: inside the window recess or outside it. This choice affects every measurement that follows.

A recess fit sits within the opening of the window. It is neat, practical and works well where there is enough depth for the blind and any window handles. For this option, measure the opening itself. The manufacturer will normally make the deductions needed for the blind to operate without rubbing against the sides.

An exact fit is fitted outside the recess, usually to the wall, window frame or ceiling. You tell us the finished width and drop you want. This is often the better choice for shallow recesses, outward-projecting handles or windows where you want stronger light control. A wider blind gives better coverage beyond the glass, which is particularly useful in bedrooms.

Do not order a recess measurement as an exact fit without thinking it through. A blind made to the full opening width may not fit between the walls. Equally, an exact-fit blind measured only to the glass can look undersized and allow unwanted gaps.

Blind sizing questions about measuring a recess

Should I measure the glass or the whole recess?

For a recess fit, measure the whole internal opening, not the glass. Take the width between the two side walls or frames, and the drop from the top of the recess to the sill. The glass size does not account for the space needed around it, so it is not a reliable blind size.

Measure the width in three places: near the top, middle and bottom. Old properties in particular can have recesses that are not perfectly square. Use the smallest width measurement so the blind has the best chance of fitting cleanly. For the drop, measure left, centre and right, then use the shortest measurement if the sill or top of the recess is uneven.

Use a metal tape measure rather than a cloth tape. Record measurements in millimetres where possible. It avoids confusion between 104.5 cm and 1045 mm, and it gives a clearer result when ordering a custom product.

Do I need to make a deduction for a recess blind?

Usually, no. If a product is ordered as a recess fit, provide the exact recess size requested in the measuring instructions. The blind needs small operating clearances, and these should be calculated by the manufacturer. Making your own deduction as well can leave a larger-than-expected gap at each side.

Product types vary, so always follow the measuring guidance for the specific blind. Roller blinds have brackets and a control mechanism to consider, while vertical blinds have a headrail and slats that need enough room to turn and hang freely.

How deep should the recess be?

Depth matters as much as width. Check for handles, air vents, tiles, alarm sensors and window catches that sit within the opening. A roller blind may fit in a narrow recess but still be held away from the wall by a handle. Vertical blinds also need sufficient clearance for the headrail and for the slats to rotate.

If anything projects into the recess, fitting outside it may be simpler. It gives you more freedom to position the brackets above the obstruction and choose a width that covers the whole window.

How much wider should an exact-fit blind be?

There is no single figure that suits every room, but allowing extra width beyond each side of the recess or glass generally improves privacy and light control. For most windows, an overlap of around 50 mm to 100 mm on each side is a sensible starting point, provided there is clear wall space.

For blackout blinds, wider coverage is usually worth having. Blackout fabric blocks light through the material, but light can still enter around the edges. Extending the blind beyond the window makes a noticeable difference in a bedroom, nursery or room used for shift work.

The available fixing space decides the final size. Check that brackets will not land on uneven plaster, a tile edge, cupboard doors or a picture rail. If you are fitting above the recess, allow enough height for the blind to roll up without covering too much glass when raised.

For the drop, decide where you want the blind to finish. A blind fitted outside the recess often looks best when it reaches below the sill. In kitchens and bathrooms, you may prefer it to stop above a worktop, radiator or splashback. Measure to the point where the blind can hang without getting in the way.

Sizing roller blinds correctly

Roller blinds are straightforward once you separate fabric width from bracket-to-bracket width. These are not always the same. The brackets and control side add to the overall fitting width, while the fabric sits slightly inside them.

This matters when your recess is tight. A roller blind can have fabric that looks as though it should fit, but the complete mechanism may be too wide for the opening. Check the product specification and measure the usable width between the surfaces where the brackets will be fixed.

Think about which side the control chain should sit on before ordering. Put it where it is easy to reach and least likely to interfere with a window handle or furniture. Keep chains safely secured and out of reach of children.

PVC waterproof roller blinds are a practical option for kitchens and bathrooms because they can be wiped clean. Their sizing process is the same, but moisture-prone rooms often contain tiles, extractor switches and mirrors, so inspect the fitting area carefully before choosing recess or exact fit.

Sizing vertical blinds and replacement slats

Vertical blinds need accurate width and drop measurements, but there are a few extra details to check. First, decide whether the headrail will sit inside or outside the recess. For a recess fit, make sure the slats will not rest on a deep sill, radiator cover or protruding handle when they hang down.

For an exact fit, measure the finished headrail width you want, including the extra coverage either side of the window. Then choose a drop that gives enough clearance above the floor, sill or furniture. Slats should not drag along the floor, but leaving them too short can make the blind look unfinished.

If you only need replacement vertical slats, measure one existing slat from the very top to the very bottom rather than estimating from the headrail. Also check the slat width and the hanger style. Replacing worn or marked slats can refresh a room for far less than fitting a complete new blind, especially in rental homes or larger properties.

Common measuring mistakes to avoid

The most frequent problem is assuming all windows are square. Measure every window individually, even when two windows look identical. Extensions, bay windows and older walls regularly vary by a few millimetres.

Another mistake is measuring around a blind that is already fitted. Remove the old blind from the calculation and measure the actual recess or the intended new fitting area. Existing blinds may have been cut down, fitted poorly or made to a different size.

It is also easy to forget obstacles. Open the window fully and look at the route of the handle. Check where cupboard doors open, where radiators sit and whether curtain poles, tiles or sockets affect the brackets. A quick check now is easier than moving fixings later.

Finally, do not round measurements up because you want better coverage. For recess blinds, use the smallest exact measurement. For exact-fit blinds, build in the extra coverage deliberately and write down the final finished width and drop before ordering.

A final check before you order

Put your measurements beside the fitting type one last time. Ask yourself: am I giving the recess opening size, or the finished blind size? That single check prevents most ordering errors.

If you are unsure, take a photo of the window, note any handles or obstructions, and measure again before placing the order. Queen Blinds makes custom sizing affordable, but a careful five minutes with a tape measure is what makes your new blind look properly made for the window.