How to Measure Blind Windows for a Neat Fit

A blind that is only a few millimetres too wide can catch on the frame, while one that is too narrow leaves unwanted light gaps. Knowing how to measure blind windows properly before ordering saves wasted money, delays and the frustration of trying to make an ill-fitting blind work.

The first decision is not the blind style. It is whether the blind will sit inside the window recess or outside it. That choice changes every measurement you take, as well as how much light and privacy the finished blind will provide.

Start by choosing recess or face fixing

A recess fit means the blind is fitted inside the opening of the window. It gives a neat, built-in appearance and works well where the recess is deep enough for the brackets and blind mechanism. It is a popular option for roller, Venetian and vertical blinds, but it will not completely block light around the edges.

A face fit, also called an exact fit or outside-recess fit in some product guides, places the blind on the wall, ceiling or window frame outside the recess. This gives greater coverage and is often the better choice for bedrooms, shallow recesses and windows with handles, vents or other obstructions inside. It can also make a small window appear larger.

Before measuring, open the window fully and check where handles sit. Look for tile edges, alarms, vents, picture rails and protruding sills. A blind may look simple once installed, but its brackets and control chain need clear space to work safely.

What you need to measure blind windows

Use a rigid metal tape measure, not a fabric tape or a ruler. Measure in millimetres wherever possible, as most blind suppliers and fitters work to millimetre accuracy. Keep a pencil and notepad handy, and write down the width and drop separately for every window.

A spirit level is useful if you are face fitting above a window, particularly in older properties where ceilings, plaster and window openings are not always perfectly level. If you are measuring several rooms, label each note clearly, such as “front bedroom, left window, recess fit”. Two similar-looking measurements are easy to mix up later.

Do not round a measurement up because it sounds neater. Record what the tape says. If an online supplier asks for whole centimetres, follow its instructions, but do not make your own deductions unless the manufacturer specifically tells you to do so.

How to measure a recess blind

For a recess blind, take three width measurements: across the top, centre and bottom of the recess. Use the smallest measurement. Window openings are often slightly narrower at one point due to plaster, tiling or uneven reveals, and the smallest width is the one that tells you whether the blind will physically fit.

Then measure the drop on the left, in the centre and on the right. Use the shortest drop if you are ordering a blind that must sit fully within the recess. This prevents the bottom bar from fouling a sloping sill or tight edge.

Measure from the point where the brackets will be fixed, not from the front edge of the wall. For a top-fixed blind, that is usually the underside of the top recess. For a blind fixed to the sides, allow for the bracket positions and check the product fitting guide before ordering.

Most made-to-measure suppliers apply their own deduction to recess measurements so the blind has operating clearance. In that case, provide the actual recess width and drop, with no deductions. Ready-made blinds work differently: the stated size may refer to the fabric width, the headrail width or the overall bracket-to-bracket width. Always check which measurement the product uses. This is one of the main reasons a blind can appear too wide even when the window was measured correctly.

Check the recess depth

Depth matters as much as width. A slim roller blind may fit comfortably in a narrow recess, while a Venetian blind or a blackout roller blind cassette needs more room. If the blind projects beyond the recess, it may still operate, but it will not have the flush look you expected.

Handles are another common issue. Measure how far they project into the opening when the window is closed and opened. If they clash with the blind, consider a face fit, extension brackets or a blind with a different mounting position.

How to measure blinds fitted outside the recess

For a face-fitted blind, start with the width of the area you want to cover, rather than the width of the glass alone. As a general rule, add at least 50 mm of overlap on each side of the recess. In bedrooms or rooms where privacy and light control matter, 75 mm to 100 mm each side is often better, provided there is enough clear wall space.

For the drop, decide where the top of the blind will sit. Fitting it 50 mm to 100 mm above the recess is common because it gives space for the brackets and reduces light entering above the fabric. Measure down to where you want the blind to finish: just below the sill, level with the sill, or further down the wall.

A blind fitted to the ceiling can be a practical answer where there is no space above the window or where a full-height look is wanted. Measure from the ceiling to the finished drop, but make sure the ceiling fixing is suitable. Plasterboard alone may not support the weight of some blinds without proper fixings or a secure timber point above.

Remember that the operating mechanism can make the overall blind wider than the fabric. Check whether the advertised width includes the brackets and chain control, particularly on ready-made roller blinds. Leave enough space for the chain to hang freely and keep it safely tensioned with the supplied child-safety device.

Measure differently for each blind type

The basic process is similar, but blind types have different practical considerations. Roller blinds are usually straightforward, although you must confirm whether the stated width is fabric or mechanism width. The fabric is commonly narrower than the full fitting width, which creates small side gaps.

Venetian blinds need enough depth to tilt and raise without catching the window handle. Their headrails are also heavier than a lightweight roller blind, so the fixing surface must be checked before installation.

Vertical blinds need careful drop measurements. If they stop above a radiator, sill or furniture, ensure there is room for the louvres to turn and hang straight. Roman blinds stack up at the top when raised, so allow for the folded fabric if it could obstruct an opening sash or reduce the visible glass area.

Perfect-fit and clip-in blinds are measured to the glazing and bead rather than the wall recess. They are designed for suitable uPVC windows and doors, but not every frame profile accepts the clips. Measure the glass, bead depth and frame type exactly as required by the product instructions. Guessing on this type of blind is rarely successful.

Common measuring mistakes to avoid

The most frequent mistake is measuring only once. Taking three measurements shows whether the opening is square and helps avoid a blind that binds at one end. The second is using the largest recess width because it looks more generous. Inside a recess, the narrowest point controls the fit.

Another issue is forgetting the sill, handle or tiles. A blind may clear the window when raised but strike an obstruction as it comes down. It is also worth checking which way the window opens. A recess blind can restrict access to inward-opening windows if the headrail sits too low.

Do not assume two windows in the same room are identical. New-build homes can vary from one opening to the next, and older London, Essex and Kent properties often have uneven plaster or altered window surrounds. Measure each opening separately.

Finally, avoid drilling until the blind has arrived and you have checked the supplied brackets against the actual wall or frame. Solid brick, concrete, timber and plasterboard all need the right fixings. A blind that is correctly sized still needs secure, level mounting to operate smoothly.

If you are unsure about the recess, fixing surface or the measurements, it is usually quicker to get the job checked before ordering than to replace a made-to-measure blind. We Fit All can measure and fit blinds across London, Essex and Kent, helping make sure the finished result is straight, secure and right for the room.