How to Measure Windows for Blinds: Inside and Outside Mount

An inside-mount blind sits within the window reveal, its headrail or brackets fixed to the top of the recess. It looks neat, leaves the architrave and any good stone or plaster reveal visible, and keeps the window wall clear. Its limits are depth and light: it needs enough depth for the mechanism and for the fabric or slats to clear handles, and it always leaves a slim gap at each side where light passes.

An outside-mount blind sits on the wall above the opening, or on the face of the frame, and overlaps the opening on every side. It darkens better because it covers the edges, it hides a crooked or damaged reveal, and it is the only option where the recess is too shallow. It also lets a Roman or slatted blind stack on the wall rather than across the glass. The cost is a more visible headrail and a blind that reads as added to the window rather than part of it.

Decide this first, because the two are measured differently and the numbers you send mean different things. If you are undecided, measure for both: the recess for an inside mount, and the opening plus the wall around it for an outside one.

Depth is the measurement people forget. Hook the tape on the front edge of the reveal and measure back to the window frame, then measure again to the front of the window handle, the stay or the lock, whichever projects furthest into the recess. The second figure is the one that matters, because the blind has to hang in front of whatever sticks out.

How much depth a blind needs depends on its type, and makers publish a minimum for each. A roller needs room for its roll, which grows with the drop and the fabric thickness. A zebra carries a double layer and so needs more. A Venetian needs depth not just for the headrail but for the slats to tilt, because a slat sweeps a little more than its own width as it turns; a wide slat in a shallow reveal will strike the handle every time. Roman blinds need room for the folds to stand out from the headrail.

In Goan homes, also look for what else lives in the reveal: a sliding insect net, an inward-opening timber shutter, a security grille, a curtain track already fixed at the top. Each takes its own slice of depth. If there is not enough left for the blind you want, the options are a different type with a slimmer profile, a roller set to reverse roll, or an outside mount. The US federal guide specification for roller shades distinguishes a regular roll, where the fabric drops from the back of the tube nearest the glass, from a reverse roll, where it drops from the front; reverse roll moves the fabric forward, clear of a projecting handle.

For an inside mount, measure the width of the recess at the top, the middle and the bottom, from plaster to plaster, keeping the tape level. Then measure the drop at the left, the centre and the right, from the underside of the top of the recess to the sill. Write down all six numbers in millimetres.

Here is a typical result from an older house: widths of 1,212 mm at the top, 1,206 mm in the middle and 1,198 mm at the bottom. The recess narrows by 14 mm from top to bottom. A blind made to 1,212 mm would bind on the plaster as it came down, so the governing width is the smallest: 1,198 mm. Drops of 1,505, 1,500 and 1,496 mm give a governing drop of 1,496 mm, so the blind will not sit on the sill on the low side.

Finally, check whether the opening is square by measuring both diagonals, corner to corner. In a perfectly rectangular recess they are equal. If one reads 1,915 mm and the other 1,927 mm, the 12 mm difference tells you the recess is skewed, and that matters for any blind that runs close to the sides, such as a roller in side channels or a Venetian with a small edge clearance. It is the same check our catalogue guides recommend before fitting channels.

A blind cannot be made to the full width of the recess, or it would rub on both sides. Makers therefore deduct a clearance. A US federal guide specification for window blinds gives a typical figure for horizontal blinds mounted inside the opening: a clearance of 6 mm at each jamb. On our example, that is 1,198 − 6 − 6 = 1,186 mm for the blind. Other blind types, and other makers, deduct different amounts, because a roller's brackets and end fittings sit outside the fabric and a vertical blind's headrail behaves differently again.

This is why trade order forms ask whether a figure is a recess size or an exact size. A recess size is the hole; the maker subtracts its own clearance before cutting. An exact size is the blind; the maker cuts to your number. The most common ordering mistake is to deduct a clearance yourself and then send the result as a recess size, so the clearance is taken twice and the blind ends up with wide gaps at the sides.

Our advice is to send the raw recess measurements, all six, the two diagonals and the depth, clearly labelled as recess sizes, and let whoever makes the blind apply the deduction for that type. If you are ordering an outside mount, send the exact finished size you want, and say so.

For a blind fixed outside the opening, measure the opening at its widest, not its narrowest, because the blind must cover every part of it. Then add an overlap on each side. The same federal specification asks for blinds installed outside the jambs to overlap each jamb by 20 mm or more where the wall allows. That is a minimum for neatness; for a bedroom that must be dark, much more overlap helps, as light that escapes round the side of a blind spreads across the ceiling.

Worked example: an opening of 1,212 mm at its widest, with an overlap of 75 mm chosen for each side, gives 1,212 + 75 + 75 = 1,362 mm finished width. Check there is wall on both sides to take it: a switch, a cupboard, a corner or the next window may be closer than it looks.

For the drop, decide the fixing line above the opening first. Fixing higher lifts a Roman or Venetian stack off the glass and makes the window look taller, as long as there is solid wall or a lintel to fix into. Measure from that line to where you want the blind to finish, usually a little below the sill for an outside mount, or onto the sill if there is a deep one. In Goan houses with soft laterite or old lime plaster over the lintel, also look at what the brackets will be screwed into; our guide to laterite and old walls explains the fixings.

Out-of-square openings are the norm, not the exception, in older Goan houses. Walls of laterite block or rubble with thick lime plaster were finished by hand, and decades of repainting and patching add their own unevenness. A recess that narrows by a centimetre or more from top to bottom, or leans so that its diagonals differ, is common. The three-point, smallest-wins rule handles a taper. A pronounced lean is better handled by fixing the blind outside the recess, where its straight edges are judged against the wall rather than against a crooked reveal.

Arched heads, a familiar feature of Indo-Portuguese windows, need a decision rather than a measurement. A standard blind can be fixed across the springing line of the arch, at the level where the curve starts, leaving the arch itself open as a fanlight; or fixed above the arch on the wall, covering it completely. Shaped blinds that follow the curve exist but are fixed rather than operable. Our guide to arched and odd-shaped windows goes through the choices.

Inward-opening timber shutters, oyster-shell panes and fixed iron grilles all take space in or in front of the reveal. Measure where the shutters land when folded back, and whether a blind inside the recess would stop them closing. Often the answer in a heritage house is to leave the reveal to the shutters and fit the blind or a curtain on the inner wall face.

On doors, measure the glass panel as well as the door, and note the handle position and which way the door opens. A blind fixed to the door itself must clear the handle and not hit the frame as the door swings; a blind fixed to the wall above must clear the door as it opens. For sliding doors, the federal specification asks for vertical blinds to draw one way, stacking on the side opposite the opening panel, so the stack does not block the way out. Measure how much wall there is beside the fixed panel for that stack.

Very wide windows usually need more than one blind. A single wide roller may exceed the fabric width or bow on its tube; a wide slatted blind becomes heavy to raise. Measure the full width and the position of every mullion, so the run can be split at the mullions where the joins will be least visible. The federal specification calls for intermediate support brackets on wider blinds, so note anything above the window that would get in their way, such as a pelmet or an air-conditioning pipe.

Vertical blinds are usually fixed outside the opening on the wall or to the ceiling, and finish just above the sill or just above the floor. Measure the drop to the sill or floor at several points, because floors and ceilings are rarely level either, and take the shortest. Our vertical blinds page covers stack widths and fixing options.