How to measure a straight window for curtains
The flat window is the one everyone assumes they can measure by eye. Here is the method that gets the width, the height and the drop right the first time: the same one we use before specifying.
A flat window looks like the easy one, and it mostly is, but it is also where small errors hide. Get the width or the height slightly wrong and the curtains either crowd the glass when open or stop short of the floor. Measure it in a fixed order and a straight window is no trouble at all. Here is the order we use.
First decide: recess or wall-to-wall
Before the tape comes out, settle where the hardware will sit, because it changes how you measure the width.
- Wall-to-wall (face fix): the pole or track sits above and beyond the window on the wall. This is the usual choice: curtains pull clear of the glass, the window reads larger, and the room feels taller.
- Inside the recess: the hardware sits within the window reveal. Choose this when you want the curtains contained, or when there is no wall space above the window. For a track inside a recess, the track should be cut slightly shorter than the reveal so it fits without binding.
Measure the width
With a steel tape: never fabric, which stretches. Measure the width edge to edge at roughly the height the hardware will sit. For a wall-to-wall fit, then add your stack-back allowance: 15 to 20cm each side as a starting point, so the open curtains rest on the wall and let the full window show. If a return wall, a radiator or an adjacent window limits one side, note it now rather than at the order stage.
Measuring the width but forgetting the stack-back, then wondering why the open curtains swallow a third of the glass. The pole length is the window plus the room the fabric needs to gather, not the window alone.
Set the height
Mark the pole or track 12 to 15cm above the frame where space allows. This lifts the curtain heading above the glass so no daylight is lost at the top, and gives a more generous proportion. Check the clearance to the ceiling: you need room for the heading, the rings and (on a pole) the finials, plus the depth of the bracket itself.
Measure the drop
The drop is measured from where the curtain hangs from: the bottom of the ring on a pole, or the glider on a track down to where you want the hem to finish. Three common finishing points:
- To the sill: just above it, so the fabric does not bunch.
- Below the sill: around 15cm under, a softer, less formal look.
- Floor length: roughly 1cm clear of the floor, or a deliberate break or puddle if the scheme calls for it.
Measure the drop in three places: left, centre, right, and use the shortest, because floors and sills are rarely level. Remember the drop depends on the heading: an eyelet curtain hangs from the top of the pole, a taped heading from below the ring, which changes the finished length.
Once you have width, height and drop, the heading style decides the rest, and that is where most of the look is won or lost. Our guide to heading styles covers how each one hangs and what hardware it needs.
Common questions
Should the pole go inside the recess or on the wall?
Wall-mounting is the usual choice: it lets curtains stack back off the glass and makes the window look larger. Fit inside the recess only when you specifically want the curtains contained within it, or when there is no wall space above or beside the window.
How much longer than the window should a pole be?
As a working rule, window width plus 30 to 40cm (15 to 20cm each side), so the curtains sit on the wall rather than across the glass when open. Heavier or interlined curtains need more; see our stack-back guide.
How high above the window should it sit?
Around 12 to 15cm above the frame is typical where ceiling height allows. Going higher can make a room feel taller, provided there is still room for the heading and finials below the ceiling.
Why measure the drop in three places?
Floors, sills and ceilings are rarely perfectly level. Measuring left, centre and right and taking the shortest stops the curtain catching on a high spot at one end.