How to measure for curtains and blinds
For a quote, a rough width and height of the window is enough. Your installer measures properly before anything is made. This is how they measure, if you want to do it the same way.
Measuring for curtains
A made curtain is measured from its track or rod, not from the glass. This is how an installer does it.
Width: the track or rod, end to end
Measure the full length of the track or rod, not the window.
A new track runs far enough past the window that the curtains, pulled right back, stand on the wall beside the glass. Where there is less wall than that, they cover part of the glass.
Drop: from the track or rod down to the floor
Measure straight down to the floor from the top of the track, or from the underside of the rings on a rod. For eyelets on a pole, start at the top of the pole.
If you are unsure, tell your installer where you measured from.
A floor-length curtain is made to finish 1 to 2 cm above the floor.
For a shorter curtain: to the sill, or just below it
Over a counter, a heater or a deep sill, measure down to about 1 cm above it.
A curtain that hangs below the sill usually finishes 10 to 20 cm below it.
Measuring for a blind inside the recess
Each way is measured in three places.
Width in three places, drop in three places
Measure the width at the top, the middle and the bottom. Give the narrowest.
Measure the drop at the left, the middle and the right. Give the shortest.
Give the size of the opening itself, and tell your installer that it is the opening you measured.
For a roller blind inside the recess, the recess must be at least 50 mm deep beyond the window handle (90 mm with a cassette).
Measuring for a blind outside the recess
The blind is made larger than the opening, so it overlaps the wall.
The opening, plus 100 mm each side and above
Measure the width and the drop of the opening.
Add 100 mm to each side and 100 mm above. Check that the wall is clear that far: no switch, cupboard or corner in the way.
A blind inside the recess, or outside it?
Either is right. These are the things that decide it, and "not sure" is a full answer on your quote.
Inside the recess
A neat, built-in look. The frame stays on show and nothing stands out from the wall.
A little light shows down each side, so it is not the darkest way.
The recess must be deep enough, measured clear of the window handle, and fairly square. Your installer checks it at the visit.
Outside the recess
The blind overlaps the wall, so much less light gets round the edges. The better way where you want the room dark, such as a bedroom.
It can clear handles, a fly screen or burglar bars, and hides a recess that is shallow or out of square.
It needs clear wall above and beside the window.