Measuring the track or pole (a) and not the window. The track or pole should extend beyond each side of your window to enable your curtain to be pulled well back during the day. The usual allowance is at least 15.2cm. (6")inch of track either side of the window.
First of all, decide whether you want your curtains to reach the floor, or just below the window sill. It is recommended that the ideal length curtain should:be 6" below the sill or 1/2" above the floor
|Just below sill
|Finish 6" below the sill
|Finsih 1/2" above the floor (remember to allow extra room if you plan to fit carpets)
For pencil pleat curtains, measure from the 'eye' i.e. the small metal rings hanging from the larger curtain ring, eyelet curtains, measure from the top of the curtain pole.