Width: Measure the track (A) and not the window. The track should usually extend 30cm (12″) beyond each side of your window to enable your curtains to be pulled well back during the day. If a curtain overlap is required, add (B) plus (C) for the total track width.
Length: There are three basic lengths to choose from: sill length(D), below sill(E) and floor length(F). Curtains should finish 1.3cm (1/2″) above the dill, 15cm (6″) below the sill, or 2.5cm (1″) from the floor. Measure from the point where you wish the top of the curtain to be, down to the appropriate point. The length measurement must cover the total curtain length including the the chosen heading type.