Let's start with width and then the length.

Width:
One of the most important things is to measure the track or pole and NOT the window.

For Track:
Measure the total width of the track

For Poles:
Measure the decorative pole up to the decorative end.

For Single track with overlapping bracket or track:
Make sure to measure both widths of the track and add them together.

Length:
The length will depend on the base length and your valence or curtain style length. Decide where you want the curtain to finish. Curtains can pool on the floor, be level with the floor, below the window sill or exactly to the window sill.

Allow 1cm clearance for carpets or radiators. If a carpet doesn't exist then allow 2.5cm in clearance for the depth of the carpet.

Adjusting the length for the curtain style

For Pencil pleat:
Measure from the eye on the curtain track hook or the pole ring to the where you want the curtain to end.

Tab top/Tie top:
Measure the top of the pole.

Pinch pleat and goblet:
Measure from the eye on the curtain track hook or pole ring to where the curtains finish and then add the following

For curtain pole:
Add 05.cm to your measurement. To cover track add 4cm to your measurement. If the curtain hook is placed below the measurement it might fall forward.

To hang below the curtain pole:
Add 0.5cm to your measurement.

To cover the track: add up to 4cm to your measurement. If the curtain hook is placed below this measurement, the heading will fall forward.