How to measure for Curtains

Measure in centimetres or inches and use a metal tape measure. 

Try to ensure that your track or pole is installed before taking your measurements.
Your pole should be roughly 15cm past your window at either side to allow the curtains to be drawn back.
If you prefer as much light as possible to come through your window, your pole should extend beyond the window at either side to allow the curtains to be drawn back off the window.

Width:

Measure the track or pole (between the finials), not the window. If you do not have either of these installed, measure the window and add around 15cm either side (30cm in total) to allow for the curtains to be pulled back.

Drop:

Measurements for the curtain drop should be measured from the top to the bottom of the curtain.

For pencil or pinch pleat on a pole, measure from the eye of the ring that your curtain will be hooked onto, (this allows the curtains to fold under the pole when stacked back) to where you would like the curtains to finish. This is usually 1cm clear of the carpet, floor or radiator.