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Use a steel tape measure (if it is not steel it may stretch and give inaccurate measurements)

Measuring the width

For Either a track or pole you need to decide how much stack back you require on each side of the window (how much space you require for the curtains to pull back into when open). This is normally a recommended allowance of 30 cm (12") each side.
For a track measure the width of the track, including overlap arm and returns if applicable and also allowing for stack back each side of the window if required.
For a pole measure the overall width between the finials, making sure there is then sufficient room to fit finials, if there is not room then cap finials or recess brackets may be required.
There is no rule as to how far above the window your track or pole would need to be fitted, as long as when the curtains are hooked onto the track or pole they cover the window frame so that no light shows over the top of the curtain. Also if your curtain has a stand-up on the heading then you need to make sure there is enough space between the top of the pole and the ceiling.