Step one. Measuring the Width.
If you have a pole, measure the pole width excluding the end finials. If you have a track, measure the total width.
Step two. Measure the finished length.
The finished length is the measurement from the very top of the curtain to the bottom hem. The diagram below shows suggested lengths for both sill, and floor length curtains.
Always measure the length to the left, right and centre of your pole/track. Where the measurements vary, choose the length that suits best.