Measuring for the correct size curtain is not about simply measuring the size of the frame of the windows but has a few more steps than that. Firstly, you will want to ensure that whatever type of curtain rod or track you are planning on using is already installed, to get accurate measurements. Once your rod or track is in place, 15cm above the window top and 15-20cm either side of the edges, you will want to measure for the width of the curtains. You do this by measuring the total width of the track or pole, excluding the decorative ends or finials. Half of that measurement will be the individual curtains width.

Next you will want to measure for the correct length or drop of the curtains. To do this you will start from where the pole or track starts down to where you want the curtain to drop to. If you want them at the base of the window frame, this is normally done by going to the window frame base and then take off 1cm. Or if you want it below the window sill, add 15cm. If you want floor length, measure to the floor then take away 1cm.
This is an excellent way to ensure that you order the correct size of curtains, the first time.