The height of the curtain depends on the type of the curtain rod and on the way of hanging.
Measure like:
A: From the bottom of the gliders in case of a profile
B: From the bottom of the rings or from the top of the rod itself when you have eyelets
C: From the bottom of clips when there are clips
Until the bottom of the desired height.
If the curtain is part of a shorter setup, when recommend adding 5 cm from the bottom of the windowsill, meanwhile the overhanging shall not reach the radiators.
If the curtain is part of a longer setup, the curtain should remain 3 cm above the ground, therefore the curtain will not be contaminated and it is easier to clean the floor.
If transparent and black-out curtains are ordered at the same time, the transparent should be 1 cm shorter!