Where should curtains end? Drag the hem and see

Hem:

Pole height from the floor:

Window sill height:

Try a ready-made length

Gap above the floor

Drop to order

This hem is

A simplified front view. "Drop" is measured from the bottom of the rings or hooks, where the curtain hangs, down to the hem. The sill zone is drawn as roughly 1–8 cm below the sill; these are rules of thumb, not laws.