Medium-weight fabric curtains should measure two to two and a half times the width of the space they need to cover. This space is the width of the window frame for curtains mounted inside the frame or the window frame width plus 6 inches for curtains that extend to the sides.
Because sheer fabrics are much lighter, sheer curtains should be wider than their medium-weight counterparts. Multiply the width of the space the sheer curtains will cover by two and a half to three to find the correct width for the sheer curtains.
Curtains should measure two to three times the width of the space that they are intended to cover. This provides enough fabric to overlap in the center and drape in luxurious folds.
If needed, curtains can be as narrow as one and a half times the width of the space.