You will need
If the curtains are planned on a fabric hinge, then the width in this case, you need to calculate by multiplying the width of the cornice 1.5 or 2. Curtains of this kind do not require strong draping, and is enough to double the width.
The curtains on the grommet or on a standard curtain tape is better to buy too in the double size of the width of the ledge.
Curtains with pencil pleat or puff need to buy ranging from double-wide. The optimum width of the curtains with fancy folds starts from multiplying the width of the eaves is 2.5.
When sewing curtains should also consider their length. For example, if the curtains are planned to the window sill, the length is better to do this to curtains 1 cm is not reached to the window sill, to better glide along the eaves. If the length of the curtains is planned to sex, there are two possible alternatives. Or less than 1 cm, or if the fabric of the curtains tight, you can make it cascading to the floor.
If the curtains are patterned, the width of the curtains you need to add the width of the repeat to fit the pattern.
Remember that on both sides of the curtains need to consider the bending about 10 cm.