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Calculate the Width
Add for extra width
Debbie Colgrove, Licensed to About.com
Many windows will need more fullness than one length of fabric will achieve.
A simple rule of thumb is that a curtain is one and a half to two times the width of the area it is meant to cover. A heavier fabric will provide fullness that a light weight fabric will not supply. The lighter the weight of the fabric the more fullness it will require.
For appearance and proper hanging, it is best to have at least a half width of the original fabric when you need to add width to a panel. If you only want a couple of inches more than the original length of fabric will offer, cut away from the first length of fabric and add it on with the second length seamed to the first length of fabric.
Add additional panel lengths to achieve the desired width to the original length of the panels to purchase the proper amount of fabric.
A one inch hem is usually sufficient for the sides of curtains.