To measure windows for new curtains, begin by using a measuring tape to measure the width of the window. Measure from side to side, and write down the width to the nearest 1/8 of an inch. Then measure for length by measuring from the top of the curtain rod to the bottom of the window, and record the length.
To figure out the curtain width, first consider whether the curtains should open far enough to reveal the entire window. This is not usually necessary if the windows are large, but consider installing the curtains outside the edge of the window if the windows are smaller. This makes the window appear larger. To do this, add 6 to 8 inches to the measurement on either side or a total of 12 to 16 inches. Multiply the width measurement by two for standard fullness or by up to three for very full curtains. Divide this number in half to get the width of one of the two curtain panels.
The steps for measuring curtain length depend on the type of curtains desired. For window sill length curtains, add 1/2 inch to the length measurement. Floor length curtains should barely graze the floor and therefore the length should be a 1/4 to 1/2 inch longer than the length from the curtain rod to the floor.