Window curtains are measured first by width and then by length, and for this reason the first step is determining window width. Use a metal tape measure for the most accurate results, according to CurtainWorks.
Curtain width should be one and a half to three times the window width in order to correctly fill out the window. Less fabric gives the curtains a fitted style, while using more fabric will lend them a more plush style. The length of the curtain is a matter of personal preference. Common curtain lengths include measuring to the window sill, roughly six inches beneath the sill, and to the floor. Measure sill and floor lengths at several points, and use the lowest measurement in order to account for any unevenness, advises home furnishing retailer Dunelm.
When using an outside curtain mount the measuring process is slightly different. Window width for these mounts should be measured from just outside the window moldings. Add 16 to 20 inches to that measurement in order to determine the size curtain rod or track needed. This kind of mount is commonly hung 1 to 3 inches above the window recess. Curtain length, therefore, should be measured from the top of the rod, notes CurtainWorks.