All you need is the width and length of the window and it’s easy to determine the allowances for different curtain styles. The basic formula is to double the width of the window to add fullness to closed curtain. Length depends largely on style and personal taste. For best results, use a metal tape measure with a locking device, or have someone help hold the measuring tape in place. Generally, it’s best to measure the window with the curtain rod in place.
If no hardware is installed, measure to total length of the window, from the top of the frame to the bottom of the window sill. When the hanging rod is installed it can be moved up or down a tiny bit, to compensate for any discrepancies. Sometimes utility rods will be mounted onto the window frame, if they are, measure the distance between the mounting brackets. For windows that have traverse rods, measure the total length of the track and add twelve inches. Curtain poles with decorative finials are usually installed several inches wider and higher than the window frame. Installing a pole three to five inches wider than the window ensures the curtains can be pushed back fully, which makes the room seem bigger and brighter.
Width is a relatively simple measurement to take; length can be a little trickier. Depending on the style, curtains may extend to the window sill or all the way to the floor. Rustic and country prim curtains look fitting with a drop at the window still or to the apron, the piece of wood directly under the sill. Semi-formal lace curtains and curtains with tie-backs can generally extend about six to eight inches below the window apron. If curtains are on the shorter side, the bottom will be above the window sill when tied back. Formal floor length curtains should extend a minimum of one inch below the floor level. Think of floor length curtains like pants. A while back, it was hip to show a little bit of sock at the ankle, now, it’s more fashionable to have a little bit of extra length. Many styles look particularly opulent when allowed to pool at the bottom for a classic look, like the red velvet curtains in Gone With the Wind.
The length measurement, or curtain drop, depends on the type of curtain rod and hanging method. Traverse rod curtains, the type with tracks and pull cords, should have an inch or two extra, above the track, to hide the mechanical components. Some traverse rods have self-contained hanging rings. For curtains that will be hung from rings, the top of the curtain will be just below hanging rod. Length measurement on curtain packaging includes solid rod pockets. Depending on the manufacturer, hanging tab measurements will be included or added separately. Curtains with ruffles that go above the rod will mention the header separately from the length measurement.
The fullness of curtains depends on the style and thickness of the material. Contemporary curtains are generally one and a half times the window width. Traditional ruffled curtains are usually twice the width of the window. Doubling the window width is the easiest formula to remember, each panel should equal the window width. Lightweight glass curtains can be two or three times the window width. Increasing the width makes the curtains fuller when closed. For sheer curtains that are kept closed most of the time, it makes sense to double the width. Tripling the width will make the curtains even fuller and more opaque.