Curtain length is primarily about personal preference, but if the panels are too long, the hems can wear, and if they're not long enough, they look like awkwardly short pants on lanky legs. Get the measurements right to create neatly flanked walls that seem taller than they are, or to properly frame the windows, whether you're trying for various standard-length panels or pretty puddles.
Where the curtain rod sits isn't necessarily the point from which you'll measure for curtain length, unless you're using rod-pocket drapery. For other curtain styles, use the rod-to-floor measurement as a guide, and then deduct from it for curtain rings, grommets, ties or tabs, and a half-inch for floor clearance. If the rod isn't yet in place, position it according to the curtain length you prefer; cater to a fitting ready-made curtain length to avoid hemming. Hang curtains wide to expose all the glass, optimizing a good-looking view and natural light. For proper fullness when the curtains are pulled closed, measure curtain width, factoring for 1 1/2 to 2 times the span.
A Long, Tall Look
Standard curtains typically hang, again, from about a half-inch from the floor to halfway between the window trim and average-height ceiling. But to maximize visual height, let the walls determine your curtain's length; in a living room, library, den or bedroom, the longer the panels, the better. Fabric panels that flow from almost ceiling height to just above the floor not only add abundant soft texture but make a low, 7-foot ceiling seem average, an average 8- or 9-foot ceiling look taller, or a 10-foot ceiling appear even more substantial. Standard curtain lengths, including 84-, 95-, 108- and 120-inch-long panels, accommodate typical wall heights. For a vaulted, cathedral or overly high ceiling, mount the rod at least 4 to 6 inches above the window frame.
To Puddle or Not
Curtain puddles may be luxurious, but they're not always practical. Puddle curtain fabric only if they're decorative, not if you'll be opening and closing them routinely, which will dirty and wear the hems. An extra inch will do, if you're going for a trouser effect, like pant hems resting on shoes. If you want a bit more elegance, puddle 2 to 4 inches of fabric. For an all-out elaborate look, fan 6 to 8 inches of excess fabric on the floor.
When Shorter Is Better
Floor-length or puddled curtains aren't the norm in bathrooms, where wet floors can cause unsightly mold-growth on hems, or in the kitchen, where a window sits over the sink. In these cases, hang the panels to the window sill to show off attractive trim, or to the bottom of ho-hum or narrow trim to help hide its insignificance. Cafe-style curtains, hung from the sill to halfway or two-thirds up the window, allow some privacy without blocking all natural light.