Curtain Size Calculator: Rod Length, Panels and the Length to Buy

Most curtain size calculators multiply your window by two and stop there. They don’t know that grommets hang shorter than the package says, that clip rings drop the panel, that linen shrinks in the first wash, or that panels only come in a handful of lengths. This curtain size calculator does. Put in your window and your rod height, and it tells you the rod length, how many panels to buy, which standard length to pick and where the hem will actually land.

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Curtain Size Calculator

Measure in inches. If the rod isn’t up yet, mark where it will go and measure from that mark.

Last checked October 22, 2026. The calculator uses about 1¼ inches for grommets, ½ inch for rod pockets and 4% shrinkage for unwashed natural fabric. Check one panel’s listing before you order, since some sizes are sold per panel and some per pair.

What to Measure Before You Use the Curtain Size Calculator

You need three numbers from the wall and two from the curtain listing. Measure the window first, then decide where the rod goes, because every other number depends on the rod height.

• A, the frame width: outside edge to outside edge of the trim, or of the opening if there’s no trim.
• B, how far past the frame: 8 to 10 inches on each side lets open curtains rest on the wall instead of the glass. Less is fine where a corner or a cabinet is in the way.
• C, floor to the top of the rod: measure at both ends and in the middle, because floors are rarely level. For a float, use the shortest number; for a break or puddle, the longest.
• From the listing: the width of one panel and how it attaches. With clip rings, measure one ring from the top of the rod to the bottom of the clip.

Not sure where the rod should go? My guide to how to hang curtains walks through the height and width first, which is the order that keeps you away from the 84-inch trap.

How This Curtain Calculator Works

No black box. These are the rules behind every number, and where each one comes from.

That last rule matters most. Most online curtain calculators give you an ideal length, say 92½ inches, and leave you to guess which package to buy. However, nobody sells 92½-inch panels. This one picks the real size and then shows where its hem will land, so you can see whether a 95 floats, kisses or falls short.

For the full reasoning behind the length rules, see my guide to how long curtains should be.

When Not to Use a Curtain Size Calculator

A calculator is only as good as the window is ordinary. In these cases, I’d measure by hand or ask a shop that makes custom drapes:

• Bay and corner windows. Each section needs its own measurement, and the rod bends, so one width doesn’t describe it.
• Arched or slanted tops. The rod height changes along the window, so there’s no single floor-to-rod number.
• Pleated custom drapes. The workroom calculates fullness and returns itself; give them the rod length and height instead.
• Something under the window. A radiator, a counter or a bed changes the length you want before any math starts. My guide to curtains for small windows covers those spots.
• Blackout in a bedroom. The calculator gets the size right, but darkness also depends on which rod the blackout layer hangs on. See my guide to layering sheer and blackout curtains.

Questions People Ask

How do I calculate what size curtains I need?

Add the extension on each side to the frame width to get the rod length. Multiply that by 2, or 2.5 for sheers, to get the total fabric width, then divide by the width of one panel. For length, measure from the top of the rod to the floor and adjust for the header: grommets hang about 1¼ inches shorter than listed, clip rings drop the panel by the ring.

How many curtain panels do I need for one window?

Usually two for a standard window, and four for a wide one. As a quick check, a 36-inch window with a rod running 9 inches past each side needs a 54-inch rod and about 108 inches of fabric at standard fullness, so two 54-inch panels.

What if the calculator says I’m between two lengths?

Buy the longer one. Extra length can be raised with the rod, folded under clip rings or hemmed. A panel that’s too short can only be helped with rings or a sewn-on band.

Does the curtain width include both panels?

The calculator counts the total fabric across the whole rod, so all panels together. Some listings give the width of one panel and some of a pair, so check which before you order.