Free Curtain Size Calculator

What is a curtain size calculator?

A curtain size calculator is a free online tool that tells you the exact width, length, and rod position needed for curtains on any window. It factors in window dimensions, fullness preference, and ceiling height so you buy the right size the first time — no signup needed.

Getting curtain measurements wrong is one of the most frustrating home decor mistakes because you usually do not discover the problem until you have already hung the rod and unpacked the curtains. I have done it myself — bought 84-inch curtains for what I thought was a standard window, only to find they hovered awkwardly above the floor because I mounted the rod lower than the calculator recommended. Curtain sizing involves four variables most people do not consider together: rod overhang beyond the window, rod height above the window frame, fullness factor for the fabric width, and the curtain length style (floor, sill, apron, or puddle). This calculator handles all four simultaneously.

How it works

The calculator takes your window width, window height, and ceiling height. First, it determines rod width: window width plus 12-24 inches of overhang (6-12 inches on each side). For a 48-inch window with 12-inch total overhang, the rod width is 60 inches. Next, it multiplies rod width by your chosen fullness factor — 2x (standard) gives 120 inches of total fabric width, which means two 60-inch panels. Rod height is calculated as the window top plus your chosen rod-above distance (4-6 inches is standard, or you can mount to ceiling). Curtain length depends on your chosen style: floor length means the curtain reaches to within 1/2 inch of the floor, sill length ends at the window sill, and puddle length adds 1-3 inches of fabric pooled on the floor. For a standard 96-inch ceiling with a window starting at 12 inches from the floor and the rod 6 inches above the window, the curtain length works out to approximately 90 inches — which means you buy 95-inch curtains and hem them, or 84-inch panels with a different rod position.

When to use a curtain size calculator

Use this calculator any time you are buying curtains for a new home or redecorating a room. It is especially valuable when your window dimensions are non-standard — most curtain panels come in set lengths (63, 84, 95, or 108 inches), and knowing which size to buy without the guesswork saves an entire return trip. I use it whenever I am considering a different curtain length style than I have used before, like switching from sill-length to floor-length. Interior designers use it to spec curtains for clients without having to visit the site. The calculator is also useful when comparing mounting options: a rod mounted 6 inches above the frame vs a ceiling-mounted rod changes the required curtain length by 12-18 inches, and the calculator shows you both options side by side.

Common mistakes when measuring for curtains

The most common mistake I see is mounting the rod too low. Many people mount the rod right at the top of the window frame, which makes the ceiling look lower and the window look smaller. Mounting 4-6 inches above the frame — or ideally halfway to the ceiling — makes the room feel significantly taller. A second mistake is buying panels that are exactly the window width. Curtains need fullness to look good: flat panels that barely cover the window look like bedsheets. The minimum fullness is 1.5x the window width, and 2x is the standard that looks best for most rooms. A third mistake is forgetting about the rod overhang. Curtains that stack directly over the window block light even when fully open, so you lose 10-15 inches of glass on each side if your rod ends at the window frame.

How to use this calculator

1. Enter window dimensions — Input window width, height, and ceiling height.
2. Choose style preferences — Select fullness, length style (floor, sill, apron), and rod position.
3. Read the measurements — See rod width, panel widths, curtain length, and fabric recommendations.