Curtain Size Calculator

Why Buying to Match the Window Ruins the Look

Buying curtains to match window width, ending up with two flat panels that stretch tight and never fold, is the error that spoils more windows than any other. Curtains are supposed to be generous, a pair that just covers the glass when closed looks mean and lets light leak down the sides. Always start from rod width, multiply by the fullness a heading needs, and buy or sew to that total, even when it means twice as many panels as expected. The rod should also reach well past the window so open curtains clear the glass. Get the fullness right and even inexpensive fabric looks rich; get it wrong and the most expensive fabric still looks skimpy. The DOE guide on energy-efficient window treatments confirms the fullness and stackback principles underpinning professional curtain sizing.

What the Curtain Size Calculator Actually Does

This tool works out how wide curtains need to be, how many panels to buy, and how much fabric to purchase for sewing, all from rod width and heading style. Enter the curtain rod's width, choose a heading such as pencil pleat or eyelet, and the calculator applies the right fullness ratio to find total curtain width, then translates that into ready-made panels or sewing yardage. As the Home Depot guide to measuring curtains explains, curtains must be far wider than the window so they gather into folds rather than hanging flat. This calculator serves both shoppers and makers, buy mode tells you how many ready-made panels to purchase, sew mode returns fabric widths, cut length, and total yardage.

Fullness: Why Curtains Are Wider Than the Window

Fullness, the ratio of flat curtain width to rod width, is the single most important concept in curtain sizing. A fullness of 2 means curtains are twice the rod width; 2.5 means two and a half times. Hang curtains that merely match the window and they look thin and stretched, the most common curtain mistake. The calculation is simply rod width multiplied by fullness ratio, giving the total flat width all panels must add up to.

As Terrys Fabrics describes, matching fullness to heading is what makes curtains look custom rather than skimpy, which is why measuring the rod rather than the window always matters.

Buying Ready-Made Versus Sewing Your Own

Once total width is known, the calculator answers the practical question two ways. Buy mode divides total flat width by a single ready-made panel's width and rounds up, often surprising people since two standard panels rarely provide enough fullness for a wide window, and the calculator shows the fullness those panels actually deliver. Rod overhang matters here too, since the rod should extend 8 to 12 inches beyond each side of the window before measuring. Sew mode turns the same total width into a fabric order, dividing by fabric width to find how many widths to seam together, then adding header and hem allowances to the finished drop for cut length, multiplying for total yardage. This is the same width-nesting logic powering our Fabric Calculator for general projects, and for patterned fabric extra should be added for matching the repeat across widths, just as with our Quilt Calculator backing.

Getting the Length Right

Width sets the fullness, length sets the style, and the two decisions are separate. Curtains measure from the rod down to where the hem should fall, so mounting the rod first matters, typically 4 to 6 inches above the window frame, or higher to make the window appear taller. From there, hem position defines the look: sill length for kitchens and bathrooms, floor length for a tailored room, or a dramatic puddle for formal spaces. Always measure the actual drop in the room rather than assuming a standard panel length will fit, since window heights and mounting positions vary widely, a panel too short looks accidental while one cut to the right drop looks deliberate. As curtain measuring guidance from Islington Council's sewing course notes, finished length should be decided on the wall, not from the packaging.

Accuracy and Limitations

This calculator applies standard fullness and yardage formulas exactly, so for a straightforward window the panel count and fabric figures are reliable. Fullness ratios align with guidance from the Decorators Best curtain fullness guide, recommending 1.5x to 3x fabric width depending on heading style and fabric weight, the panel calculation rounds up to ensure adequate gather, and sewing yardage includes header and hem allowances. It assumes a single rod across a flat window and does not plan for bay windows, corner windows, or track-wrapping curtains, which need each section's width measured and added. Sewing yardage does not automatically add for pattern repeats, so patterned fabric needs extra allowance to match the design across seamed widths. Fabric weight matters too, very heavy fabrics hang well with slightly less fullness while sheers can take more, so treat the heading-based ratio as a sound default rather than an absolute, and confirm finished drop by measuring on the wall after the rod is mounted.