Curtain Size Calculator

Background.

Buy curtain panels that add up to the width of the rod and they will close, but only by stretching flat across the glass like a bedsheet. Curtains need surplus cloth to form folds, and that surplus is set as a multiple of the width being covered, called fullness. Singer's 1960 trade manual How to Make Draperies, the source for every allowance used here, puts the average drapery at about twice the width, medium-weight over-draperies at 2 to 2½ times, and gives "2½ or 3 times the width" as its general allowance. For sheers it offers no figure at all, only that they should be "full enough to hang in easy, graceful folds".

The width being multiplied is the rod's, not the window's. Singer measures from the outer edges of the brackets and then adds the return, the short leg where the rod bends back to the wall, at each end. A 64-inch rod with 3½-inch returns has 71 inches to cover.

Length is a separate measurement, taken from the top of the rod down to one of the three end points the manual calls correct: the sill, the lower edge of the apron beneath it, or the floor. To that finished drop you add cloth for the heading and hems, and here the sources part company. Singer allows between 5 and 11½ inches depending on how the curtain is made, while a modern custom workroom typically allows about 18, doubling both the header and the bottom hem. Both are offered rather than averaged into a figure nobody uses.

What is curtain size calculator?

Sizing a curtain means fixing two dimensions and one multiplier. The flat width of each panel is the rod width plus both returns, times the fullness ratio, divided by the number of panels; that cloth is gathered or pleated so it spans the rod when closed. The finished drop runs from the top of the rod to the chosen end point, shortened to clear the floor or lengthened to puddle on it. The cut length adds the heading and hem allowances and any extra needed to match a pattern, and the yardage is the number of fabric widths times that cut length.

How to use this calculator.

1. 01Fit the rod or track first and measure it bracket to bracket, then measure the return from the curve of the rod back to the wall. Enter the return once, since it is added at both ends, or 0 for a rod with no return.
2. 02Choose a fullness ratio, 2 for a flatter, tailored look or 2.5 to 3 for deep folds, and the number of panels, usually 2 for a pair that meets in the middle.
3. 03Measure from the top of the rod down to the sill, the bottom of the apron or the floor at both ends and in the middle, and enter the shortest. Use the floor adjustment for clearance (negative, such as −0.5) or a puddle (positive).
4. 04Pick the construction you will actually sew, or Custom to type your own total allowance for the heading and hems.
5. 05Enter the usable fabric width, which is less than the printed width if the selvages will be trimmed off, and the vertical pattern repeat from the fabric's label, or 0 for a plain cloth.
6. 06Check the achieved fullness and its verdict, then round the yardage up to the shop's cutting increment.

The formula.

Width comes first. The cover width is R + 2r, the rod plus a return at each end. The total flat width of curtain is (R + 2r) × F, and each panel gets that divided by the number of panels n.

Length is next. The finished drop D is your rod-to-end measurement plus the signed floor adjustment, and the cut length is L = D + a, where a is the construction allowance. With a pattern repeat p, L is raised to the next whole multiple of p so that every cut starts at the same point in the design.

Yardage comes last. The number of widths is ⌈(R + 2r) × F ÷ w⌉, rounded up because a full-length drop uses the whole width of the bolt, and Y = widths × L ÷ 36.

That rounding has a side-effect. The fullness you actually get is widths × w ÷ (R + 2r), usually higher than F. At 2.5× on a 64-inch rod, 160 inches of cloth is 2.96 widths of 54-inch fabric; three widths give 162 inches and an achieved fullness of 2.53×. The verdict reads that achieved figure: under 2× is skimpy, 2× to just under 2.5× standard, 2.5× to just under 3× full, and 3× or more very full.

No width is set aside for side hems or for the seams that join widths, so those small losses come out of the fullness. Shrinkage is not modelled either, so wash the fabric first if the curtain will ever be laundered.

A worked example.

Frequently asked questions.

References& sources.

1. [1]The Singer Company, How to Make Draperies, Book No. 102 (1960) — Project Gutenberg ebook 63282. PRIMARY SOURCE for every constant on this page: the fullness figures ("2 to 2½ times the width", "2½ or 3 times the width", "averages about twice the width"), the three correct lengths (sill, lower edge of the apron, floor), the return being added at either end, the per-construction heading and hem allowances (lined 1+4+3½; double hem 6 instead of 3½; unlined 1+4½+3½; interlined 1+3+3; pleater tape 1½+3½), the fabric-width list, and the worked example (64" window, 4 widths of 48" fabric at 3:1, 9⅓ yd plain and 10⅔ yd with a 24" repeat) that this page reproduces exactly. Retrieved and every quotation confirmed 2026-07-30.
2. [2]ASTM International, ASTM D3774-18, Standard Test Method for Width of Textile Fabric — scope and Option A (full roll or bolt): "Unless otherwise specified, measurements shall include the selvages when present." Cited for the scope limit that a bolt's nominal width includes selvage, so usable width is less; not for any computed value. PAYWALLED full standard; scope and title read on the publisher's own store page, 2026-07-30.
3. [3]AATCC, TM135, Test Method for Dimensional Changes of Fabrics after Home Laundering (current edition TM135-2025) — four wash temperatures, three agitation cycles and four drying procedures for measuring length and width change. Cited for the pre-wash caveat only. PAYWALLED full method; scope read on AATCC's own store listing, 2026-07-30.
4. [4]Cooperative Extension Service, University of Hawaiʻi — Vera Y. Reid, Make Your Own Draperies, held in ScholarSpace, University of Hawaiʻi at Mānoa. Cited as a corroborating extension-service treatment of the same measure-and-multiply drapery method. ARCHIVED SCAN: page images only, with no OCR text layer available to automated retrieval, so no figure on this page is attributed to it. Retrieval attempted 2026-07-30.