Curtain Size Calculator

Background.

A curtain size calculator answers three questions from one set of measurements: how wide each panel should be, how long the fabric has to be cut, and how many yards to buy. The width question is the one people get wrong, because a curtain that measures the same as its rod does not hang — it stretches flat across the window with no folds at all. Curtains need surplus width, called fullness, and fullness is a multiplier on the width being covered rather than a fixed number of inches.

Every constant on this page comes from one primary document: The Singer Company's trade manual How to Make Draperies, Book No. 102, published in 1960 and now open in full text at Project Gutenberg. Singer states fullness three separate ways, and the page prints all three rather than picking one for you: "The fullness of draperies averages about twice the width of the space to be covered"; "Over-curtains or draperies of medium weight fabrics require a fullness of 2 to 2½ times the width"; and "An allowance of 2½ or 3 times the width should be made for fullness." For sheer fabrics Singer gives no number at all — only "full enough to hang in easy, graceful folds" — so this calculator does not invent one. Fullness is an input with a 2.5 default, not a hidden constant.

The same manual supplies what to measure. For width: "Measure from edge to edge of window frame or from outer edges of fixture brackets. To this measurement add the 'return' at either end; that is, the length from curve of rod to the wall." The return is added at both ends, which is why a 64-inch rod with a 3.5-inch return needs 71 inches of coverage, not 64. For length: "Measure from the top of the rod down — to the sill — to the lower edge of apron — or to the floor." Those three are the lengths Singer calls correct, and full-length draperies "should just clear the floor or be long enough to crush on the floor" — a preference with no inch figure attached, so the floor clearance is a signed input you set yourself.

The construction dropdown is where the page is most useful and most honest. Singer's per-construction allowances are exact and quoted in the option labels — 8.5 inches for a lined drapery with a single bottom hem, 11 with a double hem, 9 unlined, 7 interlined, 5 with ready-made pleater tape. A modern custom workroom uses roughly 18 inches, because it doubles both the header and the bottom hem where Singer used a single 4-inch top hem over a buckram strip. Those two authorities disagree by about nine and a half inches on the same curtain. Rather than average them into a number belonging to nobody, both ship as selectable modes and the page names which is which.

Three limits sit beside the result, not behind an accordion. First, every figure is an estimate from measurements you took, so measure the installed rod and check the drop at three points along the window, because floors are rarely level. Second, the nominal fabric width printed on a bolt includes the selvages — ASTM D3774 measures it that way — so if your selvage is unusable, enter the usable width instead. Third, wash the fabric before you cut. Curtain fabric shrinks; AATCC TM135 exists because dimensional change after home laundering is real, fabric-specific and unpredictable, and a hemmed full-length curtain that shrinks two percent is nearly two inches short with no way back.

What is curtain size calculator?

Curtain size is two independent dimensions and one multiplier. The width dimension is the rod or track width plus the return at each end — the distance the rod bends back to the wall — and that cover width is then multiplied by a fullness ratio to get the total width of finished curtain needed. Divide by the number of panels and you have the finished width each panel must be. Fullness is what turns flat cloth into folds: at 1× a panel is a taut sheet, and Singer puts the average drapery at about twice the width of the space covered, rising to 2½ or 3 times for a fuller look.

The length dimension is measured from the top of the rod down to whichever of Singer's three correct end points you want — the sill, the lower edge of the window apron, or the floor — adjusted by however much you want the curtain to clear or puddle. That finished drop is not the cut length. The cut length adds the heading, the top hem and the bottom hem, and if the fabric carries a vertical pattern repeat the cut is raised up to the next whole repeat so the pattern lines up across every panel.

Yardage then follows from both dimensions together. Fabric is sold in fixed widths, so the total finished curtain width is divided by the fabric width and rounded UP to whole widths — you cannot buy two-thirds of a width for a full-height drop. Multiply those whole widths by the cut length, divide by 36, and you have yards. Because of that rounding up, the fullness you actually achieve is normally higher than the ratio you asked for, and this page reports it separately.

How to use this calculator.

1. 01Mount the rod first, then measure it from bracket to bracket. The rod width, not the window width, is what the curtain has to cover.
2. 02Measure the return — the distance the rod curves back to the wall — and enter it once. The calculator adds it at both ends.
3. 03Choose a fullness ratio. Singer's figures: about 2× is the average, 2–2½× for medium-weight over-draperies, 2½–3× for a fuller look. Sheers have no sourced figure; use your own judgement and say so.
4. 04Measure from the top of the rod down to the sill, the lower edge of the apron, or the floor — Singer's three correct lengths. Measure at both ends and the middle and use the shortest.
5. 05Set the floor clearance: negative to clear the floor, positive to puddle. Zero means the hem lands exactly on your measurement.
6. 06Pick the construction that matches how you will actually sew it. The allowance shown is the total added for the heading and both hems.
7. 07Enter the fabric width — the usable width, not the printed width, if the selvage has to go — and the vertical pattern repeat, or 0 for a plain fabric.
8. 08Read the finished width per panel as your headline answer, then check the achieved fullness: if it came out higher than you asked for, that is the rounding up to whole fabric widths, and it is a good thing.
9. 09Round the yardage up to your shop's cutting increment, and wash the fabric before you cut a single length.

The formula.

Width first. The cover width is the rod width plus the return at each end: R + 2r. Singer is explicit that the return is added at either end, so a 64-inch rod with a 3.5-inch return covers 71 inches. Multiply the cover width by the fullness ratio F to get the total finished curtain width, then divide by the number of panels n for the width each panel must finish at.

Length second. The finished drop D is your rod-to-end-point measurement plus the signed floor adjustment — negative to clear the floor, positive to puddle. Add the construction allowance a, and if the fabric has a vertical repeat p, raise the result up to the next whole multiple of p. That step is a ceiling and never a rounding: cutting a length that lands short of the next repeat makes the pattern impossible to match, and unlike buying too much it cannot be undone.

Yardage last. Divide the total finished width by the fabric width w and round UP to whole widths, because a full-height drop uses the full width of the bolt. Multiply those widths by the cut length and divide by 36 for yards. Rounding happens at exactly two places, both of them ceilings on quantities that are physically integral — whole fabric widths, and whole pattern repeats. Everything else is carried at full precision and rounded once at the end.

That rounding up to whole widths has a consequence worth reading: the fullness you actually get is (widths × fabric width) ÷ cover width, which is normally more than the ratio you asked for. Singer's own published example is the clean case. A 64-inch window at 3 to 1 fullness needs 192 inches of finished curtain; at 48-inch fabric that is exactly 4 widths, so 4 × 48 ÷ 64 gives an achieved fullness of exactly 3.0 with nothing wasted. Change the fabric to 54 inches and the same window needs ceil(192 ÷ 54) = 4 widths again, but now 4 × 54 ÷ 64 = 3.375× — a third more fabric than the ratio asked for, and a fuller curtain than planned. This is why the achieved figure is shown separately, and why the verdict beside it is computed from the full-precision achieved value rather than from the number displayed.

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.