MathBench

Curtain & Drapery Calculator

Enter the rod width and finished length, choose a fullness, and get the number of fabric widths, the cut length (repeat-matched if patterned), and the yardage to buy.

Result

—fabric to buy
—widths (drops)
—cut length per drop
—widths per panel

How this calculator works

Curtain yardage is decided by three numbers most people have never met: fullness (how much wider the fabric is than the rod), drops (how many widths of fabric you must seam together to reach that fullness), and the cut length (the finished length plus hem and header allowances: rounded up to whole pattern repeats when the fabric has a print). This calculator walks that exact workroom sequence and shows each intermediate number, so you can check the final yardage.

Measure the rod, not the window (curtains cover the rod's span), and measure the finished length from where the fabric will hang below the rod (ring bottoms for ring-hung panels) to where you want the hem: sill, below-sill, or a ⅜″ float above the floor. The overlap-and-returns default adds the workroom's 12″ for a pair that closes fully and wraps the rod ends. Set it to zero for a decorative non-closing pair.

The formula

total width = rod width × fullness + overlap & returns
drops       = ⌈ total width ÷ fabric width ⌉
cut length  = finished length + hem/header allowance
              (rounded ↑ to whole vertical repeats if patterned)
yardage     = drops × cut length ÷ 36

These are the standard professional-workroom conventions (fullness ratios, the 16″ combined allowance (4″ double hems top and bottom), and repeat-rounding), as published in drapery workroom references such as the Rowley Company's workroom guides. The arithmetic itself is on display in the result note, one step at a time.

Worked example

Dress a 72″ rod with an 84″-long pair at 2× fullness, 54″ solid fabric (the defaults):

  1. Total width: 72 × 2 + 12 = 156″
  2. Drops: ⌈156 ÷ 54⌉ = 3 widths, 1½ widths per panel
  3. Cut length: 84 + 16 = 100″
  4. Yardage: 3 × 100 = 300″ = 8⅜ yards

Now give the same fabric a 24″ repeat: each cut rounds up to 120″ (5 repeats), and the job becomes 360″ = 10 yards. The print cost 1⅝ yards of matching allowance. That's the number to know before falling in love with the damask.

Assumptions & tips

  • Hang the rod before measuring. Every length decision follows from real hardware at its real height. Measure with a steel tape, in three places, because floors are not level.
  • Half-widths go to the outside edges. When a panel is 1½ widths, seam the half to the side that stacks against the wall. The seam vanishes into the folds.
  • Weight the hems. Drapery weights or a chain in the bottom hem is why custom drapes hang in columns and ready-mades flare. The double hem the allowance pays for is where the weight lives.
  • Railroading sheers saves seams. 118″ sheer fabric run sideways covers most windows seam-free: the calculator's 118″ width option models that, with the "drops" collapsing to one.
  • Order all drops from one bolt. Dye lots vary enough to see at a seam. One order, one lot, and a quarter yard spare for squaring the first cut.

Frequently asked questions

What fullness ratio should I use?

Fullness is finished curtain width divided by the space it covers, the source of the gathered look. Workroom convention: 2× is the standard for pleated and gathered drapery, 2.5× reads luxurious and suits sheers (which look skimpy without it), and 1.5× gives a relaxed, barely-gathered modern look. Below 1.5 a curtain hangs flat, like a bedsheet.

How high and wide should I mount the rod?

Higher and wider than instinct says: typically 4 to 6 inches above the frame (or two-thirds of the way to the ceiling for drama) and 6 to 12 inches beyond the frame each side, so open curtains stack against wall instead of blocking glass. Remember to measure the rod, not the window. This calculator wants the rod length, because that is the width the fabric covers.

What is a pattern repeat and why does it eat so much fabric?

The repeat is the vertical distance before a print's motif recurs. To pattern-match across drops and hang motifs at the same height on every panel, each drop must be cut at a whole number of repeats. A 100-inch cut length with a 24-inch repeat becomes 120 inches, and that 20 inches of "waste" per drop is the price of panels that match at the seams. Solids and tiny textures have no repeat. Large damasks can add yards.

Sources

  1. How to Make Draperies. Singer Sewing Machine Company (Singer Sewing Library); public-domain text, Project Gutenberg ebook 63282. gutenberg.orgThe workroom sequence this calculator walks: mount the rods before measuring, add the returns at each end to the width, allow fullness averaging about twice the space to be covered (2½ to 3 times for sheers), add heading and hem allowances to every cut length, and buy extra yardage so a patterned repeat matches across all the lengths.
  2. EC62-1160: Preparing to Make Draperies. Magdalene Pfister, University of Nebraska–Lincoln Extension, 1962. digitalcommons.unl.eduExtension guidance on the preparation this page assumes before any yardage is bought: deciding the window treatment, using good hardware, and taking careful measurements. Linked to the repository record; the scanned circular itself is behind a download link.
  3. Curtain Making Guide (project sheet PS RS0011). Moda Fabrics. my.modafabrics.comA fabric maker's current version of the same arithmetic: cut width equals the space covered times 1.5 to 2 for fullness, cut height equals finished height plus a fixed hemming allowance, and yardage follows from how many panels fit across the bolt width.

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