QuiltMath

How to Calculate Quilt Backing Yardage by Hand

Four steps: size the backing, count the fabric widths, multiply by the cut length, convert to yards. The step everyone skips — the seam-allowance correction — is the one that leaves you 2 inches short at the fabric store.

Step 1: Size the backing

A backing is never the same size as the quilt top. It must overhang on all four sides so there is something to load onto a longarm frame, pin for basting, and trim after quilting. Add the margin twice per dimension:

Use 4" per side if a longarmer will quilt it (most require it), 2–3" per side for home-machine or hand quilting. A 60" × 80" throw with a 4" margin needs a 68" × 88" backing.

Step 2: Count the fabric widths

Quilting cotton is 42–44" wide, so one width rarely covers a backing. Divide the dimension the seams will span by your fabric width — then correct for the seams themselves: every seam consumes twice your seam allowance (¼" from each piece = ½" per seam at the standard ¼" allowance).

The trap: an 88" backing length divided by 42" fabric looks like 2.1 widths, so "3 to be safe" feels wasteful and "2, trimmed creatively" feels tempting. Two widths actually give you 42 + 42 − ½ = 83½" — nearly 5 inches short of 88". You need 3 widths. This is exactly the mistake that sends people back to the store.

You can piece in two directions, and they are not equivalent:

Step 3: Multiply pieces by cut length

Same quilt, 28 inches apart. Always run both layouts before you buy — which one wins depends on your quilt's proportions, and it flips from quilt to quilt.

Step 4: Convert to yards, rounding up

Divide inches by 36, then round up to the nearest ⅛ yard (fabric is cut in ⅛-yard increments, and the store will not sell you 4.889 yards):

The wide-back shortcut

Wide backing fabric (usually 108") skips Steps 2–3 whenever one width covers either backing dimension: lay the 108" width across the 88" side of the example backing and cut just the other dimension — 68" = 1.89 yd → buy 2 yards, no seams at all. Only kings outgrow it: a 118" × 116" king backing needs two wide widths (2 × 116" = 6.44 yd → buy 6½). See wide-back vs pieced backing for when each wins on price.

Before you leave the store