Quilt Math Guide
Most quilt math comes down to a few repeated formulas. This guide collects them in one place with a quick-reference table, so you can check your numbers before you cut.
The two numbers behind everything
Seam allowance: quilting uses a 1/4-inch seam, so a finished block is always 1/2 inch smaller than its cut size. A 12.5-inch cut block finishes at 12 inches.
Usable fabric width: a yard of 44-inch-wide fabric usually has about 42 inches of usable width after removing selvedges. Every yardage estimate starts with this number.
Quick-reference formulas
| What you need | Formula | Quick example |
|---|---|---|
| Quilt top yardage | Total piece area ÷ usable width = strips; strips × cut height ÷ 36 = yards | Five 12" blocks across, six down → 60" × 72" top, roughly 3.5-4 yd of assorted fabric |
| Binding | Perimeter + 12" = binding length; length ÷ fabric width = strips; strips × cut width ÷ 36 = yards | 60" × 80" quilt → 292" of binding → about 0.5 yd of 2.25" straight strips |
| Backing | Quilt size + 8" each way, then divide into panels of fabric width | 60" × 80" quilt → cut 68" × 88", about 5 yd of 44" fabric |
| Batting | Quilt size + 4-6" each side | 60" × 80" quilt → batting 68" × 88" to 72" × 92" |
| Block cut size | Finished size + 1/2" (1/4" seam each side) | Finished 12" block → cut 12.5" |
| Half-square triangle | Finished HST + 7/8", or + 1" and trim | Finished 4" HST → cut squares 4 7/8" |
| Precut math | Charm 5", layer cake 10", fat quarter 18" × 22" | A yard of 44" fabric yields about 56 five-inch charm squares |
| Yards ↔ meters | Yards × 0.9144 = meters; meters ÷ 0.9144 = yards | 5 yd ≈ 4.57 m |
Where the formulas come from
Yardage is just area divided by width, converted into yards. The allowances above (12 inches for binding joins, 8 inches for backing overhang, 1/2 inch per block) are the standard conventions used in quilting reference books and university extension guides. They exist because straight math leaves no room for seams, miters and trimming.
For exact numbers on any single step, the calculators on this site follow the same formulas: quilt fabric, backing, binding, batting, sizes and half-square triangles.
If you only remember two rules, make them these: add 1/2 inch to any finished size to get the cut size, and plan fabric around a 42-inch usable width rather than the bolt's 44 or 45 inches. Nearly every estimate on this page is those two rules applied to a different shape.
Common mistakes to avoid
Forgetting the seam allowance: cutting 12-inch blocks to finish 12 inches makes the whole quilt 1/2 inch smaller per block.
Using the full fabric width: selvedges are unusable, so calculate with 42 inches, not 44 or 45.
Skimping on binding: the perimeter alone is never enough; the final join and corner miters eat the extra 12 inches.
Buying exactly the estimate: prewashing, print matching and slightly different seams all consume fabric. Buy 10-15% extra on your first quilt.
Common questions
What is the formula for quilt fabric yardage?
Work in inches: find the total area of the pieces you need, divide by your usable fabric width to get the number of strips, multiply by the cut height per strip, then divide by 36 to get yards. Most quilting uses a 42-inch usable width.
How many 5-inch charm squares are in a fat quarter?
A fat quarter (18" × 22") holds 12 charm squares (3 across, 4 down) with a little leftover. A yard of 44" fabric holds about 56.
Do I add seam allowance to backing and batting?
Backing and batting don't have piecing seams, but they need overhang for the quilting frame: 8 inches total for backing, 4-6 inches each side for batting.
Is a queen quilt 90 by 102 inches exact?
No — standard sizes are ranges. Queen quilts commonly finish between 84-92 inches wide and 90-106 inches long. Use your pattern's measurements.
Why do calculators disagree with each other?
Different assumptions about usable width, seam allowance and waste produce different answers. Pick one consistent set of assumptions and stick with it.