Online Tool Store
Quilt Block Calculator
Calculate cutting sizes and fabric quantities for common quilt blocks, with seam allowances included so finished blocks land on size.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Fun & Misc
Quilt Block Calculator
Frontend preview — no upload or external service.
Cutting list
12-inch finished nine patch: cut 4.5-inch squares (4 inches finished plus half-inch seam allowance). For 20 blocks that is 180 squares, about 1.6 metres of 110 cm fabric.
How the Quilt Block Calculator works
- Enter the finished block size — the size after it is sewn in, not the size you cut.
- Choose the block type, since each has its own seam allowance arithmetic.
- Cut a test block first and measure it before cutting the whole quilt.
The method
Cutting size is the finished size plus a half inch, because a quarter-inch seam allowance is taken from each of the two opposite edges.
cut size = finished size + 0.5 inch (for squares); HST cut = finished + 0.875 inch
A 4-inch finished patch is cut at 4.5 inches. Half-square triangles need 7/8 inch extra because the seam runs on the bias diagonal.
FAQ
Why is my block coming out small?
Almost always the seam allowance. A quarter-inch seam that is a thread or two wide across dozens of seams accumulates into a visibly undersized block — which is why a test block matters.
Why do half-square triangles need 7/8 inch?
Because the seam crosses the diagonal, so the allowance is measured across a longer edge. The 7/8 figure is the standard rounding of that geometry.
Should I add extra for trimming?
Many quilters cut HSTs oversized and trim to exact size, which absorbs small seam inaccuracies. It costs fabric and saves considerable frustration.
How we compare
| Feature | Online Tool Store | Doing it by hand | A single-purpose app |
|---|---|---|---|
| Seam allowance included | ✓ | Manual | Yes |
| Yardage for the whole quilt | ✓ | Manual | ✓ |
| Block-specific arithmetic | ✓ | ✗ | ✓ |
| No account | ✓ | ✓ | ✗ |
Quilt Block Calculator builds the seam allowance into every figure — and still recommends a test block, since a scant quarter inch is what makes quilts shrink.