🔲 Block Size Calculator

Enter a block size and your seam allowance to convert between the finished (sewn) size and the unfinished (cut) size — the standard quilting seam allowance is 1/4 inch on each side.

Twice the seam allowance, added or taken away

A block loses seam allowance on both opposite edges, so the conversion doubles whatever is in the seam box:

finished   = unfinished - (2 × seam allowance)
unfinished = finished   + (2 × seam allowance)

default seam allowance: 0.25 in per side, so 0.5 in in total

The page opens on 6.5 in unfinished and returns 6.000 finished; switch direction and it goes back. At the standard quarter inch that is the whole rule: half an inch between the size you cut and the size that shows.

Running it on the pieces, not just the block

The same conversion applies to every unit inside a block. Divide the finished block by its grid, then convert:

BlockGridUnit finishedUnit cut
6 in four-patch2 × 23 in3.5 in
6 in nine-patch3 × 32 in2.5 in
9 in nine-patch3 × 33 in3.5 in
8 in sixteen-patch4 × 42 in2.5 in

Half-square triangles break the pattern: two come from one square cut on the diagonal, and need seven eighths of an inch, not a half. Put 0.4375 in the seam box and the tool gives it - a 3 in HST wants 3.875 in squares.

The arithmetic is exact; your seam is the variable. Many quilters sew a scant quarter inch because the fold of a pressed seam eats a little width, and a true quarter leaves blocks small. Test-sew two strips, measure, and put your real seam allowance in the box before cutting.

Frequently asked questions

What is the difference between finished and unfinished block size?

Finished is what the block measures in the quilt once sewn to its neighbours. Unfinished is what you cut and what it measures before joining - larger by two seam allowances.

How much bigger should I cut a 6 inch quilt block?

Half an inch at the usual quarter-inch seams, so cut it 6.5 in. That half inch goes into the seams around it, a quarter on each edge.

What is a scant quarter inch seam?

A seam sewn a thread or two under a quarter inch, leaving room for the thickness the fabric takes up when pressed to one side. It is why a block sewn at an exact quarter can measure under.