What is the difference between finished and unfinished block size?

Finished size is the block after it is sewn into the quilt, with seam allowances hidden. Unfinished size is the block before it is joined, so it includes a seam allowance on all four sides. With a quarter-inch seam, an unfinished block is half an inch larger than its finished size.

Why add the seam allowance twice?

Each block has a seam on the left and the right, and on the top and the bottom. So you add the seam allowance twice per dimension: a 12-inch finished block with a quarter-inch seam is cut at 12.5 inches square unfinished.

How is the quilt top size found?

Finished quilt width is the finished block size times the number of columns; height is the block size times the number of rows. Sashing and borders, if any, add to this and should be calculated separately.

How accurate is the fabric estimate?

It is a planning estimate based on the unfinished block area plus a cutting-waste allowance, converted to yards of 42-inch-wide fabric. Pieced blocks, directional prints, and fussy cutting can increase real usage, so always buy a little extra.

Does this account for sashing and borders?

No. This tool sizes the block grid only. Add sashing strips between blocks and border widths to the reported quilt-top dimensions to get the full size.

Quilt Block Size & Fabric Calculator

Every quilt starts with knowing two block sizes — the finished size you want and the larger unfinished size you actually cut and sew. This calculator converts between them, sizes the whole quilt top from your layout, and gives a rough yardage estimate so you can shop with confidence.

How it works

The seam allowance sits on all four edges of each block, so it is added twice per dimension:

unfinished block = finished block + 2 × seam allowance
total blocks     = rows × columns
quilt top width  = finished block × columns
quilt top height = finished block × rows

Fabric is estimated from the unfinished block area plus a cutting-waste factor, then converted to yards of 42-inch-wide fabric (area in² / (42 × 36)).

Example and tips

A throw of 12-inch finished blocks, 4 across and 5 down, uses 20 blocks cut at 12.5 inches square (with a quarter-inch seam). The finished top measures 12 × 4 = 48 inches wide by 12 × 5 = 60 inches tall, before sashing or borders. Always pre-wash and buy 10–15% extra fabric: directional prints, squaring up, and the occasional mis-cut eat into yardage faster than the bare area suggests.