Quilt Block Calculator
Tell the calculator the finished block size from your pattern and how many blocks you need. It returns the exact cut size with seam allowance and a fabric estimate.
Tip: use URL parameters to pre-fill, e.g. ?blockSize=12&blockCount=30.
Finished size vs. cut size
A finished block is the size you see after the quilt is assembled. A cut block is the fabric square before sewing. Because every side of the block loses a 1/4-inch seam allowance, the cut size is always the finished size plus 1/2 inch: a 12-inch finished block is cut at 12.5 inches.
Cutting exactly to the finished size is the most common mistake in quilt math. It makes every block 1/2 inch too small, and the error multiplies across rows.
Fabric estimate
The estimate works out how many cut squares fit across a 42-inch usable fabric width, counts the rows needed, and converts the total to yards with a small safety margin. For a 12.5-inch cut block, three fit across 42 inches, so 20 blocks take seven rows of about 12.5 inches — roughly 2.4 yards of fabric.
If your blocks are pieced from multiple fabrics, treat the estimate as the total and split it by color. For full-quilt yardage including sashing, borders and backing, see the quilt fabric calculator and backing calculator.
Common questions
Do I add seam allowance to every piece inside a block?
Yes — every piece that gets sewn needs its own allowance. This calculator gives the outer block size; for internal pieces, add 1/4 inch per seam to each piece.
What is a scant quarter inch?
A seam that is just shy of 1/4 inch. It prevents blocks from growing when pressing, and it is the default recommended by most modern patterns.
Can I cut blocks from fat quarters?
An 18x22 fat quarter fits one 12.5-inch block per fat quarter with room to spare. Smaller blocks like 8.5 inches fit four per fat quarter.