Half Square Triangle Calculator

Enter the finished HST size you want and get the exact square to cut — no more guessing at the fabric store.

Method Finished HST size (inches) Number of HSTs you need (optional)

What this calculator does

A half square triangle (HST) is one of quilting's most-used units: a square made of two right triangles. This tool tells you exactly what size to cut your starting squares so the finished HST (the size it measures once it's sewn into your quilt, seams hidden) comes out right. Pick a method, type your finished size, and you get the cut square size and how many triangles each square produces.

The two methods

2-at-a-time is the classic single-unit method: cut a square that is your finished size plus 7/8", pair two fabrics right sides together, sew 1/4" on each side of one diagonal, and cut apart. One pair of squares gives you 2 matching HSTs. 8-at-a-time is faster for repeats: cut a square equal to finished size × 2 plus 1 3/4", sew around both diagonals, and cut into eight identical HSTs. Both add a 1/4" seam allowance for you, and both leave a little wiggle room so you can square each unit up cleanly.

How to use it

Choose the 2-at-a-time or 8-at-a-time method from the dropdown.
Type the finished HST size you want (the size in the finished quilt, not the cut size).
Optionally enter how many HSTs your pattern needs.
Read the cut square size, the yield per square, and how many squares to cut of each fabric.

FAQ

Finished size vs. unfinished size?: Finished is what it measures sewn into the quilt with seams hidden. Unfinished (trim-to) size is finished + 1/2". Enter the finished size here.
Why the extra fabric?: The 7/8" and 1 3/4" allowances cover two 1/4" seams plus a small margin so you can trim each HST square and true.
Should I trim my HSTs?: Yes. These methods run slightly oversized on purpose. Trim each unit to finished + 1/2" for crisp points and accurate blocks.
Is my data uploaded?: No. Everything runs in your browser — nothing is sent anywhere. Cutting sizes are estimates; always test one unit before cutting a full batch.