Half-Square Triangle Calculator
Find the cut square size and number of squares to make any count of half-square triangles at a finished size.
How it works
2-at-a-time cut = finished + ⅞ in (2 per pair) · 4-at-a-time cut = finished + 1¼ in (4 per pair) · 8-at-a-time cut = (finished + ⅞) × 2 (8 per pair) · Pairs = ⌈needed ÷ yield⌉
A half-square triangle is a patchwork square split diagonally into two contrasting triangles. You never cut single triangles — instead you sew two squares together and cut them apart, which keeps the bias edges stable. Each method trades speed for the number of identical units it spits out: 2-at-a-time is the most controllable, while 8-at-a-time from one oversized square is fastest when you need a pile of matching HSTs. The extra seven-eighths of an inch is the standard allowance for two quarter-inch seams plus the diagonal.
Worked example
To make 24 finished 3 in HSTs 2-at-a-time, cut squares at 3 + 0.875 = 3.875 in. Each pair of squares yields 2 HSTs, so you need ⌈24 ÷ 2⌉ = 12 pairs: 12 squares of each fabric, 24 squares in all, producing exactly 24 HSTs.
Frequently asked questions
What does "finished size" mean for an HST?
The finished size is how big the triangle unit is in the completed quilt after its seams are sewn — the sewn-in edges are hidden. The unsewn block is half an inch larger on each side, which is why the cut square adds seam allowance to the finished number.
Which method should I choose?
Use 2-at-a-time for the most accurate, easily pressed units and when you need only a few. Choose 4- or 8-at-a-time when a design calls for dozens of identical HSTs and you want to save time — just expect to trim each unit to size afterward.
Should I trim HSTs after sewing?
Yes, especially with the 4- and 8-at-a-time methods, which distort slightly. Cut the squares a touch larger and trim each finished unit down with a square ruler for crisp, consistent points.
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This calculator is for educational purposes. Double-check important results before acting on them.