Half square triangle calculator

Cut your squares ⅞″ bigger than the finished half square triangle: for a 3″ unit that is 3⅞″, two at a time. The calculator below gives all three methods at any size.

Any finished size

Finished size of one half square triangle, in inches

Squares to cut for 3″ finished half square triangles.

Two at a timecut 2 squares, yields 2 units: 3⅞″
Four at a timecut 2 squares, yields 4 units: 5½″
Eight at a timecut 2 squares, yields 8 units: 7¾″

Scroll sideways to inspect every label.

Starting-square comparison for 3″ finished half square triangles: two, four and eight at a time, with cut lines and yields.

2 at a time: cut 2 squares at 3⅞″ to yield 2 units. 4 at a time: cut 2 squares at 5½″ to yield 4 units. 8 at a time: cut 2 squares at 7¾″ to yield 8 units.

The three methods, side by side

Scroll sideways to see every size.

Squares to cut for half square triangles, by method and finished unit size
Method	2″	2½″	3″	4″	6″
Two at a timecut 2 squares, yields 2 units	2⅞″	3⅜″	3⅞″	4⅞″	6⅞″
Four at a timecut 2 squares, yields 4 units	4⅛″	4¾″	5½″	6⅞″	9¾″
Eight at a timecut 2 squares, yields 8 units	5¾″	6¾″	7¾″	9¾″	13¾″

Column headings are the FINISHED size of one half square triangle — what it measures once it is sewn in. Every number in the table is the size of the SQUARE to cut. All three methods start from 2 squares; what changes is how many units come off them.

Of these three methods, four-at-a-time is the only one whose square never lands exactly on a ruler mark, so it is always rounded up to the nearest ⅛″. The two- and eight-at-a-time squares are exact.

The arithmetic, worked through

Every number above comes from the same rule, worked here for a 3″ unit so you can check the method against your own size — as computed by Quilt Block Atlas, never copied from a published chart.

Two at a time: Finished size 3″ + ⅞″ = 3⅞″, cut once on the diagonal. 2 squares like this yield 2 units.
Four at a time: Finished size 3″ plus ½″, run through the diagonal, plus ½″ again: 5½″. 2 squares like this yield 4 units, the one size on this page that never lands exactly on a ruler mark, so it is always rounded up.
Eight at a time: Twice the two-at-a-time square: 2 × 3⅞″ = 7¾″. 2 squares like this yield 8 units.

Cutting for a batch

Scroll sideways to see every count.

Squares to cut for a batch of 3″ half square triangles, by method
Method	8 units	16 units	24 units	32 units	48 units
Two at a timesquares at 3⅞″	8	16	24	32	48
Four at a timesquares at 5½″	4	8	12	16	24
Eight at a timesquares at 7¾″	2	4	6	8	12

Starting squares always come in pairs, one of each fabric. Each method's square count is therefore TWO per set: the batch size divided by how many units one pair yields, rounded up to a whole pair, then doubled. A batch that is not a multiple of the yield leaves spare units, not spare cutting. For a different finished size, read the sizes from the calculator above; the counts stay the same.

Blocks in this library that use half square triangles

11 of the blocks here are built from them, each with its own chart.

Common questions

What size do I cut for a half square triangle at 3 inches finished?

Two at a time: cut two squares at 3⅞″ — the finished size plus ⅞″ — sew, and subcut on the diagonal for two units. The table above has the four- and eight-at-a-time squares for the same size.

Does this include seam allowance?

Yes. The ⅞″ the two-at-a-time rule adds covers the seam allowance on every edge plus the diagonal seam, and the four- and eight-at-a-time formulas carry their own equivalents. Never add anything to a number this page gives you.

Which method do these numbers belong to — 2, 4 or 8 at a time?

Each result belongs to its named Two at a time, Four at a time, or Eight at a time column and must not be interchanged. The half square triangle quilt blocks page explains the yield and bias-edge trade-offs.

Why does the four-at-a-time size never come out even?

Because its formula runs through the diagonal of a square, and a diagonal is never a neat fraction of the side. The calculator rounds it up to the nearest ⅛″ for you — up, never down, so the unit finishes big enough to trim true rather than too small to fix.

Published 2026-07-28 · Last updated 2026-08-15