Half-Square Triangle Calculator

Half-square triangles are quilting's hardest-working unit — and the unit whose cut sizes nobody can remember, because each construction method starts from a different square. Enter the finished size you need and get the cut size for all three standard methods at once.

Half-Square Triangle Calculator

Work in finished sizes — what the HST measures sewn into the quilt. The calculator also shows the unfinished (trim-to) size, because every method here is designed to be trimmed. These units end up in blocks, so the natural companion is the quilt block calculator for counting how many the quilt needs.

1 Enter your measurements

The size the unit measures in the finished quilt. A 2 1/2 in finished HST is cut and sewn as a 3 in unfinished unit.

How it's calculated

The 7/8 in in the classic formula is geometry, not superstition: a diagonal seam consumes the seam allowances of a square cut corner to corner, and 7/8 in is precisely what a finished unit needs added to its starting squares. The Magic 8 method simply scales it — two HSTs along each edge, so 2 × (finished + 7/8). The 4-at-a-time method has no seam on the diagonal at all; its squares are sewn around the edges and cut on both diagonals, so the starting square is the diagonal of the unfinished unit plus the 1/2 in that the perimeter seam consumes as the squares are sewn and cut apart: (finished + 1/2) × √2 + 1/2, rounded up to the nearest 1/8 in so there is always material to trim back to a perfect unit. Leave the final 1/2 in out and the units come out about 3/8 in too small — too small to trim to size at all.

The three methods, compared

Two at a time is the classic: two squares, two units, minimal waste, and the most accurate of the three because the seams are sewn before cutting and every unit gets trimmed. Its cost is repetition — a quilt needing 200 HSTs needs 100 rounds of it. It is the right default for small batches and precious fabric.

Four at a time sews two squares together around all four edges, then cuts both diagonals for four units with no diagonal seam at all — fast, but every outer edge of the unit is on the bias, so units are stretchier and benefit from starch and gentle handling. Magic 8 applies the classic formula at double scale: eight units from two big squares, all with stable-grain edges, and the quilter's favorite for production runs of one size and color pair.

Why every method ends at the trimming board

Sewing is not a precision instrument at the 1/64 in level, and HSTs multiply errors: one unit 1/16 in small makes its block 1/16 in small, and a row of them drifts visibly. Every method on this page therefore builds in excess — the 7/8 in formula and the rounded-up 4-at-a-time figure both leave you something to trim.

Trim with the seam running corner to corner under the ruler, the ruler's 45-degree line on the seam, and the unfinished size's marks on two edges. Two cuts, rotate, two cuts. It feels slow for the first dozen units and becomes the fastest, most meditative part of the process by the second dozen.

How to use this calculator

  1. Unfinished (trim-to) size = finished + 1/2 in.
  2. 2 at a time: cut squares at finished + 7/8 in; sew 1/4 in either side of a diagonal, cut apart, press.
  3. 4 at a time: cut squares at (finished + 1/2) × √2 + 1/2 in, rounded up to the nearest 1/8 in; sew all four edges, cut both diagonals.
  4. Magic 8 (8 at a time): cut squares at 2 × (finished + 7/8) in; sew either side of both diagonals, cut into eight.

Worked example

For HSTs that finish at 3 in (3 1/2 in unfinished):

2 at a time: 3 + 7/8 = 3 7/8 in squares. Magic 8: (3 + 7/8) × 2 = 7 3/4 in squares — the same 7/8 in allowance, doubled, exactly as the published Magic 8 formula states. 4 at a time: (3 + 1/2) × √2 + 1/2 = 5.45 → cut 5 1/2 in squares and trim the units to 3 1/2 in after pressing. The final + 1/2 in is the allowance the perimeter seam consumes; without it the units come out too small to trim.

HST cut-size chart

Finished HSTUnfinished (trim to)2 at a time — cut4 at a time — cutMagic 8 — cut
1 1/2 in2 in2 3/8 in3 3/8 in4 3/4 in
2 in2 1/2 in2 7/8 in4 1/8 in5 3/4 in
2 1/2 in3 in3 3/8 in4 3/4 in6 3/4 in
3 in3 1/2 in3 7/8 in5 1/2 in7 3/4 in
3 1/2 in4 in4 3/8 in6 1/4 in8 3/4 in
4 in4 1/2 in4 7/8 in6 7/8 in9 3/4 in
4 1/2 in5 in5 3/8 in7 5/8 in10 3/4 in
5 in5 1/2 in5 7/8 in8 3/8 in11 3/4 in
5 1/2 in6 in6 3/8 in9 in12 3/4 in
6 in6 1/2 in6 7/8 in9 3/4 in13 3/4 in

Cut sizes are for two starting squares (one of each fabric) per method. The 4-at-a-time figures are the diagonal of the unfinished unit plus the 1/2 in the perimeter seam consumes, rounded up to the nearest 1/8 in; trim every unit to the unfinished size after pressing.

Related calculators

For the quilt these triangles are going into, our quilt backing calculator handles the back.

Frequently asked questions

Why 7/8 of an inch? Why not a tidy 1 inch?

Because the diagonal seam's geometry says so: when two squares are sewn on either side of a diagonal and cut apart, each resulting unit's seam allowances total exactly 7/8 in beyond the finished size. Cut at finished + 1 in instead (some quilters do, deliberately) and you simply have more to trim — a valid, fabric-hungrier variation called oversizing. This calculator uses the classic exact formulas.

Which HST method is most accurate?

Two at a time, trimmed — it has the least bias edge and the most trimming room. Magic 8 is a close second and far faster in bulk. Four at a time is the speed champion and the least forgiving: all-bias edges demand starch and a light touch. For a heirloom quilt, sew two at a time; for a baby quilt due Saturday, nobody will know you used Magic 8.

My pattern says '3 inch HSTs'. Is that finished or unfinished?

Patterns are inconsistent, which is genuinely unhelpful of them. If the pattern's block math adds up with 3 in units (twelve 3 in units across = a 36 in block), it means finished — enter 3. If the instructions say to trim units to 3 in before sewing them together, it means unfinished — enter 2 1/2. When in doubt, the trim instruction is the tell.

Can I make HSTs from charm squares or layer cakes?

Yes, with the sizes pre-cuts give you. A 5 in charm square pair, sewn 2 at a time, yields units that trim to 4 1/2 in unfinished (4 in finished). A 10 in layer cake pair yields units that trim to 9 1/2 in unfinished (9 in finished). Run the calculator backwards: pick the finished size, check the cut square it asks for against your pre-cut, and let the trim absorb the difference.

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