Half Square Triangle Calculator
Add ⅞ inch to the finished size and cut two squares — that is the whole 2-at-a-time rule. This half square triangle calculator works out the starting square size for the 2-at-a-time, 4-at-a-time and 8-at-a-time methods, with the seam allowance built in, and tells you how many squares of each fabric to cut for the number of HSTs you need.
Size in the finished quilt — fractions welcome: 2 ½
Cut
8-at-a-time: cut 8 squares (4 pairs, one of each fabric) at 6 ¾″. You will trim out 32 HST units that finish at 2 ½″.
▸How this was worked out
method: 8-at-a-time — cut square = 2 × finished + 1¾″
cut size = 2 × 2.5 + 1.75 = 6.75″
pairs = 32 ÷ 8 per pair = 4 → 4 pairs
squares = 4 × 2 = 8
Round the cut size UP to the ⅛″ and trim to 3″ unfinished after sewing — oversized is fixable, undersized is not.
Shopping list
- Fabric A (light)or one layer cake square per pair4 × 6 ¾″ squares
- Fabric B (dark)4 × 6 ¾″ squares
CraftCalc · Half-square triangle cutting
8-at-a-time · finished 2 ½″ HSTs · need 32
Cut 8 squares at 6 ¾″ (4 of each fabric) → 32 HST units, trim to 3″ unfinished
craftcalc.io — free maker calculators with the arithmetic shown
What size squares make a finished HST?
A half-square triangle unit is two triangles sewn on the diagonal, and the diagonal seam is what the math pays for. In the classic 2-at-a-time method you sew ¼ inch either side of one drawn diagonal, cut apart, press — and the starting square is finished size + ⅞ inch. That ⅞ is the ¼-inch seam on each side plus the stretch lost on the bias diagonal, rolled into one number quilters have used for decades.
Batch methods trade fabric for speed. 8-at-a-time (the “magic 8” method: two large squares, sew both diagonals on both sides, then cut horizontally, vertically and diagonally) follows the published chart cut size = 2 × finished + 1¾ inches — two 6¾-inch squares yield eight 2½-inch HSTs. 4-at-a-time squares follow the geometry of the diagonal cuts: √2 × (finished + 1), rounded up to the nearest ⅛ inch. Whichever method you pick, cut generously and trim to the exact unfinished size with a square ruler — every batch method assumes you will square up.
Worked example: 32 HSTs at 2½ inches finished
A throw pattern calls for 32 half-square triangles finishing at 2½ inches. Two-at-a-time means 16 pairs of squares cut at 2½ + ⅞ = 3⅜ inches — 32 squares total, and 32 diagonal seams to press. The magic-8 method instead cuts 2 × 2½ + 1¾ = 6¾-inch squares: 4 pairs, 8 squares, and the same 32 units from four rounds of sewing and cutting. For a whole quilt of identical HSTs the 8-at-a-time method is dramatically faster; for a handful of scattered units, 2-at-a-time wastes less fabric.
Either way, press the first units dark-side over light, then square each one to 3 inches unfinished (finished + ½ inch) with the diagonal seam running corner to corner. Chain-pressing in batches keeps them consistent — and consistent unfinished size is what makes the quilt top assemble flat.
Frequently asked questions
Why add ⅞ inch for the 2-at-a-time method?
A straight-seamed square loses ½ inch total to two ¼-inch seams; a diagonal (bias) seam loses slightly more to stretch, and the classic rule rounds that up to ⅞ inch total. Cut at finished + ⅞″ and the unit trims to exactly finished + ½″ unfinished.
What is the 8-at-a-time (magic 8) method?
Layer two squares right-sides-together, draw both diagonals, sew ¼ inch on both sides of both diagonals, then cut along the drawn diagonals plus horizontally and vertically through the centre. Eight HST units fall out of two squares — ideal when a quilt needs dozens of identical units.
Should I cut bigger and trim down?
Yes — that is the assumption behind every formula here. Oversized units can be squared to an exact unfinished size with a square ruler; undersized ones cannot be rescued. If your seam allowance runs scant or generous, add another ⅛ inch.
What is the difference between finished and unfinished size?
The unfinished size is the unit as sewn, seam allowances included — finished + ½ inch. A 2½-inch finished HST is cut and sewn as a 3-inch square before it goes into the quilt top.
How do I make HSTs from layer cake squares?
Use the 8-at-a-time method: two 10-inch layer cakes yield eight HSTs at (10 − 1) ÷ 2 = 4½ inches finished. Trim to 4 inches unfinished for a clean 3½-inch finished unit, or leave at 4½ finished for a bold block.
Formulas and standard tables are sourced in the methodology. Results are estimates — measure twice, cut once.