Half-square triangle cut size: the ⅞″ rule
Cut the squares ⅞″ larger than the finished half-square triangle. For a 3″ HST cut 3⅞″ squares, for 4″ cut 4⅞″, for 6″ cut 6⅞″. Pair a light and a dark square, draw the diagonal, sew ¼″ either side of it and cut on the line: two HSTs, no trimming. Quarter-square triangles (hourglass units) add 1¼″ instead.
Get every triangle's cut size from your own layout →
Why ⅞″ and not ½″
A square patch needs ½″ added to its finished size: ¼″ of seam on each side. A triangle needs more, because its third side is a diagonal. Push all three sides of a right-angled triangle out by ¼″ and each short side grows by ¼″ × (2 + √2), about 0.854″. The nearest ruler line above that is ⅞″ (0.875″), so that is what gets added. The rounding is why many quilters cut a little larger again and trim.
Half-square triangle cut size chart
| Finished HST | Cut squares (two at a time) | Unfinished HST |
|---|---|---|
| 1½″ | 2⅜″ | 2″ |
| 2″ | 2⅞″ | 2½″ |
| 2½″ | 3⅜″ | 3″ |
| 3″ | 3⅞″ | 3½″ |
| 4″ | 4⅞″ | 4½″ |
| 5″ | 5⅞″ | 5½″ |
| 6″ | 6⅞″ | 6½″ |
The unfinished size, finished + ½″, is what the sewn unit should measure before it goes into a block.
How many squares to cut
One pair of squares makes two HSTs, so for each fabric pairing cut half as many squares as you need HSTs, rounded up. A 12″ Friendship Star has four 4″ HSTs, so two 4⅞″ squares of the star fabric and two of background. For a whole quilt, multiply by the number of blocks before you divide. Sashing's cutting chart does this across every block and prints the total, for example “subcut (40) 4⅞″ squares”. How to read a quilt cutting chart explains that notation.
Other ways to make HSTs
- Cut oversized and trim: squares finished + 1″, then trim each HST to finished + ½″. Slower, and more forgiving if your seam wanders. Several pattern designers work this way.
- Eight at a time: two squares of 2 × (finished + ⅞″), sewn ¼″ either side of both diagonals and cut into eight. For 3″ HSTs that is 7¾″ squares.
- Cut triangles first: squares finished + ⅞″ cut once on the diagonal, then sewn in pairs. Same cut size; you sew along bias edges, so handle them gently.
Quarter-square triangles: add 1¼″
A quarter-square triangle has its long side on the outside of the unit, which costs more seam allowance. The exact figure is about 1.207″, so the rule is finished + 1¼″, cut twice on the diagonal. A 12″ Ohio Star has 4″ star-point units, so its squares are cut 5¼″.
Flying geese, for comparison
The stitch-and-flip method needs no diagonal allowance at all. Cut a rectangle the goose's finished size + ½″ each way, plus two squares its finished height + ½″. A 3 × 6″ goose is one 3½″ × 6½″ rectangle and two 3½″ squares, sewn on the diagonal and trimmed. The four-at-a-time method uses different numbers again; flying geese cut size charts both methods side by side.
If a pattern's numbers disagree with these, check its method first. Different methods give different cut sizes, and both can be right. Every triangle in a whole quilt adds up, which is why a quilt of stars takes more fabric than one of squares.
Sashing
A quilt design app for Android. Lay out classic blocks with sashing and borders in photos of your own fabrics, then get the finished size, the yardage for every fabric and a cutting chart with the ¼″ seams added. Designing and seeing the quilt is free; the yardage, cutting chart and PDF are part of Sashing Pro, a subscription with a 3-day free trial.