Quick answer: how to calculate HSTs
A half-square triangle, or HST, is a square made from two right triangles, sewn together on the diagonal, with two different fabrics. For the two-at-a-time method, cut square size equals finished HST size plus 7/8 inch, which is 0.875 inches. Squares per fabric equals HSTs needed divided by 2, rounded up to a whole number.
For example, a 6 inch finished HST needs 6.88 inch cut squares, which is 6 and 7/8 inches. If you need 14 HSTs, you need 7 squares per fabric, 14 squares total, and you make 14 HSTs exactly.
Use the HST Calculator with your own finished size and HST count, to check cut size and squares needed before you cut.
How to calculate HSTs
What you need to measure
You need two inputs only: finished HST size, and HSTs needed. Finished HST size is the size of one HST in the finished quilt, after seams are sewn, not the cut size. HSTs needed is a whole number, greater than zero. Count HSTs by fabric pair, not by quilt, if different fabric pairs use different finished sizes.
Formula and calculation rule
Cut square size equals finished HST size plus 7/8 inch, which is 0.875 inches, for the two-at-a-time method. The extra 7/8 inch covers seam allowances on the diagonal seam and trimming to finished size. Squares per fabric equals HSTs needed divided by 2, rounded up to a whole number, because each pair of squares makes 2 HSTs, and a partial pair cannot be sewn. Total squares equals squares per fabric times 2, because each pair uses one square of each fabric. HSTs made equals squares per fabric times 2, because each pair makes 2 HSTs.
Step by step example
Take finished HST size 6 inches, HSTs needed 14.
Cut square size: 6 plus 0.875 equals 6.88 inches, which is 6 and 7/8 inches.
Squares per fabric: 14 divided by 2 equals 7 squares per fabric.
Total squares: 7 times 2 equals 14 squares.
HSTs made: 7 times 2 equals 14 HSTs, exactly the number needed, with no extra HST.
HST cut size chart
| Finished HST size | Cut square size |
|---|---|
| 2 inches | 2.88 inches (2 and 7/8 inches) |
| 2.5 inches | 3.38 inches (3 and 3/8 inches) |
| 3 inches | 3.88 inches (3 and 7/8 inches) |
| 3.5 inches | 4.38 inches (4 and 3/8 inches) |
| 4 inches | 4.88 inches (4 and 7/8 inches) |
| 5 inches | 5.88 inches (5 and 7/8 inches) |
| 6 inches | 6.88 inches (6 and 7/8 inches) |
| 8 inches | 8.88 inches (8 and 7/8 inches) |
Use this chart for common finished sizes. For other finished sizes, use the calculator above, with the same formula: finished size plus 7/8 inch.
Important variations and exceptions
- Odd HST count. If HSTs needed is an odd number, squares per fabric is rounded up, and HSTs made will be one more than HSTs needed, which is one extra HST, not a mistake. For example, 21 HSTs need 11 squares per fabric, and make 22 HSTs, with one extra HST.
- Other HST methods. This guide and calculator use the two-at-a-time method only. Other methods, such as four-at-a-time or eight-at-a-time, use different cut sizes and different square counts, and they are not calculated here. Do not use the two-at-a-time cut size for another method, or your HSTs will be the wrong size.
- Directional prints. If your fabric has a directional print and triangles must all run one way, check triangle orientation before cutting, because diagonal cutting turns print direction. This calculator counts squares, it does not check print direction.
- Trimming. HSTs made by the two-at-a-time method are trimmed to finished size after sewing, before they are sewn into a block. Trimming is part of the method, not a correction for a wrong cut size, and it is already covered by the 7/8 inch in the formula.
Beginner safety net: test pair, oversize, and trimming
Before cutting a full batch, check the 0.25 inch seam allowance on scrap fabric, then sew one test pair, press, and trim to finished size plus 0.5 inches, which is the unfinished size. If the test pair measures correctly after trimming, the batch will too. If it does not, fix the seam allowance first, never change the cut size to compensate, or every later unit will drift the same way.
Oversize shortcut, stated plainly: some quilters cut finished size plus 1 inch instead of plus 7/8 inch, then trim back to finished plus 0.5 inches. That gives extra trimming room, at the cost of extra fabric and extra trimming time. This calculator gives the exact plus 7/8 inch cut, not the oversize cut. If you choose oversize cutting, add the extra yourself, and still trim to finished plus 0.5 inches. Do not mix the two cuts in one batch.
Trimming order matters: sew both sides of the diagonal, cut on the marked diagonal, press, then trim with the diagonal line on the ruler aligned with the seam. Trimming before pressing, or pressing without aligning the diagonal, can stretch the bias edge and distort the unit.
How to make sure your result is correct
- Check finished size, not cut size. Measure finished HST size in the finished quilt, after seams are sewn, not cut size before seams are sewn. Using cut size in the formula gives a cut size too large by 7/8 inch.
- Check units. Finished size and cut size must be in the same unit, inches or centimetres. In centimetre mode, 7/8 inch equals 2.22 centimetres, rounded at two decimal places, because 0.875 times 2.54 equals 2.2225 centimetres.
- Check HST count by fabric pair. Count HSTs by fabric pair, not by quilt, if different fabric pairs use different finished sizes. Adding HST counts for different fabric pairs into one calculation gives the wrong square count for each fabric pair.
- Check HSTs made against HSTs needed. HSTs made equals squares per fabric times 2. If HSTs made is less than HSTs needed, you have too few squares. If HSTs made is one more than HSTs needed, and HSTs needed is odd, that is expected, not a mistake.
Try it with the calculator: Use the HST Calculator with your own finished size and HST count. It uses the same steps as this guide, and it shows cut square size, squares per fabric, total squares and HSTs made, together.
Worked example: 6 inch finished HST, 14 HSTs needed
This worked example is different from the tool page example, which uses 4 inch finished HST and 20 HSTs needed.
Cut square size: 6 plus 0.875 equals 6.88 inches, which is 6 and 7/8 inches.
Squares per fabric: 14 divided by 2 equals 7 squares per fabric.
Total squares: 7 times 2 equals 14 squares.
HSTs made: 7 times 2 equals 14 HSTs, exactly the number needed, with no extra HST.
Calculate your own HSTs now
Use your finished HST size and HSTs needed in the calculator, and check the result against the steps in this guide.
Open the HST CalculatorFrequently asked questions
For the two-at-a-time method, cut square size equals finished HST size plus 7/8 inch, which is 0.875 inches. For example, a 6 inch finished HST needs 6.88 inch cut squares, which is 6 and 7/8 inches.
A half-square triangle, or HST, is a square made from two right triangles, sewn together on the diagonal, with two different fabrics. Its finished size is the size in the finished quilt, after seams are sewn.
For the two-at-a-time method, each pair of squares, one of each fabric, makes 2 HSTs. Squares per fabric equals HSTs needed divided by 2, rounded up to a whole number. For example, 14 HSTs need 7 squares per fabric, 14 squares total, and make 14 HSTs exactly.
If HSTs needed is an odd number, squares per fabric is rounded up, and HSTs made will be one more than HSTs needed. For example, 21 HSTs need 11 squares per fabric, and make 22 HSTs, with one extra HST. That is expected, not a mistake.
No. Other methods, such as four-at-a-time or eight-at-a-time, use different cut sizes and different square counts. Do not use the two-at-a-time cut size for another method, or your HSTs will be the wrong size.
Related guides and calculators
Next, read How to Calculate Flying Geese and How to Calculate Quarter-Square Triangles for the other triangle units. For HSTs themselves, use the HST Calculator.