Half Square Triangle Calculator
Two squares sewn along the diagonal and cut apart give two HST units. That square has to carry the finished size plus the two side allowances (0.6 × 2 = 1.2 cm) and the diagonal seam (√2 × 0.6 ≈ 0.85 cm), so 12.05 cm. Set the allowance to an exact 1/4 in (0.64 cm) and the answer becomes finished + 7/8 in (2.2 cm) — the rule quilters memorise.
Your numbers
Cut squares at
12.05cm
Cut width
11.2cm
Formula
Cut squares at = Finished HST size + 2 × Seam allowance + √2 × Seam allowance
Cutting slightly generous and trimming beats cutting exactly. The diagonal is on the bias and grows as you handle it, so exact cutting tends to finish small. Trim the sewn unit to 11.2 cm — finished plus two allowances — and the block will fit.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Finished HST size (cm)The finished half-square triangle, seam allowance excluded. | 10 | 1 and up |
| Seam allowance (cm)Allowance on one edge; it is added on both, so twice over. | 0.6 | 0.2 ~ 1.5 |
Step by step
Quick reference table
Results when only Finished HST size (cm) changes and everything else stays put.
| Finished HST size (cm) | Cut squares at (cm) | Cut width (cm) |
|---|---|---|
| 5 | 7.05 | 6.2 |
| 7.5 | 9.55 | 8.7 |
| 10 | 12.05 | 11.2 |
| 15 | 17.05 | 16.2 |
| 20 | 22.05 | 21.2 |
What each result means
| Result | At default values |
|---|---|
| Cut squares at (cm)The size to cut the starting squares. | 12.05 |
| Cut width (cm)The width to cut — finished size plus allowance. | 11.2 |
Common mistakes
Cutting slightly generous and trimming beats cutting exactly. The diagonal is on the bias and grows as you handle it, so exact cutting tends to finish small. Trim the sewn unit to 11.2 cm — finished plus two allowances — and the block will fit.
Glossary
- Finished HST size
- The finished half-square triangle, seam allowance excluded.
- Seam allowance
- Allowance on one edge; it is added on both, so twice over.
- Cut squares at
- The size to cut the starting squares.
- Cut width
- The width to cut — finished size plus allowance.
Frequently asked questions
Q. How is Half Square Triangle Calculator calculated?
Cut squares at = Finished HST size + 2 × Seam allowance + √2 × Seam allowance — Two squares sewn along the diagonal and cut apart give two HST units. That square has to carry the finished size plus the two side allowances (0.6 × 2 = 1.2 cm) and the diagonal seam (√2 × 0.6 ≈ 0.85 cm), so 12.05 cm. Set the allowance to an exact 1/4 in (0.64 cm) and the answer becomes finished + 7/8 in (2.2 cm) — the rule quilters memorise.
Q. Can you walk through an example?
With Finished HST size 10cm, Seam allowance 0.6cm, the answer is Cut squares at 12.05cm.
Q. What do I need to enter?
Enter Finished HST size, Seam allowance. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Finished HST size (cm) moves the answer to Finished HST size (cm) 5 → Cut squares at (cm) 7.05 and Finished HST size (cm) 20 → Cut squares at (cm) 22.05. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
Cutting slightly generous and trimming beats cutting exactly. The diagonal is on the bias and grows as you handle it, so exact cutting tends to finish small. Trim the sewn unit to 11.2 cm — finished plus two allowances — and the block will fit.
Related calculators
Materials differ in density and absorption. If your label states a figure, use it — the defaults here are only common values.