Half square triangle (HST) calculator
Enter the finished size of each half square triangle and how many you need. Each row shows the square size to cut and how many squares to cut.
| Method | Cut squares at | Squares to cut | You get |
|---|
The formulas
- 2 at a time: cut size = finished + ⅞".
- 4 at a time: cut size = (finished + ½") × 1.414 + ½", rounded up to the next ¼" to leave a little to trim. For 3" finished that is 5½".
- 8 at a time: cut size = 2 × finished + 1¾".
Each method starts with a pair of squares: one light, one dark. In centimetres the calculator converts the same formulas exactly. The 4-at-a-time method leaves bias edges on the outside, so cut a little larger and trim.
Frequently asked questions
Why add ⅞ inch for half square triangles?
With the 2-at-a-time method, the diagonal seam uses up extra fabric. Adding ⅞ inch to the finished size gives an exact cut. Many quilters add 1 inch instead and trim the HST down to size for more accuracy.
Which HST method is best?
2 at a time is the easiest and keeps the outer edges on the straight grain. 4 and 8 at a time are faster for large numbers, but leave bias edges on the outside, so handle them gently or starch first.
Should I trim my HSTs?
Trimming makes every unit exactly the same size, which helps points match. If you plan to trim, cut the squares about ⅛ to ½ inch larger than shown.
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