Half-Square Triangle Calculator
Enter the finished HST size and how many you need, then pick a method. Sashing tells you the cut size for your starting squares and how many squares to prep.
Cut squares
3 7/8"
What size do I cut for half-square triangles?
Every method starts with squares that are bigger than the finished triangle so the seam allowance has somewhere to go. The two-at-a-time method adds 7/8 inch to the finished size, draws one diagonal, and sews on both sides for two HSTs. The four-at-a-time method adds 1 1/4 inch, sews around all four edges, and cuts both diagonals for four HSTs. The eight-at-a-time method starts from a larger square, (finished + 7/8 inch) times 2, and yields eight.
| Method | Add to finished | HSTs per square |
|---|---|---|
| Two at a time | 7/8" | 2 |
| Four at a time | 1 1/4" | 4 |
| Eight at a time | (finished + 7/8") x 2 | 8 |
Want the reasoning behind the numbers? Read the half-square triangle math explained.
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