Calculator
Half Square Triangle (HST) Calculator
Cut sizes for every finished HST size, two-at-a-time and four-at-a-time methods, and yield per fat quarter.
The HST Formula
Two-at-a-time: Cut size = finished HST size + 7/8″. Mark diagonal, sew 1/4″ on both sides, cut on diagonal → 2 HSTs per pair.
Four-at-a-time:Large square = (finished × 2) + 7/8″. Small squares = finished + 7/8″. Yields 4 HSTs per large square.
HST Cut Size Reference
| Finished HST | Cut (2-at-a-time) | HSTs per FQ |
|---|---|---|
| 1" | 1-7/8" | 198 |
| 1.5" | 2-3/8" | 112 |
| 2" | 2-7/8" | 72 |
| 2.5" | 3-3/8" | 50 |
| 3" | 3-7/8" | 40 |
| 3.5" | 4-3/8" | 32 |
| 4" | 4-7/8" | 24 |
| 4.5" | 5-3/8" | 18 |
| 5" | 5-7/8" | 18 |
| 6" | 6-7/8" | 12 |
| 8" | 8-7/8" | 4 |
| 10" | 10-7/8" | 2 |
FQ yield based on 18″×22″ fat quarter, two-at-a-time method. Trim all HSTs after sewing for accuracy.
HSTs per Common Block
Frequently Asked Questions
What size squares do I cut for a 3-inch finished HST?
For a 3-inch finished half square triangle using the two-at-a-time method, cut squares at 3-7/8 inches. Mark a diagonal line on the wrong side of one square, sew 1/4" on both sides of the line, and cut on the diagonal. You get 2 HSTs per pair of squares, each finishing at 3 inches.
What is the HST formula?
For the two-at-a-time method: cut size = finished HST size + 7/8". For the four-at-a-time method: large square cut size = (finished HST size × 2) + 7/8"; small squares cut size = finished HST size + 7/8". Both yield HSTs that trim to the exact finished size.
How do I make 4 HSTs at once (four-at-a-time method)?
Cut one large square (finished size × 2 + 7/8") and four small squares (finished size + 7/8"). Place two small squares on opposite corners of the large square, sew 1/4" on both sides of the diagonal, cut on the diagonal. Place remaining small squares on the resulting pieces, sew and cut again. You get 4 HSTs from each large square.
Why are my HSTs not square?
HSTs usually need to be trimmed after sewing. Use a square ruler with a 45-degree diagonal line: align the line on the seam and trim two sides, then rotate and trim the remaining two sides. The most common causes of off-size HSTs are an inaccurate 1/4" seam allowance and not pressing before trimming.