Quilting
Half-Square Triangle Calculator
Enter your finished half-square triangle size and seam allowance to get the exact square to cut, plus how many squares you need for any number of HSTs.
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- Inches or centimetres
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Calculator
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What size squares do I cut for 3" half-square triangles?
For 2 half-square triangles that finish at 3" with a 1/4" seam allowance:
You enter
- Finished HST size
- 3" (7.62 cm)
- Seam allowance
- 1/4" (0.64 cm)
- HSTs needed
- 2
You get
- Cut squares
- 3-7/8" × 3-7/8" (9.9 cm)
- Unfinished size
- 3-1/2" (8.9 cm)
- Squares per fabric
- 1
The formula: cut size = finished size + 2 × seam allowance + 3/8" (metric: + 0.95 cm).
What size squares do I cut for other HST sizes?
Cut sizes for the two-at-a-time method with a 1/4" seam allowance, worked out by the calculator above. Trim each pressed HST to the "trim to" size.
| Finished HST | Cut squares | Trim to |
|---|---|---|
| 1" (2.54 cm) | 1-7/8" (4.8 cm) | 1-1/2" (3.8 cm) |
| 1-1/2" (3.81 cm) | 2-3/8" (6.1 cm) | 2" (5.1 cm) |
| 2" (5.08 cm) | 2-7/8" (7.3 cm) | 2-1/2" (6.4 cm) |
| 2-1/2" (6.35 cm) | 3-3/8" (8.6 cm) | 3" (7.6 cm) |
| 3" (7.62 cm) | 3-7/8" (9.9 cm) | 3-1/2" (8.9 cm) |
| 3-1/2" (8.89 cm) | 4-3/8" (11.2 cm) | 4" (10.2 cm) |
| 4" (10.16 cm) | 4-7/8" (12.4 cm) | 4-1/2" (11.4 cm) |
| 4-1/2" (11.43 cm) | 5-3/8" (13.7 cm) | 5" (12.7 cm) |
| 5" (12.70 cm) | 5-7/8" (15.0 cm) | 5-1/2" (14.0 cm) |
| 6" (15.24 cm) | 6-7/8" (17.5 cm) | 6-1/2" (16.5 cm) |
Frequently asked questions
What size do I cut squares for half-square triangles?
Add 7/8" to the finished size when you sew a 1/4" seam — a 3" finished HST starts from 3-7/8" squares. The extra covers the seam allowance on every side plus the fabric taken up by the diagonal seam. For other seam allowances the rule is finished size + 2 × seam + 3/8", which is what this calculator works out.
How many half-square triangles do two squares make?
Two. Layer a light and a dark square right sides together, sew a seam either side of the drawn diagonal, and cut on the line: each pair of squares gives two HSTs. For N HSTs you need N ÷ 2 squares of each fabric, rounded up.
Should I cut HST squares bigger and trim them down?
Many quilters add 1/8" to 1/2" to the cut size and trim each HST after pressing. Trimming fixes small cutting and sewing errors, so blocks line up better. Trim to the unfinished size shown in the results (finished size plus both seam allowances).
What is a half-square triangle in quilting?
A square unit made of two right triangles sewn together along the diagonal. It's one of the most-used quilt units: pinwheels, sawtooth stars, chevrons, and many traditional blocks are built from HSTs.
Related tools
Which quilting machine should I use for this project?
Three picks from starter to step-up, sold in Canada and the US. Want a match for your budget? Take the machine picker.
The first quilter
Baby Lock Jubilant
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80 built-in stitches · Computerized · 1-step buttonhole
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Janome MC9850
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The forever upgrade
Janome Skyline S9
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