Half square triangles are squares made of two right triangles, the building block of countless quilt patterns. Sewing two squares together and cutting them apart is faster and more accurate than cutting triangles. Cut slightly oversize and trim to the unfinished size (finished + ½ in) for perfect points.
How to use the Half Square Triangle Calculator
- Method
- Finished HST size
- HSTs needed
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the method, enter the finished size of each HST and how many you need.
Tips
- Measure over thin clothing with a soft tape held snug but not tight, and keep the tape level all the way round.
- Take each measurement twice; if you are between sizes, the larger size is usually the safer choice.
- Size charts vary by brand and country – check the maker’s own chart when it is available.
- For sewing projects, pre-wash fabric before cutting and buy 10% extra for shrinkage, pattern matching and mistakes.
Common mistakes to avoid
- Mixing inches and centimetres between the tape and the size chart.
- Forgetting seam and hem allowances when cutting fabric.
- Measuring an old, stretched garment instead of your body.
Inputs explained
| Field | What to enter |
|---|---|
| Method | Choose one: Two at a time, Four at a time, Eight at a time |
| Finished HST size | Units: in, cm |
| HSTs needed | A number |
What each result means
- Cut squares (fraction) = [Cut squares]÷0.0254
- Pairs of squares needed = ceil([HSTs needed]÷[Method] – 0.000001)
- Trim each HST to = [Finished HST size] + 0.5×0.0254
Example values
How the main result changes when Finished HST size changes and everything else stays at the example values above:
| Finished HST size (in) | Cut squares (in) |
|---|---|
| 1.5 | 2.375 |
| 2.25 | 3.125 |
| 3 | 3.875 |
| 3.75 | 4.625 |
| 4.5 | 5.375 |
| 6 | 6.875 |
Formula
Two at a time: square = finished size + ⅞ inFour at a time: square = finished × 1.414 + 1¼ in (trim to finished + ½ in)Eight at a time: square = 2 × finished + 1¾ inWorked example
Inputs — Method: Two at a time; Finished HST size: 3 in; HSTs needed: 48.
Results — Cut squares = 3.875 in; Cut squares (fraction) = 3 7/8; Pairs of squares needed = 24; Trim each HST to = 3.5 in.
Key terms
- HST
- Half square triangle – a square made of two right-angled triangles.
Frequently asked questions
How big do I cut squares for a 3 in HST?
3⅞ in for the two-at-a-time method, or cut 4 in and trim to 3½ in.
Why add ⅞ inch?
½ in for the two ¼ in seam allowances plus about ⅜ in lost along the diagonal seam.
Which method is most accurate?
Making them slightly oversize and trimming, whichever method you use.
What formula does the Half Square Triangle Calculator use?
Two at a time: square = finished size + ⅞ in.