---
title: "Half-Square Triangle Calculator (HST) | Loom Mosaic"
description: "Square sizes for half-square triangles made two, four or eight at a time, in inches or cm, with how many squares to cut, trimming size and fabric needed."
url: https://loommosaic.cfd/half-square-triangle-calculator/
---

Half-square triangle calculator

# What size squares for half-square triangles?

Enter the finished size and how many you need. You get the square to cut for each batch method, how many squares of each fabric, what to trim to and how much fabric it takes.

## Choosing a method

A half-square triangle (HST) is a square made of two right-angled triangles. All three methods start with two squares of different fabrics placed right sides together.

- **Two at a time**: the classic finished size + 7/8″. The outer edges stay on the straight grain, so units keep their shape. Best for small batches and directional prints.
- **Four at a time**: sew round the edges, then cut both diagonals. Quick, but every outer edge is on the stretchy bias, so press gently and trim.
- **Eight at a time**: sew either side of both diagonals, then cut into eight. The fastest for large numbers, with straight-grain edges.

### Where the numbers come from

Each unit must measure the finished size plus two seam allowances before it is sewn into a block. The diagonal seam uses up more: with 1/4″ seams the triangle legs need to be the finished size + 0.854″, which rounds up to 7/8″. Four at a time uses the square's half-diagonals as those legs, so the square is √2 (about 1.414) times bigger; eight at a time cuts the square in half both ways first, so it is twice as big.

### Worked example

For 24 HSTs finishing at 3″, two at a time needs 12 pairs of 3 7/8″ squares, four at a time 6 pairs of 5 1/2″ squares and eight at a time 3 pairs of 7 3/4″ squares. Every unit is trimmed or checked at 3 1/2″.

## Sources

- Utah State University Extension, [Half Square Triangles, Broken Dishes Block, Pinwheel Block](https://extension.usu.edu/sewing/research/half-square-triangles-broken-dishes-block-pinwheel-block): two-at-a-time squares are the finished size + 7/8″, sewn a scant 1/4″ either side of the diagonal; a 3″ finished unit measures 3 1/2″.
- Patchwork Posse, [Four half-square triangles at a time](https://www.patchworkposse.com/four-half-square-triangles-at-a-time/) and [8 half-square triangles at once](https://www.patchworkposse.com/8-half-square-triangles-at-once-tutorial/): how the batch methods are sewn and cut, and the advice to make a test unit and square every unit up.
- The four- and eight-at-a-time sizes are worked out here from the geometry above rather than copied from a chart, so they may differ by 1/8″ from charts that round differently.

## Trimming half-square triangles

1. Press the seam (to the darker fabric, or open) before trimming.
2. Lay a square ruler on the unit with its 45° line exactly on the seam.
3. Line up the trim size (for 3″ finished, 3 1/2″) so the unit sticks out a little on all sides, and trim the two edges at the top and right.
4. Turn the unit round, line the trimmed edges up with the ruler's trim-size marks and the 45° line with the seam, and trim the other two edges.
5. Snip the dog ears, the little triangles of seam allowance at the corners, so they do not add bulk.

## Half-square triangle questions

**Why add 7/8″ and not 1/2″?**

A plain square only loses two 1/4″ seam allowances. A half-square triangle also has the diagonal seam, which takes about 0.354″ more along each edge. Added together that is 0.854″, so quilters round up to 7/8″.

**Should I cut exact or oversized squares?**

Exact squares give the right size if your 1/4″ seam is accurate. Oversized squares (two at a time: finished + 1″) leave a little to trim, which makes every unit the same size even if seams wander, at the cost of a few minutes trimming. For four at a time, trimming is strongly advised because of the bias edges.

**Which method uses the least fabric?**

It depends on the size and how the squares fit across your fabric, so the calculator works out all three and marks the one with the shortest length of fabric. For small numbers the difference is usually only a strip or two.

**Can I use metric seam allowances?**

Yes. Choose centimetres and a 0.75 cm seam allowance if your pattern uses one. The same geometry applies, and sizes are rounded up to the next millimetre. With a 1/4″ presser foot, keep the 1/4″ option and read the sizes in centimetres.

**How many HSTs do I get from a fat quarter?**

The calculator assumes a usable fat quarter of 18 × 21″ and counts how many whole squares fit. For two at a time with 3 7/8″ squares that is 20 squares, enough for 40 HSTs when paired with another fat quarter.