This tool tells you what size to cut squares for half square triangles at any finished size, using the 2-at-a-time, 4-at-a-time or 8-at-a-time method. It also counts how many square pairs you need, so there is no hunting through a printed chart.
Enter the finished size and the number of HSTs to see cutting sizes. Sizes are rounded up to the nearest 1/8 inch and assume a 1/4 inch seam. Each pair is one light and one dark square. The 4-at-a-time size is slightly generous, so square up the units after pressing.
What Size to Cut Squares for Half Square Triangles
Cut your squares at the finished size plus 7/8″ for the 2-at-a-time method. A 3″ finished HST needs 3 7/8″ squares and trims to 3 1/2″ unfinished. The other methods use bigger squares but make more units per pair. This half square triangle size chart is rounded up to the next 1/8″:
| Finished HST | 2 at a time | 4 at a time | 8 at a time | Unfinished |
|---|---|---|---|---|
| 2″ | 2 7/8″ | 4 1/8″ | 5 3/4″ | 2 1/2″ |
| 3″ | 3 7/8″ | 5 1/2″ | 7 3/4″ | 3 1/2″ |
| 4″ | 4 7/8″ | 7″ | 9 3/4″ | 4 1/2″ |
| 5″ | 5 7/8″ | 8 3/8″ | 11 3/4″ | 5 1/2″ |
| 6″ | 6 7/8″ | 9 3/4″ | 13 3/4″ | 6 1/2″ |
The Half Square Triangle Formulas
Each method has one formula, built so the finished unit measures what you typed once it is sewn into the quilt. A half square triangle, or HST, is a square made of two right triangles sewn along the diagonal. You almost never cut triangles directly, because bias edges stretch, so you pair two squares right sides together, sew, and cut them apart.
Formula: 2 at a time: Cut square = Finished size + 7/8″. 4 at a time: Cut square = Finished size × 1.414 + 1 1/4″. 8 at a time: Cut square = (Finished size + 7/8″) × 2. Unfinished HST = Finished size + 1/2″. Square pairs = HSTs needed ÷ yield (2, 4 or 8), rounded up. Cut sizes are rounded up to the next 1/8″.
- Pick the finished size: 3″.
- Apply the 8 at a time formula: (3″ + 7/8″) × 2 = 7 3/4″.
- Divide the HSTs needed by the yield: 24 ÷ 8 = 3 pairs.
- Sew, cut, press, then trim each unit to 3 1/2″.
- Method: how many HSTs one pair of squares produces. 2 at a time: sew 1/4″ either side of one diagonal and cut on the line. 4 at a time: sew around all four edges and cut both diagonals. 8 at a time: sew either side of both diagonals, then cut horizontally, vertically and on both diagonals.
- Finished HST size: the size once sewn into the quilt, without seam allowance, in inches.
- HSTs needed: the total number of units the pattern calls for.
Example Scenarios
| Method | Finished size | HSTs needed | Cut squares at | Square pairs | HSTs made |
|---|---|---|---|---|---|
| 2 at a time | 3″ | 24 | 3 7/8″ | 12 | 24 |
| 4 at a time | 3″ | 24 | 5 1/2″ | 6 | 24 |
| 8 at a time | 3″ | 24 | 7 3/4″ | 3 | 24 |
| 2 at a time | 4 1/2″ | 10 | 5 3/8″ | 5 | 10 |
| 8 at a time | 2″ | 20 | 5 3/4″ | 3 | 24 |
| 4 at a time | 6″ | 8 | 9 3/4″ | 2 | 8 |
How to Make Half Square Triangles for Beginners
Start with the 2-at-a-time method and a little extra fabric. Many quilters add a full 1″ instead of 7/8″ and trim each unit down afterwards. Trimming costs a minute per block but gives crisp points while you are still learning a consistent 1/4″ seam.
- Use the 8 at a time setting for large batches. Three pairs of 7 3/4″ squares give you 24 units at 3″ finished, with far less marking than 12 separate pairs.
- Half square triangles 4 at a time have bias edges on all four sides. Starch the squares before sewing and handle the units gently so they do not stretch.
- Press first, then trim. Line up the 45-degree line of your ruler with the seam and square to the unfinished size shown in the results.
- Test one unit before cutting the whole quilt. If your seam runs a thread wider than 1/4″, the units will come out a touch small.
The tool only handles the geometry. Check your pattern for directional prints, which can end up running sideways in some methods. For background on piecing traditions and block construction, see Wikipedia’s article on patchwork and piecing.
Common questions about cutting half square triangles
What size do I cut squares for half square triangles?
For the 2-at-a-time method, cut squares at the finished size plus 7/8″. A 3″ finished HST starts from 3 7/8″ squares, and a 4″ HST from 4 7/8″.What is the formula for 8 at a time half square triangles?
Cut each square at (finished size + 7/8″) × 2. For 3″ finished units that is 7 3/4″, and one pair gives you eight HSTs.Why add 7/8 inch for half square triangles?
A finished HST needs 1/4″ of seam allowance on every side, which adds 1/2″. The diagonal seam uses up close to another 3/8″, since the triangle tip is lost in the allowance. Together that makes 7/8″.How does the 8 at a time method work?
Place two squares right sides together, draw both diagonals, and stitch 1/4″ on each side of both lines. Cut the square in half horizontally and vertically, then along the drawn diagonals, to get eight HSTs.Which method is best for beginners?
Start with 2-at-a-time, which has the fewest bias edges. Adding 1/8″ to the cut size and trimming each unit after pressing hides small seam errors.Which HST method wastes the least fabric?
For small quantities, 2-at-a-time is simple and wastes little. For large batches, 8-at-a-time uses fabric efficiently and saves marking time, while 4-at-a-time is fast but its bias edges need care.Related Calculators
Quilt math rarely stops at one block. Add up odd measurements like 3 7/8″ plus 5 3/8″ with the fraction calculator for adding inch measurements, convert a European pattern with the inch to cm converter, and size a quilt top or border with the rectangle area calculator. They pair well with the half square triangle calculator when you plan a full project.