The formulas behind each method
Every method pairs a light and a dark square, sews them, and cuts them apart. They differ in how many units each pair makes and how big the squares must be.
- 2 at a time: cut squares at finished size + ⅞ inch. Draw one diagonal, sew ¼ inch on both sides of it and cut on the line. One pair makes 2 HSTs.
- 2 at a time, oversized: cut squares at finished size + 1 inch, make them the same way, then trim each unit to finished size + ½ inch.
- 4 at a time: cut squares at (finished size + ⅞) × 1.414, rounded up to the next ⅛ inch. Sew around all four edges and cut on both diagonals. One pair makes 4 HSTs.
- 8 at a time: cut squares at 2 × (finished size + ⅞ inch). Sew both sides of both diagonals, cut in half both ways, then on the lines. One pair makes 8 HSTs.
Where does the ⅞ come from? A finished unit needs ½ inch for the seam allowances around its outside. Sewing ¼ inch from the diagonal also takes ¼ × 1.414 ≈ 0.354 inch from the side of the square. That adds up to 0.854 inch, so ⅞ inch leaves only a few hundredths of an inch to square up. The 4-at-a-time formula is the same idea turned 45°: the square's side becomes the unit's diagonal, so the size is multiplied by 1.414. That number is rarely on a ruler line, so we round it up to the next ⅛ inch.
The calculator works out how many pairs to cut, rounding up. Methods that make 4 or 8 at a time can leave a few spares. It also counts width-of-fabric strips for the squares, so you know how much of each fabric to buy.
HST cutting chart
Square sizes to cut for one finished size, rounded up to the next ⅛ inch. Trim every unit to the size in the second column.
| Finished HST | Trim to | 2 at a time | 2 at a time, oversized | 4 at a time | 8 at a time |
|---|---|---|---|---|---|
| 2 in | 2 ½ in | 2 ⅞ in | 3 in | 4 ⅛ in | 5 ¾ in |
| 2 ½ in | 3 in | 3 ⅜ in | 3 ½ in | 4 ⅞ in | 6 ¾ in |
| 3 in | 3 ½ in | 3 ⅞ in | 4 in | 5 ½ in | 7 ¾ in |
| 3 ½ in | 4 in | 4 ⅜ in | 4 ½ in | 6 ¼ in | 8 ¾ in |
| 4 in | 4 ½ in | 4 ⅞ in | 5 in | 7 in | 9 ¾ in |
| 4 ½ in | 5 in | 5 ⅜ in | 5 ½ in | 7 ⅝ in | 10 ¾ in |
| 5 in | 5 ½ in | 5 ⅞ in | 6 in | 8 ⅜ in | 11 ¾ in |
| 5 ½ in | 6 in | 6 ⅜ in | 6 ½ in | 9 ⅛ in | 12 ¾ in |
| 6 in | 6 ½ in | 6 ⅞ in | 7 in | 9 ¾ in | 13 ¾ in |
Other charts can differ by ⅛ inch in the 4-at-a-time column, depending on how they round. Rounding up costs a sliver of trimming but never leaves a unit too small.
Example: 48 HSTs that finish at 3 inches
Say a pattern calls for 48 half-square triangles that finish at 3 inches, cut from 42-inch cotton.
- 2 at a time: 48 ÷ 2 = 24 squares of 3 ⅞ inches from each fabric. Ten fit across a strip, so cut 3 strips 3 ⅞ inches deep: ⅜ yard of each fabric.
- 4 at a time: 48 ÷ 4 = 12 squares of 5 ½ inches from each fabric. Seven fit across a strip, so cut 2 strips: ⅜ yard of each.
- 8 at a time: 48 ÷ 8 = 6 squares of 7 ¾ inches from each fabric. Only five fit across a strip, so the sixth needs a second strip: ½ yard of each.
All three methods use about the same area of fabric. What changes is the waste at the end of each strip, how much sewing you do, and whether the units have bias edges. For exactly matching points, the oversized method needs 24 squares of 4 inches per fabric, still 3 strips and ⅜ yard.
Tips for accurate HSTs
- Check your ¼-inch seam first. The ⅞-inch formula assumes an accurate ¼-inch seam. Sew a test unit and measure it before you cut everything.
- Starch before cutting 4-at-a-time squares. Their units come out with bias edges. Starch helps them keep their shape, and pressing straight down rather than sliding the iron helps too.
- Trim with the 45° line. Lay a square ruler's diagonal line on the seam and trim two sides, then turn the unit and trim the other two.
- Press consistently. Press toward the darker fabric, or open to reduce bulk, but do every unit the same way so seams nest when you join them.
- Watch directional prints. Each method turns the triangles differently, so a one-way print will point in more than one direction. Lay out a test block before you cut it all.
Frequently asked questions
What size square do I cut for a 3-inch half-square triangle?
For 2 at a time, cut 3 ⅞-inch squares. For 4 at a time, cut 5 ½-inch squares, and for 8 at a time, 7 ¾-inch squares. Every method gives units that trim to 3 ½ inches and finish at 3 inches in the quilt.
Why do you add ⅞ inch for half-square triangles?
The unit needs ½ inch for the seam allowances around its outside edges, and the ¼-inch seam sewn along the diagonal takes about another 0.35 inch out of each side. Together that is about 0.85 inch, and ⅞ inch is the nearest ruler mark above it.
Should I cut HSTs oversized and trim them?
It is the most accurate way, and many quilters prefer it for points that must match. Cut the squares 1 inch larger than the finished size, sew and press, then trim each unit to finished + ½ inch. It costs a little fabric and the time to trim every unit.
What is the formula for 4-at-a-time half-square triangles?
Cut squares of (finished size + ⅞ inch) × 1.414, rounded up to the next ⅛ inch. Sew ¼ inch around all four edges of a light and a dark square, then cut on both diagonals. The outer edges of these units are on the bias, so handle and press them gently.
How do I make 8 half-square triangles at once?
Cut a light and a dark square at 2 × (finished size + ⅞ inch). Draw both diagonals, sew ¼ inch on both sides of each line, cut the square in half both ways, then cut each quarter on its drawn line. Each quarter acts like a 2-at-a-time square.
Last updated October 17, 2026.