Quilt math · Calculator
Half-square triangles: what size to cut
The square to cut for any finished half-square triangle, two, four or eight at a time. Type your size, or find it in the chart.
The short answer
Cut squares ⅞″ bigger than the finished size and make them two at a time. For a 3″ finished half-square triangle, cut 3⅞″ squares, or 4″ and trim to 3½″. Four at a time, cut 5″; eight at a time, 7¾″.
Work out yours
Free · no accountFinished 3″, trimmed to 3½″
For 8 units
Or 4″ to trim to 3½″. Sew ¼″ each side of the diagonal, cut on it. 4 squares of each fabric.
Sew ¼″ all around, cut both diagonals, trim to 3½″. The edges are bias: handle gently. 2 squares of each fabric.
Or 8″ to trim. Sew ¼″ each side of both diagonals, cut across the middle both ways and on the diagonals. 1 square of each fabric.
Designing a quilt? Quarter Inch finds every half-square triangle in your blocks and gives the cut, the count and the method for each fabric.
Start designing freeSame sums as the app: @quarterinch/cutting HST.exact and HST.trim · four and eight at a time from the same seams, up to the next ⅛″
Half-square triangle size chart
Find the finished size you want on the left. Every figure is the cut square, worked out by the same code that writes Quarter Inch’s cutting charts.
| Finished | 2 at a time | 2 at a time, to trim | Trim to | 4 at a time | 8 at a time |
|---|---|---|---|---|---|
| 1″ | 1⅞″ | 2″ | 1½″ | 2⅛″ | 3¾″ |
| 1½″ | 2⅜″ | 2½″ | 2″ | 2⅞″ | 4¾″ |
| 2″ | 2⅞″ | 3″ | 2½″ | 3⅝″ | 5¾″ |
| 2½″ | 3⅜″ | 3½″ | 3″ | 4¼″ | 6¾″ |
| 3″ | 3⅞″ | 4″ | 3½″ | 5″ | 7¾″ |
| 3½″ | 4⅜″ | 4½″ | 4″ | 5¾″ | 8¾″ |
| 4″ | 4⅞″ | 5″ | 4½″ | 6⅜″ | 9¾″ |
| 4½″ | 5⅜″ | 5½″ | 5″ | 7⅛″ | 10¾″ |
| 5″ | 5⅞″ | 6″ | 5½″ | 7⅞″ | 11¾″ |
| 5½″ | 6⅜″ | 6½″ | 6″ | 8½″ | 12¾″ |
| 6″ | 6⅞″ | 7″ | 6½″ | 9¼″ | 13¾″ |
| 7″ | 7⅞″ | 8″ | 7½″ | 10⅝″ | 15¾″ |
| 8″ | 8⅞″ | 9″ | 8½″ | 12⅛″ | 17¾″ |
| 9″ | 9⅞″ | 10″ | 9½″ | 13½″ | 19¾″ |
| 10″ | 10⅞″ | 11″ | 10½″ | 14⅞″ | 21¾″ |
| 11″ | 11⅞″ | 12″ | 11½″ | 16⅜″ | 23¾″ |
| 12″ | 12⅞″ | 13″ | 12½″ | 17¾″ | 25¾″ |
How it’s worked out
A half-square triangle (HST) is a square made of two right triangles sewn along the long side. When you cut a square once on the diagonal, its sides become the triangle’s short sides and the seam runs along the cut. The two short sides need ¼″ each, ½″ in all, as any square does. The long seam needs ¼″ too, but it runs into the corner at 45°, and that costs another ¼″ × √2 along the side. Together that’s 0.854″, and the next ⅛″ up is ⅞″. That’s the whole rule: finished size and ⅞″.
| The finished size | 3″ |
|---|---|
| Its size before it’s sewn in, with a ¼″ seam each side | 3½″ |
| The square to cut: the finished size and ⅞″ | 3⅞″ |
| Or cut it ⅛″ larger and trim the sewn unit | 4″ → 3½″ |
| Two squares, one of each fabric, make | 2 units |
Layer a light and a dark square right sides together. Draw the diagonal on the lighter one, sew ¼″ each side of the line, cut on the line and press. You have two units, each 3½″ square.
Four and eight at a time
Four at a time
Sew the two squares together ¼″ in from all four edges, then cut both diagonals. Here the seam is on the square’s edge and the cuts become the unit’s sides, so the square is the unit’s size before sewing, times √2 (1.414). For a 3″ finished unit, 3½″ × 1.414 is 4.950″: cut 5″ squares, the next ⅛″ up, and trim each unit to 3½″.
Every edge of these units is cut on the bias, so it stretches. Starch the squares before you cut, press gently, and sew them into the block soon after. Use this method when you need lots of units quickly, not for the points that matter most.
Eight at a time
Draw both diagonals on the lighter square and sew ¼″ each side of both. Cut the square in half across and down, then on both diagonals. Each quarter of the square is sewn and cut as a two-at-a-time unit, so the square is twice the two-at-a-time size: 3⅞″ × 2 = 7¾″, or 8″ to trim. The units’ outer edges are on the straight grain, as they are for two at a time.
Which method? Two at a time is the most accurate and the easiest to sew straight. Eight at a time is just as stable and quicker when you need many of the same pair. Four at a time is quick but stretchy.
Exact or a little large?
The exact sizes work when your seam is a true ¼″ and you press without stretching. Many quilters cut a little large instead, sew, then trim each unit to its size with a square ruler, laying the ruler’s 45° line on the seam. It costs one more step and a little fabric, and every unit comes out the same.
Quarter Inch’s cutting chart gives either. It cuts 3⅞″ squares (exact) to start with; under How units are cut, choose Change and then A little large, then trim, and it says 4″, trim to 3½″.
What changes the answer
- Your seam allowance. A scant ¼″ makes units a little big; a generous one makes them small. Sew a test unit and measure it before you cut the rest.
- Pressing. Press the seam to the dark side, or open for flatter points. Iron up and down, don’t slide: the long seam is on the bias.
- Fabric. From a fat quarter you get 20 squares of 3⅞″, so 20 pairs make 40 units with a second fabric. See fat quarter sizes.
Questions
Short answers.
Something else? A real person replies.
hello@quarterinch.app
What size square for a 3″ half-square triangle?
3⅞″ for two at a time (the finished size and ⅞″), or 4″ if you’d like to trim the unit to 3½″.
Why add ⅞″ and not ½″?
The long seam runs into the corners at 45°, so on top of the ½″ every square needs it takes another ¼″ × √2. That’s 0.854″, and the next ⅛″ up is ⅞″.
Can I add 1″ instead of ⅞″?
Yes, and many quilters do: the unit comes out ⅛″ large, and you trim it to the finished size and ½″. For 3″ finished, cut 4″ and trim to 3½″.
What’s the formula for eight at a time?
Twice the two-at-a-time square: (finished + ⅞″) × 2. For 3″ finished that’s 7¾″.