Half-square triangle size chart: 2, 4 and 8 at a time
What size squares to cut for half-square triangles, for every common finished size and all three popular methods, with and without trimming, plus how many squares you need.
Quick answer For half-square triangles two at a time, cut squares finished size + ⅞" (a 3" finished HST starts from 3⅞" squares). Eight at a time: 2 × finished + 1¾". Four at a time: (finished + ½") × 1.414 + ½", then trim. To trim for sharp points, cut every method a little bigger and trim to finished + ½".
Half-square triangles (HSTs) are in stars, pinwheels, chevrons, bear's paws and hundreds of other blocks. The cutting math changes with the method you use, and a mistake of ⅛" shows up in every point. This guide has the formulas, the full chart, which method to choose, and how many squares to cut for the number of HSTs you need.
The three methods
Two at a time. Pair a light and a dark square, right sides together. Draw one diagonal on the back of the light square, sew ¼" on both sides of the line, and cut on the line. Each pair makes two HSTs. Cut squares finished + ⅞".
Four at a time. Pair two squares, sew ¼" around all four edges, then cut along both diagonals. Each pair makes four HSTs, with no marking. The exact square is (finished + ½") × 1.414, but every edge of the HSTs comes out on the bias and stretches, so we add another ½" and trim: cut (finished + ½") × 1.414 + ½", rounded up to the next ⅛".
Eight at a time. Draw both diagonals, sew ¼" on either side of each, then cut through the middle both ways and along both diagonals. Each pair makes eight. Cut squares 2 × finished + 1¾".
HST size chart
| Finished | 2 at a time | 2 at a time, trim | 4 at a time | 8 at a time | 8 at a time, trim | Trim to |
|---|---|---|---|---|---|---|
| 1" | 1⅞" | 2" | 2⅝" | 3¾" | 4" | 1½" |
| 1½" | 2⅜" | 2½" | 3⅜" | 4¾" | 5" | 2" |
| 2" | 2⅞" | 3" | 4⅛" | 5¾" | 6" | 2½" |
| 2½" | 3⅜" | 3½" | 4¾" | 6¾" | 7" | 3" |
| 3" | 3⅞" | 4" | 5½" | 7¾" | 8" | 3½" |
| 3½" | 4⅜" | 4½" | 6¼" | 8¾" | 9" | 4" |
| 4" | 4⅞" | 5" | 6⅞" | 9¾" | 10" | 4½" |
| 5" | 5⅞" | 6" | 8⅜" | 11¾" | 12" | 5½" |
| 6" | 6⅞" | 7" | 9¾" | 13¾" | 14" | 6½" |
"Finished" is the size of the HST once it's sewn into the quilt. "Trim to" is its size before that, with seam allowances: always finished + ½".
To trim or not to trim
The exact sizes (finished + ⅞" for two at a time) leave no room for error: if your seam is a thread wider than ¼", every HST comes out small. Many quilters cut a little bigger (finished + 1" for two at a time) and trim each HST to finished + ½" with a square ruler, lining up the ruler's 45° line with the seam.
It's slower, but every unit ends up exactly the same size, and that's what gives sharp points where four triangles meet. For the four-at-a-time method, trimming isn't optional: the bias edges need it.
Which method should you use?
| Method | Best for | Watch out for |
|---|---|---|
| 2 at a time | A handful of HSTs, or every HST a different fabric pair | Marking a line on every square |
| 4 at a time | Small numbers of matching HSTs, no marking | All four edges are bias and stretch; trim them |
| 8 at a time | Lots of HSTs from the same two fabrics, such as a chevron quilt | Big squares; mark carefully |
If you're new to HSTs, start with two at a time and trim. It's the easiest to get right, and the pinwheel baby quilt is a gentle first project.
Strip-pieced and printed-paper methods exist too. They're fast for large numbers of tiny HSTs, but need their own papers or rulers.
How many squares?
Divide the number of HSTs you need by the method's yield, and round up:
- 48 HSTs, two at a time: 24 squares of each fabric
- 48 HSTs, four at a time: 12 squares of each fabric
- 48 HSTs, eight at a time: 6 squares of each fabric
Then count the strips: a 2⅞" square fits 14 across a 42" strip, so 24 squares need 2 strips of each fabric.
Pressing
Press the seam towards the darker fabric so it doesn't shadow through the light one, unless your block's instructions say otherwise. Press, don't iron: pushing the iron back and forth stretches the bias seam. Some quilters press seams open for flatter points where many HSTs meet, as in a pinwheel.
Work it out for your quilt
In the Quiltkind studio, choose the method in Settings and every HST in your quilt is counted and cut that way, paired fabric by fabric, with the trim size in the cutting plan.
Common mistakes
- Cutting the finished size + ½". That's the size of a square, not a triangle. HSTs need more, because the diagonal seam takes up fabric.
- Sewing on the drawn line. For two at a time, sew ¼" on each side of the line, then cut on it.
- Stretching bias edges. Handle cut HSTs by their straight edges and press gently.
- Trimming off-centre. Line up the ruler's diagonal with the seam, not just the corner of the unit.
- Mixing methods in one block. Different methods give slightly different results. Use one method for all the HSTs in a quilt.
Frequently asked questions
What size square do I cut for a 3" half-square triangle?
For two at a time, 3⅞" squares (or 4" if you'll trim to 3½"). For eight at a time, 7¾" squares (or 8" to trim). For four at a time, 5½" squares, trimmed to 3½".
Why add ⅞" to half-square triangles?
The seam on the diagonal uses more fabric than a straight seam. ½" covers the two ¼" seam allowances on the straight edges; the extra ⅜" covers the diagonal seam. Together that's ⅞".
What size do I trim a half-square triangle to?
The finished size plus ½". A 3" finished HST is trimmed to 3½".
How many half-square triangles does eight at a time make?
Eight from each pair of squares. For 64 HSTs, you need 8 squares of each fabric.
Which HST method wastes the least fabric?
Four and eight at a time cut the most HSTs per square, but any method wastes little. The bigger difference is how many strips you cut: the studio's cutting plan packs the squares into strips so you see the exact amount for your quilt.
Keep reading
Your first quilt: a nine-patch throw, planned to the inch
A complete beginner's plan for a 52 × 64" nine-patch throw: what to buy, how to cut every strip, the faster strip-set method, the order to sew it in, and the same quilt in baby to king sizes.
Value and contrast: why your star points disappear
Colour gets the attention, but value (how light or dark a fabric is) decides whether your quilt pattern shows. What value is, how to check it before you cut, and where contrast matters most.
What quilt can I make with 10 fat quarters?
Quilts you can make from 10 fat quarters, sized to really fit the bundle, how many squares a fat quarter gives at every size, and how many fat quarters a baby, throw, twin or queen quilt takes.