Half-square trianglecalculator
Enter the finished size of your HSTs and how many you need. You get the starting square size, the trim size, how many squares to cut and how much of each fabric to buy, for three popular methods.
QuiltHelperEnter a finished size greater than zero and at least 1 HST.
Cut squares
73⁄4"squares
Magic 8: (finished + 7/8") × 2, rounded up to the next 1/8"
Compare methods
Fabric per color for the same HSTsSew and cut
HST cut sizes chart
Starting square sizes for each method, rounded up to the next 1/8". Trim every unit to the unfinished size.
Swipe the table sideways to see every column.
| Finished | Trim to | 2 at a time | 4 at a time | Magic 8 |
|---|---|---|---|---|
| 11⁄2" | 2" | 23⁄8" | 35⁄8" | 43⁄4" |
| 2" | 21⁄2" | 27⁄8" | 43⁄8" | 53⁄4" |
| 21⁄2" | 3" | 33⁄8" | 5" | 63⁄4" |
| 3" | 31⁄2" | 37⁄8" | 53⁄4" | 73⁄4" |
| 31⁄2" | 4" | 43⁄8" | 61⁄2" | 83⁄4" |
| 4" | 41⁄2" | 47⁄8" | 71⁄8" | 93⁄4" |
| 41⁄2" | 5" | 53⁄8" | 77⁄8" | 103⁄4" |
| 5" | 51⁄2" | 57⁄8" | 85⁄8" | 113⁄4" |
| 6" | 61⁄2" | 67⁄8" | 10" | 133⁄4" |
| 8" | 81⁄2" | 87⁄8" | 127⁄8" | 173⁄4" |
How to use this calculator
- Enter the finished size. This is the size of the HST inside the quilt, without seam allowances. Your pattern usually gives it.
- Enter how many you need. Part numbers are rounded up, so you always get enough.
- Choose a method. Magic 8 makes eight HSTs from one pair of squares, 4-at-a-time makes four, and 2-at-a-time makes two.
- Cut, sew and trim. Cut the squares shown from each fabric, follow the sewing and cutting diagram, then trim every unit to the size shown.
How the HST math works
Each method starts with one light and one dark square placed right sides together. The square has to be large enough that, after the seams and diagonal cuts, every triangle still has a ¼" seam allowance. Every result is rounded up to the next 1/8", the smallest mark on most rulers.
2 at a time: square = finished + 7/8"4 at a time: square = (finished + 1/2") × 1.414 + 3/4"Magic 8: square = (finished + 7/8") × 2Trim size = finished size + ½"Pairs of squares = HSTs needed ÷ HSTs per pair, rounded upFabric per color = strips × square size, rounded up to the next 1/8 yardExample: 32 HSTs that finish at 3", from 42" fabric.
2 at a time: cut 16 squares of each fabric at 37⁄8", 10 fit across a strip, so 2 strips = 1⁄4 yard of each fabric.
4 at a time: cut 8 squares of each fabric at 53⁄4", 7 fit across a strip, so 2 strips = 3⁄8 yard of each fabric.
Magic 8: cut 4 squares of each fabric at 73⁄4", 5 fit across a strip, so 1 strip = 1⁄4 yard of each fabric.
Every unit is trimmed to 3½".
Which HST method should you use?
2 at a time is the classic method. Draw a diagonal line, sew ¼" on each side, cut on the line. It is easy to control, the outside edges are on the straight grain, and it suits scrappy quilts where every pair is different.
4 at a time means sewing all the way around the outside of the two squares, then cutting on both diagonals. It is quick, but the outside edges of each unit are on the bias, so they stretch easily. Press gently and trim with care.
Magic 8 makes eight units from one large pair. Sew ¼" either side of both diagonals, then cut through the middle both ways and along both diagonals. It is the fastest way to make many matching HSTs, and the outside edges are on the straight grain.
Trimming HSTs to size
Press each unit, then line up the 45° line of a square ruler with the seam. Trim two sides, turn the unit around, and trim the other two sides to the unfinished size, which is the finished size plus ½". Trimming takes a few extra minutes and saves a lot of frustration when blocks are joined.
Common HST mistakes
- Mixing up finished and unfinished size. A 3" finished HST is trimmed to 3½".
- Sewing a scant or wide seam. The formulas assume an accurate ¼" seam.
- Stretching bias edges. Press rather than iron back and forth, especially with 4-at-a-time units.
- Skipping the trim. Small errors add up across a quilt; square up every unit.
Questions about half-square triangles
What size square do I cut for a 3 inch half-square triangle?
For a 3" finished HST, cut 37⁄8" squares for 2-at-a-time, 53⁄4" squares for 4-at-a-time, or 73⁄4" squares for the Magic 8 method. Each unit is then trimmed to 3½".
Why do you add 7/8" to the finished size?
Cutting a square on the diagonal loses length along the seam. Adding 7/8" to the finished size gives exactly enough for a ¼" seam allowance on all sides of the triangle. Many quilters add 1" instead and trim the unit down, which is more forgiving.
Which HST method is most accurate?
2-at-a-time is the easiest to control, and its outside edges are on the straight grain, so the units hold their shape. Magic 8 also gives straight-grain edges and is much faster for large batches. 4-at-a-time units have bias edges on the outside, so handle and press them gently.
Do I need to trim half-square triangles?
Trimming, or squaring up, each unit to its unfinished size is the best way to get blocks that match. Even with an exact formula, small differences in seam width and pressing add up across a quilt.
How much fabric do I need for 32 half-square triangles?
For 32 HSTs that finish at 3", the Magic 8 method needs 1⁄4 yard of each fabric and 2-at-a-time needs 1⁄4 yard, both from 42" fabric. Enter your own size and count above.
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How we check this calculator: square sizes, pair counts and yardage are tested against hand-worked results for each method, including sizes too large for the fabric width. Fabric amounts always round up. Last checked November 2026.