Half square triangle calculator
For a 3" finished HST made two at a time, cut your starting squares at 3 7/8", or 4" if you like to trim. Enter your size, quantity, and method below and we will tell you exactly what to cut, how many, and how much fabric to buy.
Cut
HST starting square chart
The three methods trade speed for trimming. Two at a time gives straight grain edges and the classic 7/8" rule. Four at a time is quick but every edge comes out on the bias and always needs squaring up. Eight at a time is the production line: one pair of big squares yields eight identical units. Sizes below show 2 and 8 at a time exact, and 4 at a time with trim room included.
| Finished HST | Unfinished | 2 at a time | 4 at a time | 8 at a time |
|---|---|---|---|---|
| 1" | 1 1/2" | 1 7/8" | 2 3/4" | 3 3/4" |
| 2" | 2 1/2" | 2 7/8" | 4 1/4" | 5 3/4" |
| 2 1/2" | 3" | 3 3/8" | 5" | 6 3/4" |
| 3" | 3 1/2" | 3 7/8" | 5 3/4" | 7 3/4" |
| 4" | 4 1/2" | 4 7/8" | 7 1/2" | 9 3/4" |
| 5" | 5 1/2" | 5 7/8" | 9" | 11 3/4" |
| 6" | 6 1/2" | 6 7/8" | 10 1/2" | 13 3/4" |
Which HST method should I use?
Two at a time when you need precise units or many different fabric pairings: draw a diagonal line, sew 1/4" on each side, cut on the line. Four at a time when speed matters more than grain: sew around the whole square, cut both diagonals, accept the bias edges and trim. Eight at a time when one fabric pair needs to produce a small army: draw both diagonals on a big square, sew 1/4" on each side of both lines, then cut twice through the middle and twice on the diagonals.
Exact size or cut oversize and trim?
The classic 7/8" rule produces units that are exactly right if your seam is exactly a scant quarter inch and your cutting is exactly true. Most quilters round up to a whole inch, make the units, and trim each one down to the unfinished size. It costs a sliver of fabric and a few minutes with a square ruler, and it buys perfect points. The checkbox above switches between the two philosophies.
Sources and methodology
2 at a time: starting square = finished size + 7/8" (+1" when trimming). 8 at a time: starting square = (finished + 7/8") x 2 (or (finished + 1") x 2 to trim). 4 at a time: starting square = unfinished size / 0.64, plus 1/4" trim room when selected, rounded up to the next 1/4"; this method always requires squaring up. Formulas verified against multiple independent published quilting references, June 2026. Yardage assumes 42" fabric, strips cut across the width.
HST questions, answered
Geometry. When two triangles are joined on the diagonal with 1/4" seams, the seam crossing the bias eats 7/8" of the original square: 1/2" for the two seam allowances and a bit more than 3/8" lost where the seam crosses the corners at 45 degrees. It is the rare quilting number that comes from trigonometry rather than tradition.
Unfinished is the size of the unit on your cutting mat, seam allowances included. Finished is its size once sewn into the quilt, which is 1/2" smaller in each direction. Patterns usually name the finished size; rulers measure the unfinished one. This calculator asks for finished and reports both, so there is no off-by-a-half heartache.
Because every outer edge of a 4 at a time unit is cut on the bias, the stretchiest direction of woven fabric. Press by lifting the iron straight up and down rather than sliding it, handle the units as little as possible, and sew them into the quilt soon after trimming. The method is fast; the units just need gentle manners.
Exactly what the method name says: two, four, or eight per pair of starting squares. So 40 HSTs takes 20 pairs two at a time, 10 pairs four at a time, or just 5 pairs eight at a time. The calculator does that division, rounds up, and tells you how many spares you will have for the orphan block box.