Half-square triangles run on one piece of arithmetic: the square you cut is always bigger than the triangle you want, so you have room to trim down to perfect points. The amount bigger depends on the method. Once you've made a few quilts you'll have the common sizes memorized, but until then a chart at your cutting table saves a lot of second-guessing. Here are the numbers that matter, laid out so you can find your size at a glance.
How to read the chart
Every HST has three measurements, and it helps to keep them straight:
- Finished size — how big the HST is in the quilt, after it's sewn into the block with the seams swallowed. This is the number your pattern gives you.
- Cut size — the size square you actually cut from your fabric. Always larger than finished, because it includes seam allowance and trimming room.
- Trim-to (unfinished) size — the size you trim each unit down to before it goes into the block. This is always the finished size plus ½ inch.
So a 3-inch finished HST is trimmed to 3½ inches unfinished, and then loses another ¼ inch on each side when it's sewn in, landing at 3 inches in the quilt. The half inch isn't waste — it's the ¼-inch seam allowance on each of the two sides that get sewn.
Cut sizes for 2-at-a-time HSTs
For the 2-at-a-time method, the cut square is the finished size plus 7/8 inch. Pair one light and one dark, and each square you cut yields two HSTs.
| Finished size | Cut squares at | Trim each unit to |
|---|---|---|
| 1 inch | 1⅞ inch | 1½ inch |
| 1½ inch | 2⅜ inch | 2 inch |
| 2 inch | 2⅞ inch | 2½ inch |
| 2½ inch | 3⅜ inch | 3 inch |
| 3 inch | 3⅞ inch | 3½ inch |
| 3½ inch | 4⅜ inch | 4 inch |
| 4 inch | 4⅞ inch | 4½ inch |
| 4½ inch | 5⅜ inch | 5 inch |
| 5 inch | 5⅞ inch | 5½ inch |
| 6 inch | 6⅞ inch | 6½ inch |
Cut sizes for 8-at-a-time HSTs
For the 8-at-a-time method, the cut square is (finished size × 2) plus 1¾ inch. Each pair of squares yields eight HSTs, so these squares are noticeably larger — that's the trade for making so many at once.
| Finished size | Cut squares at | Trim each unit to |
|---|---|---|
| 1 inch | 3¾ inch | 1½ inch |
| 1½ inch | 4¾ inch | 2 inch |
| 2 inch | 5¾ inch | 2½ inch |
| 2½ inch | 6¾ inch | 3 inch |
| 3 inch | 7¾ inch | 3½ inch |
| 3½ inch | 8¾ inch | 4 inch |
| 4 inch | 9¾ inch | 4½ inch |
| 4½ inch | 10¾ inch | 5 inch |
| 5 inch | 11¾ inch | 5½ inch |
| 6 inch | 13¾ inch | 6½ inch |
Rule of thumb: 2-at-a-time, add 7/8 inch to the finished size. 8-at-a-time, double the finished size and add 1¾ inch. Either way, trim every unit to finished plus ½ inch.
When the chart runs out
Two things send quilters past a printed chart. The first is an odd finished size — patterns love asking for a 4¾-inch or 5¼-inch HST, and those don't fall on the tidy half-inch rows above. The second is planning yardage: knowing the cut size is only half the job if you still need to work out how many squares to cut and how many you'll get.
That's exactly where the Half-Square Triangle Calculator earns its keep. Type in any finished size — odd ones included — and how many HSTs you need, and it returns the cut size, the number of squares of each fabric, the total units produced, and the trim-to measurement, for all three methods side by side. No mental fractions, no rounding mistakes at the cutting mat.
A note on the trimming
Both charts assume you'll trim. The whole reason these squares are oversized is to give you a little fabric to shave off so every unit finishes identical with the seam landing exactly corner to corner. A square ruler with a clear 45-degree line is the tool for it — line the diagonal up with the seam and trim two sides, rotate, trim the other two. A dedicated half square triangle ruler makes the job faster, though any good square quilting ruler works.
Keep going
If you're deciding which method suits your project, the comparison of the 2, 4, and 8 at a time methods lays out the trade-offs. For a deeper look at where that 8-at-a-time formula comes from, see the math behind the 8-at-a-time method. And once your top is pieced, the standard batting sizes guide will help you buy batting that actually fits.