Half-Square Triangle Calculator — Quilting HST Cut Size
Enter the desired finished HST size and how many units you need. The calculator gives the starting cut square size (using the standard 7⁄8″ seam allowance), the number of square pairs to cut, and total squares needed from each fabric.
Unit
Starting square size for each fabric square (finished + 7⁄8")
- 1
Seam allowance
0.8757⁄8 inch standard allowance — two ¼″ seam lines plus 3⁄8″ for the diagonal cut - 2
Square pairs to cut = ⌈count ÷ 2⌉
⌈12 ÷ 2⌉ = 6 - 3
Cut square size = finished + seam allowance
3 + 0.875 = 3.875
How does this calculator work?
Cut squares at finished size + 7⁄8" (2.22 cm). Each pair of squares yields 2 HSTs. Pairs needed = ⌈N ÷ 2⌉. For 12 finished HSTs at 3″: cut 3.875″ squares, cut 6 pairs (12 squares per fabric). The formula is the same for any size — just add 7⁄8" to find the starting square.
Formula
How this is calculated
In the standard two-at-a-time half-square triangle method, you place two same-sized squares right sides together, draw a diagonal line from corner to corner, sew 1⁄4″ on each side of the line, then cut along the drawn diagonal. This yields two identical HST units. The 7⁄8″ addition to the finished size accounts for the two 1⁄4″ seam allowances (1⁄2″ total) plus an extra 3⁄8″ lost to the cut itself and the slight take-up from pressing the seams open.
For metric quilters, the equivalent addition is approximately 2.22 cm (exact: 7⁄8″ × 2.54 = 2.2225 cm). Many metric quilting references round this to 2.2 cm or 2.5 cm for ease of cutting — this calculator uses the exact conversion so you can decide your own rounding.
Since each pair of squares produces exactly 2 HSTs, the number of pairs is ⌈N/2⌉ (ceiling division). You then cut that many squares from each fabric. If you need an odd number of HSTs, one will be a spare. The unfinished HST size (finished + 1⁄2″ seam allowance only) is the measured size of the triangle before it is sewn into a block.
Frequently asked questions
The 7⁄8″ (0.875″) addition accounts for two 1⁄4″ seam allowances (one on each cut half = 1⁄2″ total) plus 3⁄8″ for the fabric lost along the diagonal cut and seam take-up when pressing. The result is the exact cut size that leaves a finished HST at the size you specify.
No — specialty rulers and triangle papers have their own cut-size rules, usually adding only 1⁄2″. This calculator uses the classic draw-and-sew method. Always check the instructions for your specific tool.
The formula rounds up to the next whole pair, so you will produce one extra HST. Keep it as a spare, use it in another project, or reduce your count by one if you don't mind having exactly the right number.
Also known as
TG we-Calculate Editorial Team. (2026). Half-Square Triangle Calculator — Quilting HST Cut Size [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/half-square-triangle-calculator
TG we-Calculate Editorial Team. "Half-Square Triangle Calculator — Quilting HST Cut Size." TG we-Calculate. 2026. https://we-calculate.com/calculator/half-square-triangle-calculator.
TG we-Calculate Editorial Team, "Half-Square Triangle Calculator — Quilting HST Cut Size," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/half-square-triangle-calculator
@misc{wecalculate_half_square_triangle_calculator, title = {Half-Square Triangle Calculator — Quilting HST Cut Size}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/half-square-triangle-calculator}}, year = {2026}, note = {TG we-Calculate} }
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