Practical tools for measured work
Half-Square Triangle Cutting Calculator
Find exact source-square counts, trim-friendly cutting size and batch instructions for two-, four- or eight-at-a-time HSTs.
Calculate the layout, check the diagram, then print the result for the shop or jobsite.
Size after the unit is sewn into the quilt.
Added to the exact source-square dimension.
Pair one light and one dark square right sides together. Mark one diagonal, sew 1/4 in on both sides, cut on the marked line, press and trim.
How it works
What to cut: cut the shown number of equal squares from each of two contrasting fabrics. The batch count rounds up so the result never leaves the project short.
Sizing: two-at-a-time uses finished size + 7/8 in before the entered trim extra. Four-at-a-time uses (finished size + 1/2 in) ÷ 0.7071 + 3/8 in for its perimeter-sewn geometry. Eight-at-a-time doubles the two-at-a-time source square. Metric input is converted around the same 1/4 in (6.35 mm) quilting seam convention.
Trimming: the optional extra makes oversized units. Press first, then trim each HST to finished size + 1/2 in unfinished size.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Measurement unit
- Inches
- Finished HST size
- 3
- HSTs required
- 24
- Construction method
- Two at a time
- Extra trim allowance
- 0.125
- Usable fabric width
- 40
Primary result: Cut square size: 4 in.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
Quarter-inch seam accuracy, thread, pressing and bias stretch change the sewn size. Make a test batch before cutting all fabric. Four-at-a-time places every outside edge on the bias. Fabric length assumes simple square rows across usable WOF and does not include washing shrinkage or cutting defects.
Frequently asked questions
What does finished HST size mean?
It is the visible square after the HST is sewn into the quilt. The unit before assembly is 1/2 inch larger in both directions because it still includes seam allowances.
Which method should I use?
Two-at-a-time is familiar and stable, eight-at-a-time is efficient for larger repeat counts, and four-at-a-time is quick but creates bias outside edges that need careful handling.
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