Sewing & crafts
Half-square triangle cut planner
Plan two-at-a-time HST starting squares with explicit trimming allowance.
A clear answer. Here’s how.
For a two-at-a-time diagonal construction, theoretical starting square side = finished side + 2s + √2s, where s is seam allowance. Adds your trim allowance to the starting square and rounds up to the next ⅛ inch. Each pair of starting squares yields two units; odd requested counts produce one spare. Results assume accurate seams and trimming, not a sewing tutorial.
See a worked example
Worked example
A fixed demonstration, separate from the values you enter above.
- Finished HST side (in)
- 4
- Seam allowance (in)
- 0.25
- Extra starting-square trim allowance (in)
- 0.125
- HST units required
- 24
Example result
- Finished HST side (in)
- 4
- Unfinished target side (in)
- 4.5
- Starting square side, rounded up to ⅛ in
- 5
- Square pairs to cut
- 12
- HST units produced
- 24
Two-at-a-time method: theoretical starting side = finished + 2s + √2s, plus supplied trimming allowance, rounded up to ⅛ inch. Each square pair yields two HST units.