Half-Square Triangle Calculator

Calculate the starting square size for half-square triangles (HSTs) based on finished size, seam allowance, and construction method.

Finished HST Size

Seam Allowance

Construction Method

Tab to move between fields

Starting Square Size

3.88 in

Unfinished HST Size3.5 in

HSTs per Pair of Squares2

MethodTwo-at-Once

Starting Squares for 20 HSTs10

Trim-to Size3.5 in

How to Use This Calculator

Enter Finished HST Size, Seam Allowance, and Construction Method.
Set Two-at-Once (standard), Four-at-Once, and Eight-at-Once.
Review the Starting Square Size result.
Use Unfinished HST Size and HSTs per Pair of Squares to inform your decision.
Use the chart to visualize the results and explore different scenarios by adjusting inputs.

How the result changes with Finished HST Size

Finished HST Size: The finished size of the half-square triangle unit in inches.
Seam Allowance: Seam allowance per side. Standard quilting is 1/4 inch.
Construction Method: Method determines the extra fabric added. Two-at-once adds 7/8 inch, four-at-once adds 1-1/4 inch.

Identify Input Parameters
Finished HST Size = 3, Seam Allowance = 0.25, Construction Method = 1 = 3 input(s) provided
Calculate Starting Square Size
Starting Square Size
3.875 = 3.875
Calculate Unfinished HST Size
Unfinished HST Size
3.5 = 3.5
Calculate HSTs per Pair of Squares
HSTs per Pair of Squares
2 = 2

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