Half Square Triangle Calculator

Enter Calculation

Finished HST size (inches)

Method Traditional (1 starting square = 2 HSTs) Two-at-a-time (1 pair of squares = 2 HSTs)

Results

Cut Each Starting Square

3.875"

inches square (cut two squares to make 2 HSTs)

Finished HST size	3"
HSTs produced per pair	2
Seam allowance added	7/8" (0.875")

What Is a Half Square Triangle (HST)?

A half square triangle is one of the most common units in quilting — a square made of two right triangles joined along the diagonal. HSTs are used in countless block patterns like pinwheels, flying geese variations, and star blocks. The tricky part is figuring out how big to cut your starting fabric squares so the finished triangle ends up exactly the right size after sewing.

A half square triangle (HST): one square divided diagonally into two contrasting triangles.

How to Use This Calculator

Enter the finished size of the HST you want — that is, the size it will measure once it is sewn into the quilt with seams hidden. The calculator instantly tells you what size square to cut. Cut two squares of contrasting fabric at this size, layer them right sides together, draw a diagonal, sew 1/4" on each side of the line, cut on the line, and press open to yield two matching HSTs.

The Formula Explained

The rule is simple: $$\text{Cut Square} = \text{Finished HST} + 7/8"$$. The extra 7/8" (0.875") accounts for two 1/4" seam allowances plus the fabric lost across the diagonal. This is the standard "add seven-eighths" method used by quilters worldwide.

The cut square is larger than the finished HST by 7/8 inch to allow for seam allowances.

Worked Example

Suppose you want finished 3" HSTs for a star block. Cut size $$= 3 + 0.875 = \mathbf{3.875"}$$ (3⅞"). Cut two 3⅞" squares, sew and slice as above, and you get two crisp 3" finished triangles.

Two layered squares sewn and cut on the diagonal yield two half square triangle units.

FAQ

Why 7/8" and not 1/2"? The 1/4" seams on both sides total 1/2", and the diagonal cut consumes additional width, so 7/8" gives the correct allowance.

Should I cut a little bigger? Many quilters add 1" or even 1¼" instead of 7/8" to allow trimming the HST down to an exact size for perfect points.

Does this work for any size? Yes — the +7/8" rule works for any finished HST size in inches.