What size do I cut squares for HSTs?

For the 2-at-once method, cut squares 2.5cm larger than the finished HST size. For a 7.5cm finished HST, cut 10cm squares. The extra allows for seam allowances and trimming.

What is the 4-at-once method?

Draw two diagonal lines on a layered pair of squares, sew 0.75cm from both sides of both lines, then cut apart on the lines. This yields 4 HSTs from one pair of squares. Cut squares 3.5cm larger than the finished size.

Should I trim my HSTs?

Yes, always trim HSTs to exact size after pressing. This ensures accuracy when joining blocks. Use a square ruler with a diagonal line for precise trimming.

How do I press HST seams?

Press seams towards the darker fabric to prevent show-through. Press flat first to set the stitches, then open the unit and press from the right side. Some quilters prefer pressing seams open for flatness.

SewCalcs

Half Square Triangle Calculator — HST Cutting Sizes

Calculate fabric and cutting sizes for half-square triangle units.

Finished HST Size

One side of finished square

Number of HSTs Needed

Construction Method

Fabric Width

Fabric Price (optional)

How We Calculate This

HST cutting sizes depend on the construction method. Each method produces a different number of HST units from a pair of squares.

Cutting formulas

2-at-once: Cut size = Finished size + 2.5cm

4-at-once: Cut size = Finished size + 3.5cm

8-at-once: Cut size = Finished size + 2.5cm (larger grid method)

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify quantities before cutting or purchasing fabric.