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 exact imperial figure is finished + 7⁄8" (2.2cm), which covers a ¼" (6mm) seam each side of the diagonal plus the cut. The 2.5cm default rounds that up so you can trim each unit down to an accurate finished size.

What is the 4-at-once method?

Layer a light and a dark square right sides together, stitch a scant ¼" (about 0.6cm) all the way around the outside edge, then cut on both diagonals. This yields 4 HSTs from one pair of squares. Cut squares to the finished size divided by 0.64 (i.e. finished × 1.5625) — for a 7.5cm finished HST that is about 11.7cm, then add a small trim margin (0.5cm default). Unlike the 2-at-once method this is a multiplicative rule, so the extra fabric grows with the HST size rather than being a fixed addition.

What is the 8-at-once (Magic 8) method?

Cut two squares sized (finished × 2) + 4.5cm — roughly double the size of a 2-at-once square — then draw both diagonals, stitch a scant ¼" each side of both lines, and cut on the drawn lines plus horizontally and vertically through the centre. One pair of these larger squares makes 8 HSTs, which is why it is the most fabric-efficient method for big batches.

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 the finished (sewn and trimmed) HST square

Number of HSTs Needed

Total finished HST units you need for the project

Construction MethodHow many HSTs you make per pair of light + dark squares

Fabric Width

Usable width of quilting cotton (UK bolt ~110-112cm)

Fabric Price (optional)

Fabric price per metre, £ (estimates cost for both colours)

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. The exact metric figures come from the imperial quilting standards (¼" / 6mm seam each side of the diagonal) — the defaults below round up slightly so you can trim each unit down to an accurate finished size.

Cutting formulas

2-at-once: Cut size = Finished size + 2.5cm (exact 7⁄8" = 2.2cm). One light + one dark square → 2 HSTs.

4-at-once: Cut size = Finished size ÷ 0.64 (= Finished × 1.5625), plus a small trim margin. One pair, stitched all four edges and cut on both diagonals → 4 HSTs. This is a multiplicative rule, so the cut square grows in proportion to the finished size rather than by a fixed amount.

8-at-once (Magic 8): Cut size = (Finished size × 2) + 4.5cm (exact 1¾" = 4.45cm). One pair of these larger squares → 8 HSTs.

Frequently Asked Questions

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Last updated: February 2026

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