Half Square Triangle Calculator

Adjust the calculator values below

Raw fabric square cut size 5.875 in
Finished HST square side length 5 in
Total cutting allowance 0.875 in
Raw square diagonal 8.309 in
Finished HST square diagonal 7.071 in
Raw square area 34.516 in²
Finished HST unit area 25 in²
Fabric square pairs required 1 pair
Raw fabric squares to cut 2 squares
Squares from fabric A 1 square
Squares from fabric B 1 square
Finished HST units produced 2 units
Extra finished units 0 units
Construction method Two-at-a-time HST method
5.875 in
Raw fabric square cut size 5 in finished HST + 7/8 in traditional cutting allowance
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Half Square Triangle Calculator

Calculate raw cut size, finished HST size, diagonals, and fabric-square quantities for two-at-a-time quilting with exact or trim-up allowances.

Size a half-square triangle quilt unit

A half-square triangle (HST) unit is a finished square made from two right triangles of contrasting fabric. This calculator is for the common two-at-a-time method, in which two equal raw squares are placed right sides together, sewn on both sides of a marked diagonal, and cut apart to produce two HST square units.

Use the correct total cutting allowance

For the traditional exact method, raw square side length equals finished HST side length plus 7/8 inch. The 7/8 inch is a total cutting-size allowance; it is not 7/8 of the selected metric unit and not the quarter-inch seam allowance itself. The oversize option adds 1 inch so the sewn units can be trimmed accurately.

Calculate forward or reverse without circular inputs

Finished-to-cut mode adds the selected allowance to the desired finished square. Cut-to-finished mode subtracts that allowance from an existing raw square. The calculator derives diagonals, areas, and cut quantities from the same consistent sizes rather than asking you to enter dependent measurements.

Round cut quantities to complete fabric pairs

Each two-at-a-time fabric pair uses one square from fabric A and one from fabric B and yields two HST units. An odd requested quantity therefore requires rounding up to the next pair and produces one extra unit. Directional prints, fussy cutting, fabric defects, test blocks, and other construction methods can require additional fabric.

Frequently asked questions

Common questions about HST finished and raw sizes, 7/8-inch and 1-inch allowances, two-at-a-time cutting, mixed units, and fabric-square quantities.

What is a half-square triangle unit?

An HST unit is a finished square made from two right triangles, commonly of contrasting fabrics. The two-at-a-time method sews two equal raw squares together and cuts them apart to create two HST units.

Why does the traditional method add 7/8 inch?

The traditional two-at-a-time cutting rule adds a total 7/8 inch to the desired finished square side to account for diagonal seams and seam allowances. It is a fixed 7/8 inch, converted when metric units are selected.

Should I use 7/8 inch or 1 inch?

Use 7/8 inch for the traditional exact cut. Use the 1-inch oversize option when you prefer extra material for pressing and squaring up, then trim each sewn HST to the exact finished size.

What does custom total allowance mean?

It is the full amount added to raw square side length beyond the desired finished size. It is not the seam allowance on one side. Use it when a pattern, ruler, sewing method, or personal test block specifies a different cut-size difference.

How many raw fabric squares do I need?

Each pair uses one square from fabric A and one from fabric B and yields two HST units. The calculator rounds an odd requested unit count up to a complete pair, producing one extra unit.

Does this work for four-at-a-time or eight-at-a-time HSTs?

No. The sizing and quantity rules here are specifically for the common two-at-a-time method. Four-at-a-time, eight-at-a-time, foundation-paper, and specialty-ruler methods use different cutting formulas and grain considerations.