Half Square Triangle Calculator

Enter your desired finished HST size (in inches) and choose your preferred quilting method — the Half Square Triangle Calculator works out exactly what size squares to cut. Get cutting measurements for 2-at-a-time, 4-at-a-time, and 8-at-a-time methods, plus the unfinished (trim) size for each approach. No more quilt math headaches.

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The size your HST will be once sewn into your project (not including seam allowance).

Results

Unfinished (Trim-To) Size

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Cut Size — 2-at-a-Time Method

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Cut Size — 4-at-a-Time Method

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Cut Size — 8-at-a-Time Method

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2-at-a-Time (nearest ¼")

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4-at-a-Time (rounded up to nearest ¼")

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8-at-a-Time (rounded up to nearest ¼")

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Cut Square Size by Method (inches)

Results Table

Frequently Asked Questions

What is a half square triangle (HST)?

A half square triangle is a square quilt unit made from two right-angle triangles sewn together along their diagonal. When you sew two HSTs together, you get a square block. They are one of the most common building blocks in quilt design.

What is the difference between finished size and unfinished size?

The finished size is the measurement of your HST once it is completely sewn into your quilt project — with all seam allowances consumed. The unfinished size is what you trim your HST to before sewing it in, which is always 0.5" larger than the finished size to account for a ¼" seam allowance on each side.

How do I calculate what size to cut squares for half square triangles?

For the 2-at-a-time method, add 0.875" (⅞") to the finished size. For the 4-at-a-time method, multiply the finished size by 1.414 (√2) and add 1.25", then round up to the nearest ¼". For the 8-at-a-time method, multiply the finished size by 2 and add 1". This calculator does all of that math automatically.

Why should I round up for the 4-at-a-time method?

The 4-at-a-time method involves cutting squares on the diagonal twice, which means any tiny measurement error can compound. Rounding up to the nearest ¼" gives you a little extra fabric so your HSTs come out the right size after trimming. You'll trim them down to the exact unfinished size before assembling your quilt.

Which method — 2-at-a-time, 4-at-a-time, or 8-at-a-time — is best?

The 2-at-a-time method is the most beginner-friendly and uses the least fabric. The 4-at-a-time method is great when you need many matching HSTs quickly. The 8-at-a-time method is the most efficient for bulk production and is popular for scrappy quilts, though it requires more careful cutting and pressing.

Is a triangle always half a square?

In quilting terms, a half square triangle refers specifically to a right isosceles triangle — cut by slicing a square from corner to corner diagonally. So yes, in the quilting context an HST is always exactly half of a square, which is why the math is based on the square's dimensions.

What are the best fabrics for making half square triangles?

Tightly woven 100% quilting cotton is the most popular choice because it presses crisply and is easy to cut accurately. Avoid stretchy or loosely woven fabrics, as the bias edges of the triangles can stretch out of shape during sewing. Pre-washing your fabric is recommended to prevent shrinkage later.

Do I need to use a special ruler to trim HSTs?

A dedicated HST ruler or a square quilting ruler with a diagonal line makes trimming much easier and more accurate. These tools let you align the seam on the diagonal line and trim all four sides at once to get a perfectly square HST every time.

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