Half Square Triangle Calculator: Cut Sizes and Yardage for 2, 4 and 8 at a Time

A half square triangle, or HST, is a square made from two right triangles sewn along the diagonal. Enter the finished size and how many you need. The calculator gives the square size to cut, how many squares of each fabric, and the yardage, for all three popular sewing methods.

Tools for trimming dozens of HSTs

Forty or more HSTs is where a slotted trimming ruler and a rotating mat start saving real time.

Intermediate

Bloc Loc

Bloc Loc Half-Square Triangle Ruler Set #2

$72.75

Slotted rulers in 2 1/2, 4 1/2 and 6 1/2 in that lock onto the seam for fast, accurate HST trimming.

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Intermediate

Olfa

Olfa 17 x 17 Rotating Cutting Mat RM-17S

$100.98

A 17 in rotating mat so you trim all four sides without moving the unit.

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Expert

AccuQuilt

AccuQuilt GO! Fabric Cutter Starter Set

$329.99

A die cutter with a value die that includes an HST shape, cutting up to six layers at once.

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Prices change often, so confirm on Amazon. As an Amazon Associate we earn from qualifying purchases.

How do you calculate half square triangle cut sizes?

Every HST method starts with two squares of fabric, one light and one dark, placed right sides together. What changes is how you sew and cut them, and that sets how much bigger than the finished size the squares must be. All sizes below are for a 3 inch finished HST, which measures 3 1/2 inches before it is sewn into a block.

1. Two at a time: finished size + 7/8 inch. Draw one diagonal, sew 1/4 inch on both sides, cut on the line. 3 + 7/8 = 3 7/8 inches, or 4 inches to trim. Each pair of squares makes 2 HSTs.
2. Four at a time: finished size x 1.414 + 1 1/4 inches, rounded up to the next 1/8. Sew 1/4 inch around all four edges, then cut on both diagonals. 3 x 1.414 + 1.25 = 5.49, so cut 5 1/2 inches, or 5 3/4 to trim. Each pair makes 4.
3. Eight at a time: twice the finished size + 1 3/4 inches. Draw both diagonals, sew both sides of each, cut vertically, horizontally and on the diagonals. 2 x 3 + 1 3/4 = 7 3/4 inches, or 8 to trim. Each pair makes 8.

Then the yardage. For 48 HSTs by two at a time you need 24 pairs, so 24 squares of each fabric. 4 inch squares fit 10 across a 40 inch strip, so 24 squares take 3 strips 4 inches wide, 12 inches of fabric, which rounds up to 3/8 yard of each fabric.

The seven eighths in the two-at-a-time formula is not arbitrary. Two 1/4 inch seam allowances make 1/2 inch, and the diagonal seam uses up the rest, because a 1/4 inch seam measured across a diagonal takes about 3/8 inch off each leg of the triangle. Four at a time needs the 1.414 multiplier because the HST ends up turned 45 degrees inside the square you cut, so its sides run along the diagonals.

What size do you cut squares for each HST size?

This chart uses the same formulas as the calculator. Each size links to a page with blocks that use it and precut options.

How much fabric do you need for 100 half square triangles?

Yardage per fabric for 100 finished HSTs, using the oversize-and-trim cut for each method and 40 inches of usable width. You need this amount of each of the two fabrics.

The methods use similar amounts of fabric. The bigger difference is time: eight at a time means cutting a quarter as many squares and marking fewer lines, while two at a time is the most forgiving for beginners. Our step-by-step guide to making half square triangles walks through each method.

Which HST method should you choose?

• Two at a time keeps straight grain on all four outside edges, so units stay square and do not stretch. It is the best method for beginners, for scrappy quilts where every HST uses a different pair, and for charm squares.
• Four at a time is fast because there is no marking, just sewing around the edge. The trade-off is that all four outside edges of every unit are on the bias and stretch easily. Starch the fabric before cutting and handle the units gently.
• Eight at a time gives the most HSTs for the least marking and cutting. Outside edges are a mix of straight grain and bias. It is ideal for quilts with lots of matching HSTs, like a queen-size chevron.

If a pattern calls for more than a couple hundred HSTs, consider cutting triangles with a die cutter instead. Die-cut HST shapes come with the seam allowance and dog ears already trimmed, which skips both the marking and the trimming steps. The results panel switches to die-cutting tools when your quantity reaches 200.

Can you make HSTs from charm packs and layer cakes?

Yes. A 5 inch charm square makes two-at-a-time HSTs up to 4 1/8 inches finished, since 4 1/8 + 7/8 = 5. A 10 inch layer cake square makes eight-at-a-time HSTs up to 4 1/8 inches finished, or two at a time up to 9 1/8. The precut fabric calculator compares packs by area, and the flying geese calculator handles the other unit that pairs with HSTs in star blocks.