How to use the half square triangle calculator
- Enter the finished unit size in inches.
- To count how many squares to cut, enter the number of units you need.
- Open More options if your seam allowance is not 1/4 inch.
- Cut the squares at the rounded size shown, pair them right sides together, sew either side of a diagonal and cut apart.
Formula
exact square = finished size + seam allowance x (2 + sqrt 2)
cut size = exact square rounded up to the next 1/8 inch
squares of each fabric = units needed / 2, rounded up
The extra seam allowance x sqrt 2 accounts for the diagonal seam, and the 2 x seam allowance covers the outer edges. With 1/4 inch seams the exact figure is finished + 0.8536 inch, so rounding up to an eighth gives the familiar finished + 7/8 inch.
A 3 inch finished unit
For a 3 inch finished unit with 1/4 inch seams, the exact square is 3 + 0.25 x 3.4142 = 3.8536 inches. Rounded up to the next eighth, you cut 3.875 inches, or 3 7/8 inches. A 2 inch unit needs 2 7/8 inches and a 2.5 inch unit needs 3 3/8 inches.
Twenty-five units need 25 / 2 = 12.5 squares of each fabric, so cut 13 of each. That makes 26 units, one more than you need. A 3/8 inch seam allowance for a 3 inch unit gives 4.2803 inches, rounded up to 4 3/8 inches.
Why the squares are cut a little large
The unit shrinks in a few ways: the seam allowance on each outer edge, the diagonal seam, and the small amount that the folded seam and pressing use. Cutting at the rounded-up size leaves a hair of extra fabric. You can trim each unit to the unfinished size, which is the finished size plus two seam allowances, for crisp points in the quilt.
Another way is to cut larger squares, such as the finished size plus 1 inch, and trim each unit down. It wastes a little more fabric but gives precise units. The page shows that size too.
Where to go next
Plan the block layout with the Quilt Block Size Calculator and the full quilt with the Quilt Sashing and Border Calculator. The Fat Quarter Cutting Calculator shows how many squares come from one fat quarter, and the Quilt Backing and Binding Calculator covers the back.
Assumptions and limits
- The two-at-a-time method is assumed: two squares make two units.
- The seam allowance is the same on every seam and starts at the example value of 1/4 inch.
- The cut size is rounded up to the next 1/8 inch to match common rulers.
- Test one unit before cutting the whole project.
Frequently asked questions
What size square do I cut for a half square triangle?
Add 7/8 inch to the finished size when you sew with 1/4 inch seams. For a 3 inch finished unit, cut 3 7/8 inch squares.
Why is it finished size plus 7/8 inch?
The outer edges need two 1/4 inch seam allowances, and the diagonal seam needs another 1/4 inch times the square root of 2. Together that is about 0.85 inch, which rounds up to 7/8 inch.
How many squares do I need for a number of half square triangles?
Each pair of squares, one from each fabric, makes 2 units. For 25 units cut 13 squares of each fabric, which gives 26 units.
Does the size change with a different seam allowance?
Yes. The page uses the seam allowance you enter. A larger allowance asks for a larger square, and the rounded size updates.