How it works
Exact: cut square = finished + 1¼"
Oversize and trim: cut square = finished + 1½", trim to finished + ½"
Squares of each fabric = round up( QSTs ÷ 2 ) (2 squares make 2 QSTs)
A quarter-square triangle unit, or hourglass, is a square made of four triangles that meet in the middle. The usual quick method starts with two squares, one of each fabric. Sew them into two half-square triangles, put those two together with the colors opposite, draw the other diagonal, sew 1/4" either side and cut. Two squares give you two QSTs.
The outer edge of each small triangle is the long side, so the cut square has to cover the finished size plus seam allowance on three edges, two of them diagonal. Worked through, that comes to the finished size plus about 1.21", which quilters round to 1 1/4". A 4" finished QST is cut from 5 1/4" squares.
Hourglass points are easy to lose, so many quilters cut 1/4" bigger, finished plus 1 1/2", and trim each unit to the unfinished size (finished plus 1/2") with the center seam on the ruler diagonal. The calculator shows both.
Since every pair of squares makes two units, you need half as many squares of each fabric as QSTs, rounded up.
Worked example
For 24 QSTs finishing at 4", cut 12 squares at 5 1/4" from each fabric. At 40" wide, 7 squares fit in one strip, so 2 strips and 3/8 yd of each fabric. To oversize and trim, cut 5 1/2" squares instead (3/8 yd each) and trim the units to 4 1/2".
Common mistakes
- Using the HST allowance (+7/8"). A QST has two diagonal seams, so it needs +1 1/4"; +7/8" squares come out about 3/8" too small.
- Cutting squares into quarters first. Cutting a square twice diagonally leaves bias edges on the outside of the unit, which stretch. The two-at-a-time method keeps straight grain on the outer edges.
- Trimming off-center. Line the ruler diagonal up with a seam and put the center of the unit on the ruler center before you trim.
- Pressing seams the same way. Press the first seams toward the darker fabric so they nest when you pair the HSTs.
Last checked October 2026 against standard quilting and knitting math. Found a mistake? Tell us and we’ll fix it.