Open Quarter-square triangles in the studio. The form on this page is that calculator.
Why hourglass blocks are cut oversized
A quarter-square triangle block, often called an hourglass, is four triangles in a square. The quick method makes four of them from two squares: place the squares right sides together, sew a quarter-inch seam around all four sides, and cut on both diagonals. Each piece opens into an hourglass.
The squares are cut 1 1/4 inches larger than the finished block, which is a quarter inch beyond the usual seam allowance. That extra is what you trim. Hourglasses go skewed if you skip the trim. A square ruler with a diagonal line is the easy way to keep the center on center.
How the hourglass square is calculated
- Cut square = finished size + 1 1/4 inches
- Squares to cut = two for every four blocks, rounded up when the count is not a multiple of four
- Trim each block to = finished size + 1/2 inch
These are not the triangles that fill the edge of an on-point quilt. Those are cut so the straight grain sits on the outside of the quilt. Use the setting triangle calculator for them. Half-square triangles, two, four, and eight at a time, are on the HST calculator.
Hourglass cutting chart
Two squares make four blocks. Trim every block, even when the count comes out even.
| Finished block | Cut each square | Trim the block to |
|---|---|---|
| 2 in | 3 1/4 in | 2 1/2 in |
| 3 in | 4 1/4 in | 3 1/2 in |
| 4 in | 5 1/4 in | 4 1/2 in |
| 6 in | 7 1/4 in | 6 1/2 in |
| 8 in | 9 1/4 in | 8 1/2 in |
How to use this calculator
- Enter the finished hourglass, the size of the block after it is sewn into the quilt.
- Enter how many blocks you need. Eight blocks is four pairs of squares. Six blocks is still four pairs, because the method makes four at a time and you will have two spare.
- Cut the squares at the size on the card, one of each fabric in each pair. Sew, cut both diagonals, press, and trim to the unfinished size.
Tips for hourglasses
- Trim to the diagonal, not just to the outside edge. A block that is the right size with the center seam off will throw the next seam.
- Press the seams open or to one side and stay consistent. The center of an hourglass is four seams meeting, and it gets bulky.
- A setting triangle is a different cut. Using this square for the edge of an on-point quilt puts the bias on the outside, and the edge stretches while you quilt it.
When the piece is part of a whole quilt. An hourglass quilt is a grid of these blocks. The studio counts the squares once the layout is drawn. The fabric calculator counts every piece in a design, then adds binding, backing, and batting. These pages answer one question at a time.