Quick answer: how to calculate QSTs
A quarter-square triangle, or QST, is a square made from four triangles, sewn together on the diagonals, usually with two different fabrics in an hourglass layout. Its finished size is the size in the finished quilt, after seams are sewn.
For the two-square method, cut square size equals finished QST size plus 1 and 1/4 inches, which is 1.25 inches. One set, with 2 squares, one of each fabric, makes 4 QSTs.
For example, a 6 inch finished QST needs 7.25 inch cut squares. If you need 18 QSTs, you need 5 sets, 5 squares per fabric, 10 squares total, and you make 20 QSTs, with 2 extra QSTs.
Use the QST Calculator with your own finished size and QST count, to check cut size and sets needed before you cut.
How to calculate QSTs
What you need to measure
You need two inputs only: finished QST size, and QSTs needed. Finished QST size is the size of one QST in the finished quilt, after seams are sewn, not the cut size. QSTs needed is a whole number, greater than zero. Count QSTs by fabric pair, not by quilt, if different fabric pairs use different finished sizes.
Formula and calculation rule
Cut square size equals finished QST size plus 1.25 inches, for the two-square method. The extra 1.25 inches covers seam allowances on the diagonal seams and trimming to finished size. Sets needed equals QSTs needed divided by 4, rounded up to a whole number, because each set of 2 squares makes 4 QSTs, and a partial set cannot be sewn. Squares per fabric equals sets needed times 1, because each set uses 1 square of each fabric. Total squares equals squares per fabric times 2, because each set uses one square of each fabric. QSTs made equals sets needed times 4, because each set makes 4 QSTs.
Step by step example
Take finished QST size 6 inches, QSTs needed 18.
Cut square size: 6 plus 1.25 equals 7.25 inches.
Sets needed: 18 divided by 4 equals 4.5, rounded up to 5 sets.
Squares per fabric: 5 times 1 equals 5 squares per fabric.
Total squares: 5 times 2 equals 10 squares.
QSTs made: 5 times 4 equals 20 QSTs, with 2 extra QSTs, because 18 is not a multiple of 4.
QST cut size chart
| Finished QST size | Cut square size |
|---|---|
| 2 inches | 3.25 inches |
| 2.5 inches | 3.75 inches |
| 3 inches | 4.25 inches |
| 3.5 inches | 4.75 inches |
| 4 inches | 5.25 inches |
| 5 inches | 6.25 inches |
| 6 inches | 7.25 inches |
| 8 inches | 9.25 inches |
Use this chart for common finished sizes. For other finished sizes, use the calculator above, with the same formula: finished size plus 1.25 inches.
Important variations and exceptions
- QST count not a multiple of 4. If QSTs needed is not a multiple of 4, sets needed is rounded up, and QSTs made will be more than QSTs needed, which is extra QSTs, not a mistake. For example, 18 QSTs need 5 sets, and make 20 QSTs, with 2 extra QSTs.
- QST versus HST. A QST is not an HST with a different name. An HST is a square made from two triangles, with cut size finished size plus 7/8 inch, by the two-at-a-time method. A QST is a square made from four triangles, with cut size finished size plus 1.25 inches, by the two-square method. Do not use HST cut size for QSTs, or your QSTs will be too small.
- Other QST methods. This guide and calculator use the two-square method only. Other methods use different cut sizes and different square counts, and they are not calculated here. Do not use the two-square cut size for another method, or your QSTs will be the wrong size.
- Directional prints. If your fabric has a directional print and triangles must all run one way, check triangle orientation before cutting, because diagonal cutting turns print direction. This calculator counts squares, it does not check print direction.
Warning: four at a time does not mean the same thing everywhere
Do not trust the phrase four at a time on its own, because different sites mean different yields by it. On this site, one set uses two starting squares, one per fabric, and makes four QSTs. On other sites, a light and dark pair goes through half-square triangles first and makes two hourglass units, not four. The cut size is the same, finished plus 1.25 inches, but the squares per fabric and the units made are different. Always check the method, squares per fabric, and units made together, never the phrase alone.
Oversize option, stated plainly: some quilters cut finished plus 1.5 inches instead of plus 1.25 inches, then trim back to finished plus 0.5 inches, which is the unfinished size. That gives extra trimming room, at the cost of extra fabric and extra trimming time. This calculator gives the exact plus 1.25 inch cut, not the oversize cut. If you choose oversize cutting, add the extra yourself, and still trim to finished plus 0.5 inches.
Trimming and centre: trim from the centre out, with the diagonal lines on the ruler aligned with the seams, not just the outside edges, or the centre will drift off. At the bulky centre, spinning the seam helps the unit lie flat. Setting triangles are a different cut from hourglass units, even though both use triangles, so do not use this cut size for setting triangles.
How to make sure your result is correct
- Check finished size, not cut size. Measure finished QST size in the finished quilt, after seams are sewn, not cut size before seams are sewn. Using cut size in the formula gives a cut size too large by 1.25 inches.
- Check units. Finished size and cut size must be in the same unit, inches or centimetres. In centimetre mode, 1.25 inches equals 3.18 centimetres, rounded at two decimal places, because 1.25 times 2.54 equals 3.175 centimetres.
- Check QST count by fabric pair. Count QSTs by fabric pair, not by quilt, if different fabric pairs use different finished sizes. Adding QST counts for different fabric pairs into one calculation gives the wrong square count for each fabric pair.
- Check QSTs made against QSTs needed. QSTs made equals sets needed times 4. If QSTs made is less than QSTs needed, you have too few sets. If QSTs made is more than QSTs needed, and QSTs needed is not a multiple of 4, that is expected, not a mistake.
Try it with the calculator: Use the QST Calculator with your own finished size and QST count. It uses the same steps as this guide, and it shows cut square size, squares per fabric, total squares and QSTs made, together.
Worked example: 6 inch finished QST, 18 QSTs needed
This worked example is different from the tool page example, which uses 4 inch finished QST and 20 QSTs needed.
Cut square size: 6 plus 1.25 equals 7.25 inches.
Sets needed: 18 divided by 4 equals 4.5, rounded up to 5 sets.
Squares per fabric: 5 times 1 equals 5 squares per fabric.
Total squares: 5 times 2 equals 10 squares.
QSTs made: 5 times 4 equals 20 QSTs, with 2 extra QSTs, because 18 is not a multiple of 4.
Calculate your own QSTs now
Use your finished QST size and QSTs needed in the calculator, and check the result against the steps in this guide.
Open the QST CalculatorFrequently asked questions
For the two-square method, cut square size equals finished QST size plus 1 and 1/4 inches, which is 1.25 inches. For example, a 6 inch finished QST needs 7.25 inch cut squares.
A quarter-square triangle, or QST, is a square made from four triangles, sewn together on the diagonals, usually with two different fabrics in an hourglass layout. Its finished size is the size in the finished quilt, after seams are sewn.
For the two-square method, each set of 2 squares, one of each fabric, makes 4 QSTs. Sets needed equals QSTs needed divided by 4, rounded up to a whole number. For example, 18 QSTs need 5 sets, 5 squares per fabric, 10 squares total, and make 20 QSTs, with 2 extra QSTs.
An HST is a square made from two triangles, with cut size finished size plus 7/8 inch, by the two-at-a-time method. A QST is a square made from four triangles, with cut size finished size plus 1.25 inches, by the two-square method. Do not use HST cut size for QSTs, or your QSTs will be too small.
If QSTs needed is not a multiple of 4, sets needed is rounded up, and QSTs made will be more than QSTs needed. For example, 18 QSTs need 5 sets, and make 20 QSTs, with 2 extra QSTs. That is expected, not a mistake.
Related guides and calculators
Next, read How to Calculate Half-Square Triangles and How to Calculate Flying Geese for the other triangle units. For QSTs themselves, use the QST Calculator.