Half Square Triangle Calculator
Convert between finished size and raw cut size for quilting.
The calculator uses a standard seam allowance of 7/8 inch, which is the common quilting reference for half square triangles.
Result
--
Run the calculation to see the cut size.
| Metric | Value |
|---|---|
| Finished size | -- |
| Raw cut size | -- |
| Seam allowance | -- |
| Raw square diagonal | -- |
Quick Answer
Add 7/8 inch to the finished square size to get the raw cut size for a half square triangle.
How to Calculate
- Choose the unit.
- Choose whether you want the raw or finished size.
- Enter the size and seam allowance.
- Click Calculate.
Formula
Raw cut size = finished size + seam allowance
Finished size = raw cut size - seam allowance
Worked Examples
Example 1: A finished 5 inch square needs a raw cut size of 5 7/8 inches.
Example 2: A finished 10 cm square needs a raw cut size of about 12.22 cm when using a 2.22 cm seam allowance.
Example 3: A raw 8 inch square with a 7/8 inch seam allowance finishes at 7 1/8 inches.
How to Interpret Your Results
| Output | Meaning | Action |
|---|---|---|
| Raw cut size | The square you should cut before sewing. | Use this for ruler marking and cutting. |
| Finished size | The size after seams are sewn and pressed. | Use this for quilt block planning. |
| Diagonal | The length across the raw square corner to corner. | Use it when checking triangle placement. |
Frequently Asked Questions
The extra allowance gives enough fabric for the seam and trimming.
Yes. You can enter any seam allowance that matches your sewing method.
Use the calculator for the square size, then cut the diagonal to create the two half-square triangles.
Related Calculators
References
- OmniCalculator reference page
- The standard rule is to add 7/8 inch to the finished square size.
- Last reviewed: March 2026.