Formula
- Use the units shown beside each field.
- Results update automatically whenever you change an input.
- Displayed values use up to four decimal places, or scientific notation for very small or large numbers. Calculations use standard floating-point arithmetic; exact integer and fraction tools identify their own precision rules.
- Geometric two-at-a-time method. Round upward to your convenient cutting increment; trim the sewn unit to the required unfinished size.
Example Calculation
- Finished square side: 4 in
- Seam allowance: 0.25 in
- Extra trimming margin: 0.125 in
A 4-inch finished square with 1/4-inch seams and 1/8-inch trim needs a starting square about 4.979 inches.
Frequently asked questions
How do I use this calculator?
Enter finished square side, seam allowance, extra trimming margin. The result updates automatically. Use Reset to restore the example values.
What method does it use?
starting square=finished side+2 seam+seam√2+trimming margin.. Geometric two-at-a-time method. Round upward to your convenient cutting increment; trim the sewn unit to the required unfinished size.
Why might a rounded result differ?
The calculation keeps full numeric precision internally, then rounds the displayed result. Rounding intermediate steps by hand can produce a slightly different answer.
Are my inputs saved online?
No. This calculator processes your inputs in your browser. A recently used list stores only calculator names on this device, not your entered values.
Check the assumptions and units before using this result. Read about calculation methods.