Quick answer
Diagonal = √(length² + width²). Equal diagonals are a useful geometry check, not a tolerance or construction-acceptance standard.
| Rectangle | Diagonal | Exact basis |
|---|---|---|
| 3 × 4 | 5.00 ft | 3-4-5 |
| 6 × 8 | 10.00 ft | scaled 3-4-5 |
| 9 × 12 | 15.00 ft | scaled 3-4-5 |
| 10 × 10 | 14.14 ft | √200 |
| 10 × 12 | 15.62 ft | √244 |
| 12 × 16 | 20.00 ft | scaled 3-4-5 |
| 12 × 20 | 23.32 ft | √544 |
| 16 × 20 | 25.61 ft | √656 |
| 20 × 20 | 28.28 ft | √800 |
How to use this chart
- Match the two measured side lengths to a listed rectangle.
- Compare the theoretical diagonal with the field diagonal measured between the same corners.
- Use the Rectangle Diagonal Calculator for dimensions not shown here or to compare two field diagonals.
Data basis
Generated exactly from the Pythagorean theorem diagonal = √(length² + width²).