About this calculator
The diagonal of any rectangle, which is the fastest way to check a slab, deck or wall layout is square, plus a scaled 3-4-5 triangle you can mark out.
How it’s calculated
C = √(A² + B²). A layout is square when both diagonals measure the same.
Worked example
With these inputs:
- Side A (e.g. length)
- 12 ft
- Side B (e.g. width)
- 16 ft
the result is:
- Diagonal (hypotenuse)
- 20'
- Angle at A
- 53.13°
- Angle at B
- 36.87°
- Area
- 96 sq ft
- 3-4-5 check marks
- 9' × 12' → 15'
Tips from the trade
- Use the biggest 3-4-5 multiple that fits. Bigger triangles are more accurate.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and local building codes before ordering.