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 the building code adopted in your province before ordering.