Formulas
Slope length = √(run² + rise²)
Soffit = slope length × width
Each side = slope length × waist + risers × ½ × R × G
Riser boards = risers × R × widthWorked Example
A flight rising 1,500 mm, max riser 175 mm, 250 mm going, 1,200 mm wide, 150 mm waist, both sides open:
- 9 risers of 166.7 mm, 8 treads, run 2.0 m, slope length 2.5 m
- Soffit = 2.5 × 1.2 = 3.00 m²
- Sides = 2 × (2.5 × 0.15 + 9 × ½ × 0.1667 × 0.25) = 1.13 m²
- Riser boards = 9 × 0.1667 × 1.2 = 1.80 m²
- Total = 5.93 m² → 3 sheets with 15% waste
Formwork for Common Flights (1.2 m wide, 150 mm waist, both sides open)
| Rise | Risers | Soffit | Sides | Risers | Total |
|---|---|---|---|---|---|
| 1.5 m | 9 | 3.00 m² | 1.13 m² | 1.80 m² | 5.92 m² |
| 1.8 m | 11 | 3.70 m² | 1.37 m² | 2.16 m² | 7.23 m² |
| 2.7 m | 16 | 5.55 m² | 2.06 m² | 3.24 m² | 10.85 m² |
| 3.0 m | 18 | 6.24 m² | 2.31 m² | 3.60 m² | 12.15 m² |
Assumptions
- One straight flight; landings are extra (use the formwork area calculator for them as slabs).
- Uses the same riser layout as the staircase calculator.