Drainage Pipe Size Calculator
Calculate the required drainage pipe diameter based on area to drain and rainfall rate for effective stormwater management.
Recommended Pipe Size
Drainage pipe sizing is based on the volume of water runoff that needs to be carried away during peak rainfall.
Runoff (cubic ft/sec) = Runoff Coefficient × Rainfall Rate (in/hr) × Area (acres) / 96.23
The required pipe diameter is then determined using Manning’s equation for gravity flow:
Q = (1.49 / n) × A × R^(2/3) × S^(1/2)
Simplified sizing guide (1% slope, smooth pipe):
- 4" pipe: Handles up to ~2,700 sq ft at 4 in/hr rainfall
- 6" pipe: Handles up to ~6,800 sq ft at 4 in/hr rainfall
- 8" pipe: Handles up to ~13,900 sq ft at 4 in/hr rainfall
- 10" pipe: Handles up to ~24,000 sq ft at 4 in/hr rainfall
- 12" pipe: Handles up to ~38,000 sq ft at 4 in/hr rainfall
Runoff coefficients:
- Roof/pavement: 0.9–0.95
- Gravel/packed earth: 0.6–0.7
- Lawn/garden: 0.2–0.4