Drainage Pipe Size Calculator

Drainage pipe sizing uses the Manning equation to determine the minimum pipe diameter needed to carry a design flow rate under gravity, given the pipe slope and material roughness. Getting this wrong causes backups, flooding, or undersized infrastructure.

Manning’s equation for full-flow circular pipe: Q = (1/n) × A × R^(2/3) × S^(1/2)

Where:

• Q = flow rate (cubic meters/second or cubic feet/second)
• n = Manning’s roughness coefficient (dimensionless)
• A = cross-sectional area of pipe = π × (D/2)² = πD²/4
• R = hydraulic radius = D/4 (for full circular pipe)
• S = pipe slope (dimensionless, e.g. 1% slope = 0.01)
• D = pipe diameter (meters or feet)

Manning’s n values by pipe material:

• PVC/HDPE smooth plastic: n = 0.009–0.011
• Concrete pipe: n = 0.011–0.015
• Clay tile: n = 0.012–0.016
• Corrugated metal: n = 0.020–0.024
• Cast iron: n = 0.012–0.014

Minimum slopes for self-cleaning velocity (≥ 0.6 m/s or 2 ft/s):

• 4-inch pipe: 1/8 inch per foot (1.04% slope)
• 6-inch pipe: 1/16 inch per foot (0.52% slope)
• 8-inch pipe: 1/10 inch per foot (0.42% slope)

Worked example: Design a PVC drain for a parking lot. Peak runoff: 0.05 m³/s. Slope available: 1.0% (S = 0.01). n = 0.010.

Using the Manning equation rearranged to solve for D: D ≈ [Q × n / (0.3117 × S^0.5)]^(3/8) D ≈ [0.05 × 0.010 / (0.3117 × 0.1)]^(3/8) = [0.0005 / 0.03117]^(3/8) = [0.01605]^(3/8) ≈ 0.22 m (8.5 inches)

Select the next standard size: 250 mm (10-inch) PVC pipe to provide capacity margin and account for partial-flow conditions.