Drainage Pipe Size Calculator

Drainage pipe sizing happens in two steps. First you estimate the peak stormwater runoff the pipe must carry. Then you pick the smallest standard pipe whose full-flow capacity meets that peak. This calculator does both.

Step 1, peak runoff (the Rational Method): Q (gallons per minute) = C × i × A / 96.23

Where C is the runoff coefficient (how much rain runs off instead of soaking in), i is the peak rainfall rate in inches per hour, and A is the drainage area in square feet. A roof or paved surface sheds almost everything (C ≈ 0.95); a lawn absorbs most of it (C ≈ 0.30).

Step 2, matching to a pipe. The calculator compares that flow against the full-flow capacity of each standard pipe size and returns the smallest one that keeps up. Those capacities come from the Manning equation below, evaluated at a typical 1% slope with smooth PVC. Steeper slopes carry more; rougher pipe (corrugated) carries less.

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: A 5,000 sq ft roof (runoff coefficient 0.95) during a storm dropping 4 inches of rain per hour:

Peak runoff = 0.95 × 4 × 5,000 / 96.23 = 197 GPM

The smallest standard pipe that carries at least 197 GPM at 1% slope is the 6-inch (250 GPM capacity), so that is what the calculator recommends. Drop the rainfall to 1 in/hr and the same roof needs only about 49 GPM, which a 4-inch pipe (80 GPM) handles comfortably.