Drainage Pipe Flow Rate Calculator
Calculate the gravity flow capacity of your drainage pipes using Manning's Equation.
Internal diameter of the pipe.
Drop in elevation per 100 ft of length (1/4″ per ft ≈ 2.08%).
PVC / Plastic (Smooth) – n=0.009
Concrete (Normal) – n=0.013
Corrugated Metal (CMP) – n=0.024
Cast Iron – n=0.012
Vitrified Clay – n=0.015
Custom n-value
Calculation Results (Full Flow)
Flow Rate (GPM):
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Flow Rate (CFS):
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Flow Velocity:
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*Results assume the pipe is flowing 100% full under gravity (no pressure).
How to Calculate Drainage Pipe Capacity
Correctly sizing a drainage pipe is critical for preventing floods, managing stormwater, and ensuring efficient sewage transport. This calculator estimates the maximum flow rate a pipe can handle using Manning's Equation, the standard formula for open-channel and gravity-fed pipe flow.
Manning's Equation Logic
The flow rate depends on the physical dimensions of the pipe, the slope at which it is laid, and the roughness of the interior material. The formula used is:
Q = (1.486 / n) × A × R^(2/3) × S^(1/2)
- Q: Flow Rate (Cubic Feet per Second).
- n: Manning's Roughness Coefficient (friction factor).
- A: Cross-sectional Area of flow.
- R: Hydraulic Radius (Area / Wetted Perimeter).
- S: Slope of the hydraulic grade line (ft/ft).
Key Input Variables Explained
1. Pipe Diameter
This is the internal diameter of the pipe. Common residential sizes include 3″, 4″ (standard for downspouts and foundation drains), and 6″. Larger municipal storm drains can range from 12″ to several feet.
2. Slope (Gradient)
Water requires gravity to move. The steeper the slope, the faster the water moves and the higher the capacity. Slope is often measured in percentage:
- 1% Slope: 1 foot of drop for every 100 feet of length (or ~1/8 inch per foot).
- 2% Slope: 2 feet of drop for every 100 feet of length (or ~1/4 inch per foot). This is a common standard for building codes regarding sewer lines.
3. Pipe Material (Roughness 'n')
The smoother the inside of the pipe, the less friction resists the water flow. Plastic pipes allow for higher flow rates than concrete or corrugated metal of the same diameter.
| Material | Typical 'n' Value | Flow Characteristics |
|---|---|---|
| PVC / Plastic | 0.009 – 0.011 | Very smooth, high efficiency. |
| Concrete | 0.013 | Standard for culverts and mains. |
| Corrugated Metal | 0.022 – 0.025 | High friction due to ridges. |
Interpreting the Results
- GPM (Gallons Per Minute): Useful for relating to rainfall intensity or pump specifications.
- CFS (Cubic Feet per Second): The standard engineering unit for hydrology and large-scale water flow.
- Velocity (ft/s): The speed of the water. generally, a minimum velocity of 2 ft/s is recommended to make the pipe "self-cleaning" (preventing sediment buildup).