Drainage Pipe Calculator
Standing water next to your foundation is a recipe for expensive problems. This calculator sizes drainage pipes using two standard engineering methods: the Rational Method to figure out how much water you need to move, and Manning's equation to verify your pipe can handle it. Enter your drainage area, rainfall rate, and pipe specs to get a clear pass or fail on capacity.
How It Works
Step 1 -- Estimate runoff (Rational Method): Q = C x i x A
Where Q is flow rate, C is runoff coefficient, i is rainfall intensity, and A is drainage area. This tells you how many gallons per minute your pipe needs to carry.
Step 2 -- Check pipe capacity (Manning's Equation): The pipe's carrying capacity depends on its diameter, slope, and surface roughness. Smooth PVC moves water much faster than corrugated plastic at the same slope.
Minimum Slopes
| Pipe Diameter | Minimum Slope |
|---|---|
| 3-4" | 1% (1/8" per foot) |
| 6" | 0.5% (1/16" per foot) |
| 8" | 0.4% |
| 10-12" | 0.3% |
Steeper is generally better for drainage as long as velocity stays under 10 ft/s.
Flow Velocity
Proper velocity is key to a pipe that works long-term:
- Below 2 ft/s -- Sediment settles in the pipe, causing blockages over time
- 2-10 ft/s -- The target range for most drainage systems
- Above 10 ft/s -- Risk of pipe erosion and joint separation
Common Runoff Coefficients
- Concrete/asphalt: 0.90-0.95
- Rooftops: 0.85-0.95
- Gravel paths: 0.40-0.60
- Lawns (flat): 0.25-0.35
- Lawns (steep): 0.50-0.75