Drainage Calculator
Stormwater Runoff (Rational Method)
Calculate peak runoff flow from rainfall and area
Drainage Pipe Size
Determine pipe size needed for a given flow
Formula
Rational Method: Q = C × I × A | Manning's Equation: Q = (1/n) × A × R^(2/3) × S^(1/2)
Frequently Asked Questions
What is the Rational Method?
The Rational Method (Q = C × I × A) estimates peak stormwater runoff flow. Q is flow in cfs, C is a runoff coefficient (0-1), I is rainfall intensity in in/hr, and A is drainage area in acres. It is the standard method for small drainage design.
What size drain pipe do I need?
Common residential drainage: 4" pipe handles about 0.3 cfs at 1% slope, sufficient for most residential lots. For larger areas: 6" handles ~0.8 cfs, 8" handles ~1.7 cfs, and 12" handles ~5 cfs at 1% slope with PVC pipe.
How much does a concrete surface increase runoff?
Dramatically. Natural lawn absorbs 60-90% of rainfall (C=0.10-0.40), while concrete and asphalt produce 85-95% runoff (C=0.85-0.95). A 1,000 sq ft concrete pad produces about 5× more runoff than the same area of lawn.
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