Stormwater Drainage Calculator

Unit System

Catchment Area

m²

Area contributing to runoff

Surface Type

Rainfall Intensity

mm/hr

Typical values: 10-100 mm/hr (0.4-4 in/hr)

Rainfall Duration

Add 15% safety margin

Typical Runoff Coefficients (C)

Surface Type	Runoff Coefficient
Concrete/Asphalt	0.75 – 0.95
Roof Surface	0.9 – 1.0
Lawns (sandy soil)	0.1 – 0.2
Lawns (clay soil)	0.3 – 0.4
Gravel	0.4 – 0.6
Bare Earth	0.2 – 0.5
Forest/Heavy Vegetation	0.05 – 0.2
Park/Open Space	0.1 – 0.3

Tip: C = 1.0 for impermeable surfaces like roofs, C = 0.1–0.3 for lawns and loose soil.

How to Use This Calculator

The runoff coefficient (C) represents the fraction of rainfall that becomes surface runoff. It ranges from 0 (all rainfall infiltrates) to 1 (all rainfall becomes runoff).

Impervious surfaces like roofs and pavement have high coefficients (0.9-1.0), while permeable surfaces like lawns have lower coefficients (0.1-0.4).

Rainfall intensity is how hard it rains, typically measured in millimeters per hour (mm/hr) or inches per hour (in/hr).

Typical values:

Light rain: 2-4 mm/hr (0.08-0.16 in/hr)
Moderate rain: 5-20 mm/hr (0.2-0.8 in/hr)
Heavy rain: 20-50 mm/hr (0.8-2 in/hr)
Extreme rain: 50+ mm/hr (2+ in/hr)

Check local weather data for region-specific intensity values.

Always add a safety margin (10-20%) for design purposes
Local codes may specify minimum drain pipe sizes
Consider the 10-year or 100-year storm event for critical infrastructure
For rainwater harvesting, design for average rainfall conditions
For flood prevention, design for extreme rainfall events

Note: This calculator provides estimates only. Consult a civil engineer for critical drainage systems.

Results

Runoff Volume

m³

Formula: V = C × I × A × t

Peak Flow Rate

L/s

Formula: Q = C × I × A

Drain Size Estimation

Minimum Pipe Diameter:

Approximate Trench Size:

mm wide × mm deep

Note: Pipe sizing assumes full flow at 1 m/s velocity. Trench size is approximate for French drains.

Runoff Analysis

Showing runoff volume at different rainfall intensities for your area and surface type.

No Calculations Yet

Enter your parameters in the left panel and click "Calculate" to see results here.