Stormwater Drainage Calculator

Rain becomes a plumbing problem the moment it hits a roof: every acre dumps a measurable flow, and undersized gutters, downspouts and storm drains prove it during a 10-year storm.

Stormwater Drainage Calculator runs the rational method to size the flow your gutters and drains must carry, and cross-checks it against a pipe size.

• Rational method: Q = C x i x A runoff estimate.
• Area units: sq ft, sq m or acres.
• Runoff coefficient: by surface type or custom.
• Flow in 3 units: GPM, L/s, m3/h.
• Preliminary pipe size: Manning cross-check at a slope.
• Downspout guidance: per-1000-sq-ft rules.
• Export: Copy or download the report.

1. Enter the catchment area and pick its unit.
2. Enter the design rainfall intensity in in/hr.
3. Pick the runoff coefficient for the surface.
4. Read the peak flow and suggested pipe / downspout.

Example - 2000 sq ft roof, 3 in/hr. With C=0.95, the roof sheds roughly 95 GPM. Two 3x4 in downspouts split the load comfortably; a 6 in storm drain at 1% carries it with room.

• Real peak flows: gutter and drain design numbers.
• Roof or plaza: any catchment, any surface type.
• Metric-ready: L/s for international specs.
• Free and private: no uploads, no logging.