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Rainwater runoff calculator

Calculate roof runoff from effective area, user-entered rainfall intensity and runoff coefficient.

Enter the project values

All values remain in your browser. Review the assumptions before using the result.

Enter the project design intensity; this calculator does not choose it.
Your result will appear here.Calculate to see the outputs, formula and substituted working.

How the calculation works

Volume balance using 1 mm rainfall over 1 m² = 1 litre.

Formula

Q(L/s) = A × r × C ÷ 3600

Worked example

Effective area500 m²
Rainfall intensity75 mm/h
Runoff coefficient1.00
Expected runoff10.417 L/s

Assumptions and limitations

Assumptions

  • Rainfall intensity and runoff coefficient are chosen by the user.
  • The effective area is already determined.

What this does not check

  • Does not choose the design storm or effective catchment area.
  • Does not size gutters, outlets, pipes, attenuation or overflow routes.
Formula version: 1.0 · Last reviewed: 16 August 2026

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Engineering basis: first-principles calculation using user-entered project data. The calculator does not choose design criteria or replace the complete project-specific engineering design.
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