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Rainwater Calculators

Runoff Calculator

Surface water runoff from a site using the rational method, for drainage design.

Enter values

Results update as you type.

Result

Peak runoff rate

6.597 L/s

Total storm volume
11,875 litres
In cubic metres
11.875
Runoff coefficient
0.95
Drain pipe needed at 1 m/s
92 mm
Soakaway volume (with 30% voids)
39.583 m³

About the Runoff Calculator

Surface water runoff from a site using the rational method, for drainage design.

Also known as: surface water runoff, rational method calculator, drainage runoff, stormwater calculator.

How to use this calculator

  1. Enter catchment area, surface type, rainfall intensity, storm duration.
  2. The result updates instantly as you type — press Calculate (or Enter) at any time.
  3. Review the breakdown shown under the main result.
  4. Use Copy result or Share to save the answer or send a link with your inputs.

Formula

Q = C × i × A (rational method)

Worked example

Using the values pre-filled in the calculator above:

  • Catchment area = 500 m²
  • Surface type: Roof or paving (0.95)
  • Rainfall intensity = 50 mm/hour
  • Storm duration = 30 min

the calculator returns:

  • Peak runoff rate: 6.597 L/s
  • Total storm volume: 11,875 litres
  • In cubic metres: 11.875
  • Runoff coefficient: 0.95