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Rainfall Runoff Calculator

Work out rainfall runoff instantly with clear inputs, formula shown and shareable results.

About Rainfall Runoff Calculator

The rational method gives peak runoff as Q = CiA/360 with intensity in mm/h and area in hectares. A 1.5 hectare catchment with 70% runoff under 25 mm/h produces 0.073 m3/s, or 73 L/s. The coefficient captures how much rain runs off rather than infiltrating - near 0.95 for asphalt and 0.1-0.3 for woodland.

Formulas

Rational method

Q = C x i x A / 360, with i in mm/h and A in hectares

Rainfall volume

volume = intensity x area x 10 m3 per hour

Worked example

With the default values below, Rainfall Runoff Calculator returns Peak Runoff Flow of 0.0729. Change any input above and every figure here is recalculated in your browser.

Inputs used

Runoff coefficient Caccepts 0–1
0.7
Rainfall intensityminimum 0 mm/h
25 mm/h
Catchment areaminimum 0 ha
1.5 ha

Results

Peak Runoff Flow
0.0729 m3/s
Same Flow In Litres Per Second
72.9
Runoff Volume Per Hour
262.5 m3
Total Rain Falling Per Hour
375.0 m3

Step by step

  1. 1Values used
  2. 2Rational method
  3. 3Rainfall volume
  4. 4Peak Runoff Flow= 0.0729 m3/s
  5. 5Same Flow In Litres Per Second= 72.9
  6. 6Runoff Volume Per Hour= 262.5 m3
  7. 7Total Rain Falling Per Hour= 375.0 m3

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What runoff coefficient should I use?

About 0.95 for paved surfaces, 0.7-0.9 for roofs, 0.4-0.6 for suburban mixed, 0.2-0.35 for grassland and 0.1-0.2 for woodland.

When does the rational method break down?

Above about 80 hectares, and where storage or routing matters. Larger catchments need a hydrograph model.

Related Calculators

Rainfall Runoff Calculator – Calculate Peak Runoff Flow