Rainfall Calculator - Volume, Weight and Runoff

Your details

Switch to reverse-solve for the catchment area you need to collect a target volume.
Length of the catchment surface. Leave blank to enter area directly.
m
Width of the catchment surface. Leave blank to enter area directly.
m
The horizontal surface area collecting rain. If you entered length and width above, this is filled in automatically.
m²
How much rain fell, measured as the depth that would accumulate on a flat surface.
mm
The surface material determines how much rain actually runs off vs. soaks in or evaporates. These coefficients follow standard engineering guidance.
Gross rainfall volumeSubstantial harvest
2,000

Total volume of rain that falls on the catchment, before losses

Volume unitL
Collectable volume (after runoff)1,800
Runoff coefficient0.9
Water weight1,796.4
Weight unitkg
Catchment area used80
Catchment area unitm²
Gross volume2,000
Collectable volume1,800

You can realistically collect 1800.0 litres from this rainfall.

  • The gross volume of rainfall on your catchment is 2000.0 litres.
  • With a runoff coefficient of 0.90, about 10% (200.0 litres) is lost to absorption and evaporation.
  • The collectable water weighs approximately 1796.4 kg - factor this into any tank or gutter load calculations.

Next stepTo design a rainwater harvesting system, check that your storage tank capacity matches the collectable volume and that gutters are sized for peak rainfall intensity.

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