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Rainfall Depth & Catchment Water Volume

Calculate theoretical water volume from rainfall depth and catchment area, with optional rectangular dimensions and volume conversions.

  • Formula & worked example
  • Private in your browser
  • No signup
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

Ten millimeters of rain over a 10 m × 10 m catchment represents 1 cubic meter, or 1,000 liters, before collection losses.

Rainfall volume = horizontal catchment area × rainfall depth; rectangular area = length × width

Worked example

Enter these known values and leave the other values blank.

Horizontal catchment length
10 m
Horizontal catchment width
10 m
Rainfall depth
10 mm
Horizontal catchment area
100 m²
Theoretical rainfall volume
1000 L

Assumptions and limitations

  • Use horizontal projected catchment area. A sloped roof’s surface area is not necessarily its horizontal rainfall-catching footprint.
  • The model assumes rainfall is uniform over the entered area. It does not apply a runoff coefficient, collection efficiency or drainage losses.
  • Zero rainfall produces zero water volume. Actual harvested water is usually lower because of wetting, leakage, first-flush diversion, evaporation and system limits.
  • This is a volume balance, not a gutter, drain, storage-safety or flood-flow design calculation.

Common questions

How much water is 1 mm of rain per square meter?

It is exactly one liter: 1 m² multiplied by 0.001 m gives 0.001 m³.

Can I enter an area without length and width?

Yes. Supply the measured horizontal area directly when the catchment is not rectangular or its dimensions are already accounted for.

References

The international acre is exactly 43560 square international feet; Bookify replaces the rounded captured area factor. Bookify replaces the captured rounded US gallon with the exact 3.785411784-liter definition.

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