Rainfall Volume Calculator
Work out how much water a given depth of rainfall drops on a paddock, roof or catchment. Enter the rainfall depth and the area (in hectares or square metres) and the tool returns the volume in litres, cubic metres, kilolitres and megalitres.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the rainfall depth in millimetres — a single storm total, a daily fall or an annual average.
- Enter the catchment or paddock area in ONE unit: hectares or square metres.
- Read the volume in litres, m³, kL and ML; for usable harvest, multiply by your capture efficiency.
How it works
One millimetre of rain over one square metre is exactly one litre, so the collected volume is depth (mm) ÷ 1000 × area (m²), which gives cubic metres. Hectares are converted at 10,000 m² each. The result is then expressed in litres (× 1000), kilolitres (1 kL = 1 m³) and megalitres (1 ML = 1000 m³). A useful benchmark is that 1 mm of rain over 1 hectare equals 10,000 L = 10 m³ = 0.01 ML.
Worked example
Worked example. 25 mm of rain over a 5-hectare paddock: area = 5 × 10,000 = 50,000 m²; volume = 25 ÷ 1000 × 50,000 = 1,250 m³ = 1,250,000 L = 1,250 kL = 1.25 ML.
Common mistakes
- Entering the area in both hectares and square metres — use one unit only.
- Treating the gross catch as usable water; roofs capture ~80–90% and bare soil far less after infiltration, first-flush and evaporation losses.
- Confusing kilolitres and megalitres — 1 ML is 1000 kL (1000 m³).
Frequently asked questions
How many litres is 1 mm of rain over a hectare?
Exactly 10,000 litres, which is 10 m³ or 0.01 ML. Scale that up by your depth in mm and area in hectares.
Is this the water I can actually collect?
No — it is the gross volume that falls. Multiply by a capture/runoff efficiency (roughly 0.8–0.9 for a good roof, much less for soil or a paddock) for the usable harvest.
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Tip: Enter any known values to calculate the remaining results.
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