Rainfall Volume & How Much Water the Sky Just Gave You
Measures rain
Enter rainfall depth and the catchment area to get the total volume of water that fell — in litres, cubic metres and per hectare.
Enter rainfall & area
Next: plan storage for up to 202 m³ (202,343 L) per event; apply a runoff coefficient (0.5–0.9) for the share you can actually capture.
1 mm of rain over 1 m² = 1 litre. Real harvest is lower after first-flush, evaporation and infiltration losses.
Rainfall volume — key facts
- Volume
- Depth × area
- 1 mm rain
- = 1 litre per m²
- 1 mm over 1 ha
- = 10 m³ (10,000 L)
- 10 mm over 1 ha
- = 100 m³
- 1 m³
- = 1,000 litres
- Area to use
- Horizontal (plan) area
- Use
- Harvesting, ponds, recharge
- Privacy
- Runs in your browser; nothing uploaded
A millimetre of rain is a litre on every square metre
Rain is reported as a depth in millimetres, but to plan with it you need a volume. The link is wonderfully simple: one millimetre of rain is one litre of water on every square metre. So the volume that fell is just the depth times the area. Spread that millimetre over a whole hectare and you get 10,000 litres — ten cubic metres — from a single millimetre, which is why even modest rain adds up to a lot of free water across a field or rooftop.
This tool turns your rainfall depth and area into the total volume in litres and cubic metres, plus the volume per hectare. Use it to plan rainwater harvesting, size farm ponds and storage tanks, estimate recharge, and see how much of your water budget the monsoon delivers for free. Pair it with the Rainwater Harvesting, Tank Sizing and Curve Number Runoff tools to turn the gross rainfall into water you can actually store and use.
See the free water
Turn rain depth into real litres and m³.
Plan harvesting
Size tanks and ponds to the volume that falls.
Per-hectare anchor
1 mm over a hectare is a clean 10 m³.
Budget the monsoon
Know how much water a season delivers.
Frequently Asked Questions
How do I convert rainfall in mm to a volume of water?+
Rainfall depth multiplied by the area it fell on gives the volume. Because one millimetre of rain is exactly one litre on every square metre, you can read the depth straight off as litres per m². Multiply by your area in square metres for the total litres, then divide by 1,000 for cubic metres. This tool does both for you.
Why is 1 mm of rain a litre per square metre?+
A millimetre is one-thousandth of a metre. Spread over one square metre, a depth of 0.001 m holds 0.001 m³ of water, and 0.001 m³ is exactly one litre. So every millimetre of rainfall puts one litre on each square metre of ground — a tidy rule that makes rainfall maths simple.
How much water is 1 mm of rain over a hectare?+
A hectare is 10,000 square metres, so 1 mm over a hectare is 10,000 litres, which is 10 cubic metres. Ten millimetres of rain on a hectare is therefore 100 m³, and 100 mm is 1,000 m³ — a useful set of anchors when you are sizing tanks, ponds or recharge structures.
What can I use the rainfall volume for?+
Knowing the volume that lands on a roof, field or catchment lets you plan rainwater harvesting, size storage tanks and ponds, estimate groundwater recharge, and budget how much of a season's water comes free from the sky. It is the starting point for almost any water-harvesting calculation.
Does all the rain that falls become usable water?+
No. Some evaporates, some soaks straight into the soil and some runs off before it can be captured. The volume here is the gross amount that fell on the area; to estimate what you can actually collect, apply a runoff or capture coefficient — see the Rainwater Harvesting and Curve Number Runoff tools.
What area should I enter?+
Use the horizontal catchment area that the rain falls on — the roof footprint for roof harvesting, the field area for a farm pond, or the whole catchment for a tank. Enter it in the units the tool offers; for sloped surfaces use the flat (plan) area, since rainfall depth is measured on the horizontal.
How does this relate to a rain gauge reading?+
A rain gauge reports depth in millimetres regardless of the gauge's size, because depth is independent of area. This tool takes that same depth and scales it up to your catchment, turning the gauge number into the real volume of water that landed on your roof or field.
Can I use it for a storm or a whole season?+
Yes. Enter the depth for whatever period you measured — a single storm, a day, a month or a whole monsoon — and the area stays the same. For seasonal planning, add up the volumes from each period, or enter the total seasonal rainfall depth directly.
Is the calculation exact?+
The depth-times-area relation is exact, so the volume the tool reports for the rain that fell is precise. Real-world uncertainty comes from the rainfall measurement itself and from how much of that water you can actually capture after evaporation, infiltration and overflow losses.