One millimetre of rain means that, if water spread evenly without soaking in or running off, it would form a layer one millimetre deep.
Across each square metre, 1 mm is exactly 1 litre of water. Therefore 20 mm over 100 m² corresponds theoretically to 2,000 litres.
Amount and duration
Thirty millimetres in a day and 30 mm in half an hour are the same total water but a very different hazard. Short duration means much greater intensity and less time for infiltration and drainage.
Flood assessment needs hourly or finer observations, soil conditions and catchment properties.
Why a rain gauge is local
A gauge measures its exact location. Thunderstorm totals may change substantially within only a few kilometres.
Satellites, radar and models fill gaps but have their own resolution and uncertainty. The best assessment combines sources.
From rain to available water
Not all water reaches a tank or stays in the soil. Wind, evaporation, slope, vegetation, sealed surfaces and collection losses alter the actual amount.
The e‑meteo calculator is a geometric conversion, not an estimate of collectable or available water.
These explanations and calculations are educational. For safety, occupational, health, marine or aviation decisions, follow the appropriate official services.