As a general rule in meteorology and weather forecasting, 1 mm (0.03 in) of precipitation equals one liter (0.21 gallons) of water per 1 square meter (10.7 sq foot) of area. The amount of precipitation is measured over a certain period of time, for example, per hour, day, a few days, a week or a month, or a year.

The most common rainfall measurement is the total rainfall depth during a given period, expressed in millimeters (mm). For instance, we might want to know how many millimeters of rain fell over the course of 1 h, 1 day, 1 month, or 1 year. You can easily get a rough measurement of rainfall depth at home.

Measurement of the Rainfall: Thus, one c.m. of rainfall means that if the rain were to fall on a level surface which does not absorb it and from which it cannot runoff or evaporate, it would form a sheet of water covering one c.m in depth.

With a rainfall of 1 mm, every square metre receives 1 litre of rain water. A rainfall of 1 mm supplies 0.001 m³, or 1 litre of water to each square metre of the field. Thus 1 ha receives 10 000 litres.