Why rainfall is measured in millimetres
Nairobi residents shield themselves from the rain in the city centre on February 22, 2026. The Kenya Meteorological Department recently issued a heavy rainfall alert for large parts of the country.
After the heavy rainfall that caused death and destruction in Nairobi early this month, the Kenya Meteorological Department issued a public statement to explain that the downpour was unusually heavy, hence the deadly flash floods.
That press release noted that “In meteorological terms, 20mm of rainfall approximately equals 20 litres of water per square metre…”
Now, while that sentence is true, it not entirely accurate. First of all, this is not a just a meteorological result, it is a mathematical fact. Secondly, 20mm of rainfall is not “approximately” equal to 20 litres of water per square metre; it is “exactly” equal to 20 litres of water per square metre. I have explained this in the past (September 2014) but let’s go over it again.
The volume of a regular shaped object is found by multiplying the area of the base by the height. Thus, if we have an area measuring one square metre and we put water to a depth of 20mm, the volume can be calculated as follows.
One metre has 1,000mm so one square metre is 1,000,000 square millimetres. Multiplying this by the 20mm depth gives a volume of 20,000,000 cubic millimetres (Don’t confuse this with millilitres, ml!). How many litres are these?
We know that one litre is equal to 1,000ml or 1,000cc (cubic centimetres). But, 1cm = 10mm; so, 1cc = 10 x 10 x 10 = 1,000 cubic millimetres (again, not to be confused with millilitres, ml!). Therefore, 20,000,000 cubic millimetres are equal to 20,000cc, or 20,000ml, or 20 litres exactly.
This is the reason why rainfall data is reported in millimetres instead of centimetres. For example, on March 7, 2026, the Wilson Airport weather station reported that it received 160mm of rainfall in the preceding 24 hours.
It might have appeared more convenient to report 16cm instead of 160mm; after all, when we hear 16cm, we immediately recall that the ruler in a student’s geometrical set is 15cm and we get a feel of what 16cm means. However, this is rainwater data and we would be more interested in the volume; this comes to 160 litres per square metre! 160mm of rain in a day is a lot of water.
With such a downpour, a typical 3-bedroomed house measuring 100 square metres will collect 160 x 100 = 16,000 litres of water. This is more than what that household consumes in one month – and it is all collected in one day!
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