A one-hundred year flood is calculated to be the maximum level of flood water to be expected in an average one-hundred-year period. Sometimes, a 500-year or 10,000-year flood is also calculated (especially, in low lying countries, such as the Netherlands). The 100-year flood is sometimes referred to as the 1% flood, since there is a 1% chance of it occurring in any year. Based on the expected water level, an expected area of inundation may be mapped out according to elevation above sea level. This area figures very importantly in building permits, environmental regulations, and flood insurance.

The mathematical field of extreme value theory was created to model rare events such as 100-year floods for the purposes of civil engineering.

In 2002 100-year floods caused by over a week of continuous heavy rains ravaged Europe, killing dozens, dispossessing thousands, and causing damages of billions of dollars in Russia, the Czech Republic, Austria, Germany, Romania, Spain, Croatia, Italy, and Bulgaria.

See also: topographic map, extreme weather