Flood Water Volume Calculator

Flood water volume is calculated as: Volume = Area x Average depth. For a flooded area, multiply the surface area by the average depth of water above the ground surface. The result can be expressed in cubic feet, cubic metres, acre-feet or gallons. This basic formula is used by FEMA, USGS and Army Corps of Engineers in floodplain mapping and hydraulic modelling.

Fresh water has a density of approximately 62.4 pounds per cubic foot (1,000 kg/m3). One US gallon of water weighs 8.34 pounds. One acre-foot of water (the volume of one acre covered to one foot depth) equals 325,851 US gallons and weighs approximately 1,359 US tons (1,233 metric tonnes). Floodwater containing sediment, debris or salt can be heavier.

An acre-foot is a unit of volume used in US water resource management equal to the volume of one acre of surface area filled to one foot depth. It equals 43,560 cubic feet, 325,851 US gallons or approximately 1,233.5 cubic metres. It is the standard unit used by USGS, the Bureau of Reclamation and Army Corps of Engineers to measure reservoir storage and flood volumes.

FEMA defines flood zones through Flood Insurance Rate Maps (FIRMs). Zone A (and sub-zones AE, AH, AO, AR) is the special flood hazard area (SFHA) with a 1% annual chance flood (100-year flood). Zone AE includes base flood elevations determined by detailed hydraulic study. Zone X is outside the SFHA with less than 0.2% annual chance. These are used to determine flood insurance requirements and floodplain development regulations.

A 100-year flood (also called the base flood or 1% annual chance flood) is a flood that has a 1% chance of occurring or being exceeded in any given year. It does not mean the flood only happens once a century. FEMA uses the 100-year flood as the regulatory standard for floodplain management under the National Flood Insurance Program (NFIP). The 500-year flood (0.2% annual chance) is used as a secondary planning benchmark.