An acre-foot is a common way in the U.S. to measure water volume and use. It is the amount of water it takes to cover an acre of land one foot deep. An acre is about the size of a football field.

An acre-foot of water equals 325,851 gallons, and historically that was enough to serve the needs of two families for a year in California.

In 2018, with many people using less water, Metropolitan Water District of Southern California’s service area reported an acre-foot was enough to supply about three homes a year.

An acre-foot of water continues to go considerably further than it used to in meeting California’s urban water needs, depending on the region, because per-capita water use has been steadily falling.

Other common acre-foot measurements:

• The federal Central Valley Project delivers about 7 million acre-feet of water annually for agricultural, urban and wildlife uses in years of normal precipitation.
• The State Water Project delivered an average of 1.8 million acre-feet annually to municipal, industrial and agricultural users between 2011 and 2020.
• Shasta Lake, California’s largest reservoir, has a storage capacity of 4.55 million acre-feet. This amount would cover the entire state in about a half inch of water.