Measurement & units
What is an acre-foot?
An acre-foot is the amount of water it takes to cover one acre one foot deep: 325,851 gallons, or 43,560 cubic feet. It is the unit the American West runs on, roughly a year of water for two average California households.
Updated Jul 9, 2026
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An acre-foot is one acre of surface covered one foot deep. Picture a football field, which is close to an acre, under a foot of water. That is about an acre-foot. In exact terms it is 325,851 gallons, or 43,560 cubic feet, and those two conversions never change, so you can treat them as fixed.
Why the West counts water this way
Western water moves in bulk. Reservoirs bank millions of acre-feet, projects deliver hundreds of thousands to farms and cities, and the contracts behind those deliveries are written in acre-feet. The eastern US gets rain through the growing season and rarely stores or moves water at that scale, so it talks in gallons or in millions of gallons per day. In the West, where a dry winter has to be rationed across a whole summer, the acre-foot is the natural unit of the ledger.
How big is an acre-foot, really
One acre-foot is 325,851 gallons. A common planning rule of thumb from Reclamation and California water agencies is that an acre-foot covers roughly one to two average households for a year, indoor and outdoor use combined. So when a district says it is short 200,000 acre-feet, that is a city's worth of annual supply, not an abstraction.
Worked example: one field against one reservoir
Alfalfa is thirsty. A grower might apply about 4 acre-feet per acre across a season. A 160-acre field, a quarter of a square-mile section, then uses roughly 640 acre-feet in a year. That sounds enormous until you set it against a major reservoir.
- Storage
Shasta Lake, charted above, holds millions of acre-feet when full. Those 640 acre-feet vanish into it: one large farm's entire irrigation season is smaller than the daily wiggle of a single reservoir's storage line. That gap, between what one field drinks and what one reservoir banks, is exactly why the West keeps its books in acre-feet instead of gallons.
The number to remember
One acre-foot is 325,851 gallons, or 43,560 cubic feet, or about a year for one to two California homes. Every other figure on this site, from reservoir storage to project allocations to end-of-year carryover, is counted in these.
Try it
Exact conversions: 1 acre-foot = 43,560 cubic feet = 325,851 gallons (an acre-foot is one acre covered a foot deep). One cubic-foot-per-second sustained for a day is about 1.9835 acre-feet. Household-years use an average US household of about 300 gallons a day (~109,500 gallons a year), per EPA WaterSense — real household use varies widely. Flow figures assume the rate holds steady.