What is an acre-foot?

An acre-foot is one acre of surface covered one foot deep. Picture an American football field, goal line to goal line, under about 11 inches of water. That is 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. To turn any other amount of water into gallons, or gallons back into acre-feet, the acre-feet to gallons converter does the arithmetic both ways.

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.

How many football fields is an acre-foot?

About nine-tenths of one. An acre is 43,560 square feet. An American football field is 360 by 160 feet with its end zones, and 300 by 160 feet between the goal lines, which is 48,000 square feet. So an acre is about 91% of the field of play and about 76% of the whole field, end zones included.

Spread one acre-foot over the field between the goal lines and the water stands about 10.9 inches deep. Over the whole field, end zones included, it stands about 9.1 inches. Turned the other way, the field of play under a foot of water holds 48,000 cubic feet: about 1.1 acre-feet, or 359,065 gallons.

The picture scales up to a reservoir. Shasta Lake, charted in the worked example below, held 2.29 million acre-feet on Oct 28, 2026, enough to cover about 2.08 million football fields a foot deep. The football field is only a way to see the unit: water agencies, contracts and this site's charts count the acre-foot itself, and the acre-feet to gallons converter turns any of those figures into gallons.

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.

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.