An acre-foot is the amount of water it takes to cover one acre of land exactly one foot deep. That works out to 325,851 gallons, or 43,560 cubic feet. Federal and state agencies across the United States use the acre-foot as the standard unit for tracking reservoir storage, dividing irrigation deliveries, and defining water rights, especially in the arid West where every acre-foot is measured, priced, and sometimes litigated.
What an Acre-Foot Looks Like
One acre covers 43,560 square feet, a square roughly 209 feet on each side, or just under the size of a football field. Flood that area exactly one foot deep and you have one acre-foot. For a more intuitive picture, imagine an entire football field, end zones included, under about nine inches of water. That gets you close.
The number sounds abstract until you weigh it. A single acre-foot of water weighs over 2.7 million pounds. Reservoirs in the Colorado River system hold tens of millions of acre-feet, and individual farms can hold rights to hundreds of acre-feet per year. The unit exists because gallons become unwieldy at that scale. Nobody wants to write contracts in billions of gallons.
Key Conversions
The conversions that come up most often in water management are straightforward:
- 1 acre-foot = 325,851 U.S. gallons
- 1 acre-foot = 43,560 cubic feet
- 1 acre-foot ≈ 1,233,482 liters (about 1,233.5 cubic meters)
Municipal water districts lean on the gallon conversion when they translate reservoir levels into something residential customers can picture. Engineers designing pipelines and treatment plants typically work in cubic feet. The metric figures matter for international agreements and scientific publications.
Converting Flow Rates to Volume
River flows and canal deliveries are measured in cubic feet per second (CFS) rather than acre-feet, so water managers constantly convert between the two. One CFS running continuously for 24 hours produces about 1.98 acre-feet. Over a 30-day month, a steady 1 CFS flow generates roughly 59.5 acre-feet. These conversions matter because water rights often specify both an instantaneous flow rate cap in CFS and an annual volume cap in acre-feet.
How Far Does an Acre-Foot Go?
Household Use
The longstanding rule of thumb is that one acre-foot supplies two suburban households for a year, covering both indoor and outdoor use. That benchmark dates from an era of less efficient fixtures and larger lawns. Conservation efforts have stretched the number considerably. Water agencies in California, Arizona, and other western states now report that one acre-foot serves three or more households annually, thanks to low-flow appliances, drought-tolerant landscaping, and tighter building codes.
Agricultural Crops
Crop water requirements dwarf residential use, which is why agriculture accounts for roughly 80 percent of consumptive water use in western states. Alfalfa is one of the thirstiest irrigated crops, with water needs commonly ranging from about two to nearly four acre-feet per acre per year in temperate climates and climbing higher in hot desert valleys with long growing seasons. Almond orchards typically require around three to four acre-feet per acre annually. Row crops like wheat and corn generally fall below two acre-feet per acre. Producers whose rights are measured in acre-feet build their planting decisions around these numbers; picking a thirsty crop against a limited allocation is a fast way to lose money.
Commercial and Industrial Uses
A standard 18-hole golf course uses a median of roughly 79 acre-feet of irrigation water per year nationally, though desert courses can consume substantially more. Data centers are an emerging source of demand. A 2026 study modeling 12 planned data centers in Nevada projected combined cooling-water consumption of about 9,650 acre-feet per year, separate from the water consumed to generate the electricity those facilities need. As these facilities concentrate in arid regions, their acre-foot demands are becoming a factor in regional water planning.
Why the Acre-Foot Matters Legally
The acre-foot is not just a convenient volume. It is the unit that makes water rights enforceable in most of the western United States, which follow the prior appropriation doctrine: first in time, first in right. Each right carries a priority date, an annual acre-foot volume, and usually a flow-rate cap in CFS. When a river or aquifer cannot satisfy everyone, the newest rights get curtailed first while senior rights keep diverting their acre-feet.
Eastern states generally follow a different system called riparian rights, where any landowner along a waterway can use a reasonable amount of water regardless of when they started. A handful of states blend both doctrines. The distinction matters here because prior appropriation rights are measured and enforced in acre-feet, while riparian rights are often less precisely quantified.
When you buy property in a prior-appropriation state, the associated water rights usually transfer with the land automatically. The deed or closing documents should identify what rights exist, including the priority date, the permitted diversion rate in CFS, and the annual volume in acre-feet. Missing this during a real estate transaction can mean discovering after closing that you bought land without the water to farm it.
Acre-Feet in Interstate Compacts
The acre-foot is also the unit that makes interstate water agreements enforceable. The most consequential example is the Colorado River Compact of 1922, which divided the river system into an Upper Basin and a Lower Basin. The Compact allocated 7.5 million acre-feet per year of exclusive beneficial consumptive use to each basin, with an additional one million acre-feet per year granted to the Lower Basin.1Bureau of Reclamation. Colorado River Compact Subsequent agreements divided each basin’s share among individual states.
The problem is that the Compact’s framers based their allocations on flow data from an unusually wet period. Long-term averages and recent drought have shown the river produces less than the total allocated volume, creating chronic overallocation. States that exceed their share face potential curtailment through negotiated agreements, court orders, or federal intervention. All of those enforcement mechanisms ultimately trace back to whether each state diverted more than its share of acre-feet.
Similar interstate compacts govern rivers across the West, each specifying allocations in acre-feet. Compliance depends on accurate measurement at state-line gauging stations and faithful reporting by each state’s water agencies. When a compact state falls short on delivery obligations, the disputes land in the U.S. Supreme Court, which has original jurisdiction over conflicts between states. These cases can take decades to resolve, with billions of dollars in agricultural and municipal water supply hanging on how many acre-feet crossed a state line in a given year.
Market Value of an Acre-Foot
Market prices for acre-feet vary enormously based on location, reliability, and whether you are buying a permanent right or a one-year lease. A study of wholesale water pricing across the Lower Colorado River Basin found prices ranging from effectively zero per acre-foot for some agricultural districts with legacy federal contracts to over $2,800 per acre-foot in coastal California. Municipal water generally commands much higher prices than agricultural water. The weighted average for municipal utilities in that study was roughly $512 per acre-foot compared to about $30 for agricultural districts. Those gaps create steady pressure to move water from farms to cities, which is why transfer applications are increasing and why state review of proposed transfers is so heavily contested.