What Is an Acre-Foot?
An acre-foot is a unit of volume commonly used in the United States to measure large quantities of water, such as reservoir capacity, irrigation supply, and river flow. It represents the volume of water needed to cover one acre of land (43,560 square feet) to a depth of one foot. One acre-foot equals exactly 325,851 US gallons (often rounded to 325,851.43).
How to Use This Calculator
Enter the number of acre-feet you want to convert and the calculator instantly returns the equivalent volume in US gallons. It works for whole numbers and decimals alike, making it useful for water managers, farmers, engineers, and students.
The Formula Explained
The conversion uses a single multiplication: $$\text{gallons} = \text{acre-feet} \times 325{,}851$$. The factor \(325{,}851\) comes from multiplying the area of one acre in square feet (\(43{,}560\)) by \(7.48052\) gallons per cubic foot, then by one foot of depth.
Worked Example
Suppose a reservoir holds 5 acre-feet of water. Multiply $$5 \times 325{,}851 = 1{,}629{,}255$$ US gallons. So 5 acre-feet equals roughly 1.63 million gallons.
Acre-Feet to Gallons Conversion Table
One acre-foot is the volume of water that covers one acre of land to a depth of one foot. The standard conversion uses \(1\text{ acre-foot} = 325{,}851\text{ US gallons}\), so the formula is:
$$\text{Gallons} = \text{Acre-Feet} \times 325{,}851$$The table below lists common acre-foot values with their exact gallon equivalents.
| Acre-Feet (ac-ft) | US Gallons |
|---|---|
| 0.25 | 81,462.75 |
| 0.5 | 162,925.5 |
| 1 | 325,851 |
| 2 | 651,702 |
| 5 | 1,629,255 |
| 10 | 3,258,510 |
| 50 | 16,292,550 |
| 100 | 32,585,100 |
| 1000 | 325,851,000 |
Real-World Acre-Feet Scenarios
Acre-feet are widely used in water management, irrigation, and reservoir planning because the unit scales naturally from a single household up to a city's annual supply. The scenarios below show how typical acre-foot volumes translate into gallons.
| Scenario | Acre-Feet | US Gallons |
|---|---|---|
| Typical U.S. household annual indoor + outdoor use | 0.5 | 162,925.5 |
| Annual supply for one to two families (common planning figure) | 1 | 325,851 |
| Small farm pond or stock reservoir | 5 | 1,629,255 |
| Irrigated field receiving 2 ft of water over 10 acres | 20 | 6,517,020 |
| Small municipal reservoir | 500 | 162,925,500 |
| Mid-size municipal annual supply | 1000 | 325,851,000 |
To explore the volume from the opposite direction, you can convert a gallon figure back to cubic feet with a gallons to cubic feet conversion, or check the surface area of an irrigated parcel using an acres-to-square-feet conversion. Note that household water-use figures vary widely by region and climate; the values above are representative planning estimates, not exact measurements.
FAQ
How many gallons are in one acre-foot? One acre-foot equals \(325{,}851\) US gallons.
Are these US or imperial gallons? This calculator uses US liquid gallons, the standard for US water measurement.
Can I convert decimals? Yes, fractional acre-feet such as \(0.25\) or \(2.7\) are fully supported.
