Acre ft to Gallons Calculator: How to Convert

This calculator converts acre-feet to gallons and back. You enter the volume in acre-feet, and it returns the equivalent in US gallons (and also in cubic feet for reference). The conversion is exact: 1 acre-foot equals 325,851.43 US gallons.

Formula

The conversion uses a fixed relationship between acre-feet and gallons:

Gallons = Acre-feet × 325,851.43

Where:

• Acre-feet (af) – a unit of volume commonly used for large water bodies, irrigation, and reservoir capacity.
• Gallons (gal) – the US liquid gallon, equal to 231 cubic inches or about 3.785 liters.
• 325,851.43 – the number of US gallons in one acre-foot.

The acre-foot itself is defined as the volume of one acre of surface area to a depth of one foot. Since one acre is 43,560 square feet, one acre-foot equals 43,560 cubic feet. Then, because one cubic foot contains 7.48052 US gallons, multiplying gives 43,560 × 7.48052 = 325,851.43 gallons.

Worked Example

Using the default value of 1 acre-foot:

1. Start with the volume: 1 acre-foot.
2. Multiply by the conversion factor: 1 × 325,851.43 = 325,851.43 gallons.
3. For cubic feet: 1 × 43,560 = 43,560 ft³.

So 1 acre-foot equals 325,851 gallons (rounded to the nearest gallon) or 43,560 cubic feet. The calculator rounds the gallon result to the nearest whole number because you cannot practically measure a fraction of a gallon in large water volumes.

Why Use Acre-Feet?

Acre-feet are the standard unit for measuring large water volumes in agriculture, municipal water supply, and reservoir management. One acre-foot is roughly enough water to cover a football field (including end zones) to a depth of one foot, or to supply a typical family of four for about one year (depending on usage).

When you see a report that a reservoir holds 50,000 acre-feet, that’s about 16.3 billion gallons. The unit is convenient because it directly relates to land area and depth, which are the two quantities you can easily measure in the field. Gallons, by contrast, are more familiar for household use but become unwieldy for large volumes.

How to Use the Calculator

Enter the volume in acre-feet in the input field. The calculator handles any non-negative number up to 100,000 acre-feet (which is about 32.6 trillion gallons – far larger than any real reservoir). The result appears instantly in gallons and cubic feet.

If you have a volume in gallons and want acre-feet, simply divide the gallons by 325,851.43. For example, 1 million gallons ÷ 325,851.43 = 3.07 acre-feet. The calculator does not do the reverse conversion directly, but you can use the formula above.

Common Mistakes and Misconceptions

The most frequent error is mixing up US and imperial gallons. The acre-foot is defined using US gallons, not imperial gallons. One imperial gallon is about 1.2 US gallons, so using the wrong factor gives a result that is off by about 20%.

Another mistake is confusing acre-feet with acre-inches. One acre-inch is 1/12 of an acre-foot, so 1 acre-inch = 27,154 gallons. If you are working with irrigation depths in inches, convert to feet first (divide by 12) before using the acre-foot factor.

Finally, remember that the acre-foot is a volume, not a flow rate. A flow rate of 1 acre-foot per day is about 226 gallons per minute, but the calculator only handles total volume.

Real-World Applications

Water districts use acre-feet to bill large agricultural customers. A farmer with a 100-acre field applying 2 feet of water per season uses 200 acre-feet, which equals 65,170,286 gallons. That same farmer might need to know how many gallons per minute a pump must deliver to apply that water over a certain number of days.

Reservoir operators track storage in acre-feet. When a reservoir drops by 1,000 acre-feet, that’s 325.85 million gallons less available. Environmental studies also use acre-feet to quantify stream flows or wetland water requirements.

For small-scale projects like ponds or cisterns, gallons are more practical. A 0.5-acre pond with an average depth of 6 feet holds 3 acre-feet, or about 977,554 gallons – useful to know for aeration sizing or fish stocking.

Limitations of the Conversion

The conversion factor is exact by definition, but the accuracy of your result depends on the accuracy of your acre-foot measurement. If you estimated the depth or area, the gallon figure inherits that uncertainty.

Also, the calculator uses US gallons. If you need imperial gallons (used in the UK and some Commonwealth countries), multiply the US gallon result by 0.832674. For example, 1 acre-foot = 325,851.43 US gallons = 271,328 imperial gallons.

Finally, the calculator rounds to the nearest whole gallon. For very large volumes, this is negligible – a difference of less than 0.0003%.

Frequently Asked Questions

How many gallons are in one acre-foot?

Exactly 325,851.43 US gallons. This is derived from 43,560 cubic feet (one acre-foot) times 7.48052 gallons per cubic foot. For practical purposes, you can remember it as about 326,000 gallons.

How do I convert gallons to acre-feet?

Divide the number of gallons by 325,851.43. For example, 10 million gallons divided by 325,851.43 equals 30.69 acre-feet. This is the reverse of the calculator’s operation.

What is the difference between an acre-foot and an acre-inch?

An acre-inch is one-twelfth of an acre-foot, equal to 27,154 gallons. It is sometimes used for irrigation scheduling because it is a smaller unit. To convert acre-inches to gallons, multiply by 27,154.

Are acre-feet used outside the United States?

Mostly in the US, Canada, and a few other countries for water resources. Most of the world uses cubic meters or liters. One acre-foot equals 1,233.48 cubic meters.

How much water does a typical household use per year?

In the US, a family of four uses about 100,000 to 150,000 gallons per year, depending on habits and location. That is roughly 0.3 to 0.46 acre-feet. So one acre-foot can supply a typical household for about two to three years.

Why is the conversion factor not a round number?

Because it comes from two separate definitions: the acre (43,560 square feet) and the US gallon (231 cubic inches). The product of the conversions (43,560 × 7.48052) yields a non-integer. It is exact, just not neat.

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in af | Volume | acre-ft | 1 | 0 | 100000
out gal | Gallons | US gal | af * 325851.43 | 0
out ft3 | Cubic feet | ft³ | af * 43560 | 0