What is an acre-foot and how many gallons is it?
An acre-foot is the volume of water that covers one acre of land to a depth of one foot. An acre is 43,560 square feet, so one acre-foot is 43,560 cubic feet, which is 325,851 US gallons (rounded to the nearest gallon) or 1,233.48 cubic meters. Because the unit ties area and depth to volume, one acre-foot spread over 2 acres is 6 inches deep.
An acre-inch uses the same idea with a 1-inch depth. It is one-twelfth of an acre-foot, or 27,154 gallons. Irrigation depths are often quoted in inches, so acre-inches turn up in field irrigation, while acre-feet are the usual unit for storage and allocations.
| Unit | Equals | Also equals |
|---|---|---|
| 1 acre-foot | 325,851 gallons | 43,560 ft³, 1,233.48 m³, 12 acre-inches |
| 1 acre-inch | 27,154 gallons | 3,630 ft³, 102.79 m³ |
| 1 million gallons | 3.069 acre-feet | 3,785.41 m³ |
| 1 hectare-meter (10,000 m³) | 8.107 acre-feet | 2,641,721 gallons |
Check your own right or contract. A water right, permit or district contract defines what volume you may divert, store or use, and how it is measured. This calculator converts units. It does not tell you what you are entitled to.
How does this acre feet calculator work?
In convert mode, your number is changed to cubic meters and then to the unit you asked for. In area × depth mode, the calculator first multiplies area by depth to get a volume, then converts it the same way.
1. Converting between units
Unit conversion
Result = Entered value × (m³ per entered unit) ÷ (m³ per result unit)
Every unit goes through one exact cubic-meter factor, so converting there and back returns your original number.
| Unit | Cubic meters per unit | Basis |
|---|---|---|
| Acre-foot | 1,233.48183754752 | 43,560 cubic feet |
| Acre-inch | 102.79015312896 | One-twelfth of an acre-foot |
| US gallon | 0.003785411784 | 231 cubic inches |
| Cubic foot | 0.028316846592 | (0.3048 m)³ |
| Liter | 0.001 | One-thousandth of a cubic meter |
2. Volume from area and depth
Area × depth
Acre-feet = Area (acres) × Average depth (ft)
Volume (m³) = Area (m²) × Average depth (m)
Other units are converted first: 1 acre = 4,046.8564224 m² (43,560 ft² × 0.09290304). The result is the volume of a flat-bottomed basin at that average depth.
3. Rounding
These are exact conversions, so results are not rounded to "about" figures. Values of 1,000 or more show whole units, smaller values show four significant figures, and values under 0.001 use powers of ten, such as 3.069 × 10⁻⁶. The number you typed is always shown as entered. In area × depth mode the result is only as good as your average depth.
When this calculator isn't the right tool
- You know a flow, not a volume. Start from the irrigation well capacity calculator or the GPM per acre calculator.
- You are applying a depth to a field. Use the acre-inches calculator or the irrigation application depth calculator.
- You have a lawn depth in inches. Use the inches of water to gallons calculator.
- You need crop demand or a legal storage figure. See the crop water requirement calculator, or use a surveyed capacity table. More tools are in the agriculture calculators.
Acre-foot calculation examples
A farm pond has a surface area of 3.2 acres and averages 6.5 feet deep. In area × depth mode, with acres, feet and the answer set to acre-feet:
- Volume: 3.2 acres × 6.5 ft = 20.8 acre-feet.
- Gallons: 20.8 × 325,851.43 = 6,777,709.71, shown as 6,777,710 gallons.
- Cubic meters: 20.8 × 1,233.4818 = 25,656.42, shown as 25,656 m³.
- Acre-inches: 20.8 × 12 = 249.6 acre-inches.
The same calculation in metric
A 1.8 hectare reservoir averages 2.1 m deep. Choose hectares, meters and the answer in cubic meters:
- Area: 1.8 ha × 10,000 = 18,000 m².
- Volume: 18,000 m² × 2.1 m = 37,800 m³.
- Acre-feet: 37,800 ÷ 1,233.4818 = 30.64496, shown as 30.64 acre-feet.
- Gallons: 37,800 ÷ 0.003785411784 = 9,985,703.58, shown as 9,985,704 gallons.
- Acre-inches: 30.64496 × 12 = 367.74, shown as 367.7 acre-inches.
How do I measure and use an acre-foot figure?
Finding the average depth of a pond
Take depth readings on a grid or along several lines across the pond, then average them. The deepest spot is a small part of most basins, so using it overstates the volume. For an evenly sloping cone or pyramid, area × maximum depth is three times the true volume. For a flat-bottomed box it is exact.
For an irregular basin, measure the surface area at several depth intervals. Each layer's volume is the average of its top and bottom areas times its thickness. Run each layer through the calculator and add the acre-feet. This average-end-area method slightly overstates layers with evenly sloping sides.
Layer volume
Layer volume = (Area at top + Area at bottom) ÷ 2 × Layer thickness
From a pump or stream flow to acre-feet
Volume is flow times time. A pump running 500 gpm for 48 hours delivers 500 × 60 × 48 = 1,440,000 gallons, which is 4.419 acre-feet in convert mode. One cubic foot per second for 24 hours is 86,400 ÷ 43,560 = 1.9835 acre-feet.
Common mistakes
- Entering imperial gallons as US gallons. Convert them to liters first.
- Leaving the depth unit on inches when you measured feet, which changes the answer by a factor of 12.
- Comparing acre-feet with an allocation stated per year, per season or as a flow rate without matching the time period.
What changes the volume of a pond or allocation?
| Factor | Effect | What to adjust |
|---|---|---|
| Basin shape | Basins that narrow toward the bottom hold less than area × depth. | Use a measured average depth or the layer method. |
| Water level | Surface area and depth both change as the level moves. | Re-measure at the level you care about. |
| Evaporation and seepage | Stored water is lost over time. The calculator does not include this. | Allow for it separately. |
| Sediment | Deposits take up storage space. | Measure depth again periodically. |
| Outlet and spillway levels | Water below the outlet or above the spillway crest may not count as usable storage. | Measure between the levels that apply. |
| Gallon definition | An imperial gallon is 4.546 L, larger than a US gallon at 3.785 L. | Convert to liters first. |
Frequently asked questions
How many gallons are in an acre-foot?
One acre-foot is 325,851 US gallons, rounded to the nearest gallon. It is the volume of water that covers one acre (43,560 square feet) to a depth of one foot, which is 43,560 cubic feet. Multiply cubic feet by 7.480519 gallons per cubic foot to get gallons. In metric, one acre-foot is 1,233.48 cubic meters.
How do I convert gallons to acre-feet?
Divide the number of US gallons by 325,851. For example, 1,000,000 gallons divided by 325,851 is 3.069 acre-feet. Multiply acre-feet by 12 to get acre-inches. This calculator does the conversion for you and also handles cubic meters, liters and cubic feet.
How many gallons are in an acre-inch?
One acre-inch is 27,154 US gallons, rounded to the nearest gallon. It is the volume that covers one acre to a depth of one inch, or one-twelfth of an acre-foot. Acre-inches suit irrigation work, where applied depth is often quoted in inches, while acre-feet suit storage and allocations.
See the acre-inches calculator for field irrigation depths.
How do I calculate the volume of a pond or reservoir in acre-feet?
Multiply the surface area in acres by the average depth in feet. A pond of 3 acres with an average depth of 5 feet holds 15 acre-feet. Use the average depth rather than the deepest point, because the bottom slopes. For an irregular basin, a survey of areas at several depths gives a better figure than a single average.
Does this calculator use US or imperial gallons?
It uses the US gallon, which is 231 cubic inches or 3.785 liters. An imperial gallon is larger, at 4.546 liters, so gallon figures from UK sources will not match. Convert imperial gallons to liters first, then enter liters.
How do I convert cubic meters to acre-feet?
Divide the cubic meters by 1,233.48. For example, 5,000 cubic meters is 4.054 acre-feet. A hectare-meter, which is 10,000 cubic meters, is 8.107 acre-feet. Enter your number with the unit set to cubic meters and the answer set to acre-feet.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Every unit runs through one exact cubic-meter factor, so US to metric and back returns your number.
- Both worked examples and the test values match the calculator's output.
- Unit definitions follow NIST conversion factors.
Sources
- National Institute of Standards and Technology. Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 edition, Appendix B conversion factors. [VERIFY URL]
- U.S. Geological Survey, USGS Water Science School. Water Science Glossary (acre-foot entry). [VERIFY URL]
- U.S. Bureau of Reclamation. Water Measurement Manual, for measuring flow before converting it to volume. [VERIFY URL]