What is an acre-inch, and why does it matter?
An acre-inch is a volume of water: the amount it takes to cover one acre of land to a depth of one inch. It's a way of turning a depth of water, the unit irrigation and rainfall are normally measured in, into a total volume for a specific area. Well drillers, irrigation districts, water-rights administrators and pump manufacturers all report water in acre-inches or the related acre-foot (12 acre-inches) because it connects directly to field size, the same way "1 inch of rain" does.
Pump output, on the other hand, is almost always given in gallons per minute (GPM). That mismatch is the main reason this calculator exists: it moves between the volume units (acre-inches, gallons, cubic feet, cubic meters) in either direction, and it turns a pump's GPM and run time directly into acre-inches.
| Unit | Equal to 1 acre-inch |
|---|---|
| Cubic feet | 3,630 ft³ (exact) |
| US gallons | 27,154 gal |
| Cubic meters | about 102.8 m³ |
| Acre-feet | 1/12 ac-ft (exact) |
If you're already working in acre-feet rather than acre-inches, the acre-feet calculator covers the same conversions at that larger scale. Both sit under the site's agriculture calculators.
How this acre-inches calculator works
Every conversion here is exact geometry and defined unit relationships, not an estimate, so the calculator never rounds to a "nice" number or shows the word "about" for a straight unit conversion. Results are shown to four significant figures, or as a whole number once a figure is in the thousands.
1. What one acre-inch equals, in cubic feet
Acre-inch to cubic feet
Cubic feet = Acre = 43,560 sq ft × 1 inch depth (1/12 ft) = 3,630 ft³
An acre is defined as 43,560 square feet. One inch is 1/12 of a foot, so the depth term is 1/12. This is exact: no rounding is involved.
2. Cubic feet to gallons and cubic meters
Cubic feet to US gallons
Gallons = Cubic feet × 7.480519 (1,728 in³ ÷ 231 in³/gal)
3,630 ft³ × 7.480519 ≈ 27,154 gallons per acre-inch, the figure published by Oklahoma State University Extension and other land-grant sources.
Acre-inch to cubic meters
Cubic meters = Acres × 4,046.856 m²/acre × 0.0254 m (1 inch)
Using the international foot (0.3048 m exactly), one acre is 4,046.856 m² and one acre-inch works out to about 102.79 m³, matching the 102.8 m³ figure from University of Florida IFAS Extension.
3. From a pump's flow rate and run time
GPM × hours to acre-inches
Acre-inches = (GPM × 60 × Hours) ÷ 27,154
GPM × 60 gives gallons per hour; multiplying by the run time gives total gallons, which convert to acre-inches at 27,154 gallons each. Oklahoma State University Extension states the same relationship the other way: a steady 1 GPM flow takes about 452.6 hours to deliver one acre-inch.
Rounding policy
Because these are exact conversions rather than estimates, the calculator shows about four significant figures for small results and whole numbers once a value passes a few hundred, so figures like "27,154 gal" print exactly instead of being smoothed to "about 27,000." Your own entered amount is always shown back exactly as you typed it, not re-rounded.
When this calculator isn't the right tool
It converts a volume, or a flow and a time, into acre-inches. Use a different tool for:
- Matching a well's flow rate to your acreage, rather than converting a volume. See the GPM per acre calculator.
- Turning a volume you've already pumped into a depth applied across your specific field, rather than the generic 1-acre unit. See the irrigation application depth calculator.
- Working out how long to run a pump to hit a target depth over a known acreage, instead of a single flow-and-time conversion. See the irrigation set time calculator.
- Volumes in acre-feet rather than acre-inches. See the acre-feet calculator.
- Checking whether a well can sustain a given flow, rather than converting a known flow. See the irrigation well capacity calculator.
Acre-inches calculation examples
A grower runs a 900 GPM well for 10 hours to finish an irrigation set.
- Total gallons pumped: 900 GPM × 60 × 10 hr = 540,000 gallons.
- Acre-inches: 540,000 ÷ 27,154.29 ≈ 19.89 acre-inches.
- Cubic feet: 540,000 ÷ 7.4805 ≈ 72,188 ft³.
- Cubic meters: 540,000 × 0.0037854 ≈ 2,044 m³.
The same tool, working from a metric volume
A farm reservoir is measured at 500 cubic meters and the manager wants that in acre-inches and gallons:
- Acre-inches: 500 ÷ 102.79 ≈ 4.864 acre-inches.
- US gallons: 500 × 264.172 ≈ 132,086 gallons.
- Cubic feet: 500 ÷ 0.028317 ≈ 17,657 ft³.
Both examples match what the calculator above shows for the same inputs. Notice how much a slow rotor-style well versus a high-output turbine pump changes the time needed to move the same volume — the flow rate, not the acre-inches, is usually the limiting factor in the field.
Measuring pump flow and using the result
The flow-rate mode is only as accurate as the GPM you enter. A pump's nameplate or catalog rating is measured under ideal test conditions and often overstates what it actually delivers once it's connected to real pipe, elevation change and a specific set of nozzles or gates.
A simple way to check your actual flow
Time how long it takes to fill a container of known volume, such as a 5-gallon bucket, directly from the pump's discharge or a single open outlet. Convert that to gallons per minute:
Bucket-and-stopwatch flow check
GPM = (Bucket volume in gallons ÷ Seconds to fill) × 60
Repeat the test two or three times and average the results; flow can vary slightly with each run.
For a full-system reading rather than a single outlet, the bucket test calculator and the irrigation system GPM calculator walk through the same idea at a larger scale.
Common mistakes
- Mixing up acre-inches and acre-feet. An acre-foot is 12 times larger, so a slipped unit can throw a water budget off by an order of magnitude.
- Using a pump's rated GPM instead of its measured output. Pressure, wear and how many outlets are open at once all change the real flow.
- Assuming flow stays constant for a long pumping run. A well's yield can drop as the water table draws down over many hours, so a short test doesn't always represent a full day's pumping.
- Applying a well's total GPM to only part of a field when some of that flow is actually split across other zones, gates or risers.
Using the acre-inch figure
Once you know the acre-inches delivered, the irrigation application depth calculator converts that volume into a depth over your actual field size, and the irrigation energy cost per acre-inch calculator puts a fuel or electricity cost on it.
Where the numbers can diverge from real-world pumping
The conversions themselves are exact. What varies is how closely a real pumping event matches the flow rate and time you enter.
| Factor | Effect | What to check |
|---|---|---|
| Actual vs. rated pump flow | Output changes with pressure, drawdown and wear, so a nameplate GPM rarely matches all-day output. | Measure actual flow with a bucket test rather than using the rating. |
| Multiple outlets or gates | Splitting flow across several risers, gates or zones reduces the rate through each one. | Measure flow at the outlet that matters, not the pump's total. |
| System losses | Leaks, evaporation, wind drift and runoff mean less water reaches the crop than was pumped. | Compare this result with a soil-moisture or catch-can check in the field. |
| Short test duration | Flow measured over a few minutes may not reflect what a well delivers over a full day. | For sizing decisions, test over as long a run as practical. |
| Elevation and friction losses | These don't change the volume pumped, but they do change the pressure and horsepower needed to pump it. | Use a pipe-friction or total dynamic head calculator alongside this one. |
Frequently asked questions
What is an acre-inch of water?
An acre-inch is the volume of water it takes to cover one acre of land to a depth of one inch. It equals 3,630 cubic feet, or 27,154 gallons. Farmers, irrigation districts and water managers use it because it ties a water volume directly to a field size and a depth of application, the way irrigation and rainfall are usually measured.
How many gallons are in an acre-inch?
One acre-inch equals 27,154 gallons. That comes from multiplying an acre (43,560 square feet) by one inch of depth to get 3,630 cubic feet, then converting cubic feet to gallons at 7.48 gallons per cubic foot.
How do I convert GPM to acre-inches?
Multiply your pump's gallons per minute by 60, then by the number of hours it runs, to get total gallons. Divide that by 27,154 to get acre-inches. A steady 1 GPM flow takes about 452.6 hours to deliver one acre-inch, so a 500 GPM pump reaches one acre-inch in a little under an hour.
Use the "Pump flow rate & time" mode of the calculator above to do this automatically.
What's the difference between an acre-inch and an acre-foot?
An acre-foot is 12 times larger than an acre-inch: it's the volume that covers one acre to a depth of one foot instead of one inch, equal to 325,851 gallons. Acre-inches are more convenient for a single irrigation event, while acre-feet are more common for describing seasonal water use, reservoir storage or water rights.
For volumes already in acre-feet, see the acre-feet calculator.
How many cubic feet or cubic meters are in an acre-inch?
One acre-inch equals 3,630 cubic feet, an exact figure since an acre is defined as 43,560 square feet. In metric terms, one acre-inch equals about 102.8 cubic meters (102,642 liters).
How long does it take to pump one acre-inch of water?
It depends entirely on your pump's flow rate. At 27,154 gallons per acre-inch, a 100 GPM pump takes about 4.5 hours, a 500 GPM pump takes about 54 minutes, and a 1,000 GPM pump takes about 27 minutes. Enter your own flow rate above for an exact figure.
Does this calculator account for pump or system losses?
No. It converts a volume, or a flow rate and time, into acre-inches using exact geometric and unit conversions, based on the water actually pumped or measured. It doesn't subtract for leaks, evaporation, wind drift or an uneven application pattern, so the acre-inches actually reaching the crop root zone is often somewhat less than this result.
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
- All four conversions and both worked examples verified by hand against the calculator's code.
- Conversion factors cross-checked against three independent land-grant extension sources.
- Unit and gallon-to-acre-inch definitions follow standard US survey and hydraulic references.
Sources
- Oklahoma State University Extension. Water Measurement Units and Conversion Factors (BAE-1501).
- University of Florida IFAS Extension. Water Measurement for Agricultural Irrigation and Drainage Systems (Circular 1495 / CH153).
- University of Missouri Extension. Measuring Irrigation Water in a Ditch, Stream or Reservoir (G1681).
- Kansas State University Research and Extension. Irrigation Formulas and Conversions (MF2404).