Irrigation Water Per Acre-Inch Calculator
Convert an irrigation depth over an area into gallons and acre-feet, and find how long your pump must run at a given flow rate.
How it works
Gallons = acres × depth (in) × 27,154 · Acre-feet = acres × depth ÷ 12 · Run time = gallons ÷ flow (gpm) ÷ 60
One acre-inch — an inch of water spread over one acre — is 27,154 US gallons, the anchor for all irrigation volume math. Multiply by your acres and desired depth to get total gallons, and divide inches by twelve to express it as acre-feet, the unit water rights and reservoirs use. Dividing gallons by pump flow in gallons per minute tells you how long the set must run, which is how growers schedule irrigation around crop water use.
Worked example
Putting 1 inch on 40 acres is 40 × 1 × 27,154 = 1,086,160 gallons, or 40 ÷ 12 = 3.33 acre-feet. A pump delivering 500 gpm moves that in 1,086,160 ÷ 500 = 2,172 minutes, about 36.2 hours.
Sources: USGS — Water measurement units
Frequently asked questions
Why is an acre-inch 27,154 gallons?
An acre is 43,560 square feet, and one inch is 1/12 foot, so an acre-inch is 3,630 cubic feet. At 7.4805 gallons per cubic foot that is 27,154 gallons. It is the standard building block for irrigation and rainfall volume calculations.
Does this account for irrigation efficiency?
No — it computes the gross water delivered at the stated depth. Real systems lose water to evaporation, drift, runoff and deep percolation, so divide by an application efficiency (often 0.6–0.9) to find how much you must pump to net a given depth in the root zone.
How much water does a crop need?
It depends on the crop, stage and climate, typically expressed as inches of crop water use (evapotranspiration) per week. Match your application depth to replace that use, using local extension ET data or a soil moisture sensor rather than a fixed schedule.
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This calculator is for educational purposes. Double-check important results before acting on them.