Crop farming · Irrigation
Acre-inch irrigation calculator.
Acres, inches to apply, pump flow and system efficiency become gallons, pumping hours and the GPM per acre needed at peak water use.
Inputs
Water and pumping time
Enter the values.
How many gallons in an acre-inch, and how long to apply it
One acre-inch is the water that covers one acre one inch deep: 27,154 gallons (K-State Research and Extension). A flow of 450 gallons per minute equals one acre-inch per hour. So hours to apply = acre-inches ÷ (GPM ÷ 450). Net irrigation equals gross irrigation times system efficiency, so gross depth = net depth ÷ efficiency.
Worked example. 130 acres, 1 inch net, 900 GPM, 80 percent efficiency. Gross depth = 1 ÷ 0.80 = 1.25 inches, which is 162.5 acre-inches or 4,412,525 gallons. Flow = 900 ÷ 450 = 2 acre-inches per hour, so the pump runs 162.5 ÷ 2 = 81.3 hours (3.4 days). Peak use of 0.25 inches per day at 80 percent efficiency needs 0.25 × 27,154 ÷ 1,440 ÷ 0.80 = 5.89 GPM per acre. That matches K-State's delivery-rate table (5.89 at 0.25 in/day and 80 percent), and over 130 acres is 766 GPM, so a 900 GPM system keeps up.
Limits. This is a water volume calculation. It does not know your soil intake rate, the depth of your root zone, runoff, or when to irrigate. For those, use local extension scheduling tools and soil moisture readings. For a garden or lawn sprinkler, the garden irrigation tools on this site are a better fit.
Frequently asked questions
How many gallons are in an acre-inch?
27,154 gallons, according to K-State Research and Extension. An acre-foot is 12 acre-inches, or 325,851 gallons.
How do I convert GPM to acre-inches per hour?
Divide GPM by 450. A 900 GPM pump delivers 2 acre-inches per hour, enough for 1 inch on 2 acres per hour.
What system efficiency should I use?
It depends on the system and conditions, so use the figure your irrigation specialist gives you. The K-State delivery-rate table runs from 50 to 100 percent. Lower efficiency means more water pumped for the same net depth.
How many GPM per acre do I need?
GPM per acre = peak inches per day x 27,154 divided by 1,440 minutes, divided by efficiency. For 0.25 in/day at 80 percent efficiency that is 5.89 GPM per acre.
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Not professional advice
Estimates only. Pump flow, losses and crop water needs vary. Confirm flow with a meter and follow local irrigation scheduling advice and water-use rules.