Irrigation calculator
Irrigation run time calculator
Estimate how many hours a pump or system needs to apply a target depth of water, from flow rate in U.S. gallons per minute.
Run time from GPM
Gross delivery; divide by system efficiency for net run time.
Formula
One acre-inch is 3,630 cubic feet, about 27,154.29 U.S. gallons — see the acre-inch to gallons converter.
Hours per acre-inch by flow rate
| Flow rate (GPM) | Hours per acre-inch (per acre) |
|---|---|
| 50 | 9.05 |
| 100 | 4.53 |
| 200 | 2.26 |
| 300 | 1.51 |
| 450 | 1.01 |
| 500 | 0.91 |
| 750 | 0.60 |
| 1,000 | 0.45 |
| 1,500 | 0.30 |
Worked examples
Applying 1 inch on a 40-acre field at 900 GPM: 40 × 1 × 27,154.29 ÷ (900 × 60) ≈ 20.1 hours of gross delivery.
If the system applies water at 85% efficiency, the net run time is 20.1 ÷ 0.85 ≈ 23.7 hours.
FAQ
How long does it take to apply one inch of water on one acre?
One acre-inch is about 27,154 U.S. gallons. At 100 GPM that takes 27,154 ÷ (100 × 60) ≈ 4.5 hours; at 450 GPM, about one hour.
What is the 450 GPM rule of thumb?
450 GPM delivers 27,000 gallons per hour, almost exactly one acre-inch per hour. It is a convenient field approximation of the exact 27,154-gallon value.
Does this calculator account for irrigation efficiency?
No. It computes gross water delivery. To estimate real run time, divide the result by your system's application efficiency (for example 0.85 for a well-run center pivot).