Irrigation Water Requirement Calculator
An irrigation water requirement calculator estimates how much water a crop needs from irrigation, after accounting for rainfall. A crop's total water use is its evapotranspiration (ET), the water it loses to the air plus what it transpires; effective rainfall supplies part of that, and irrigation must make up the rest. This tool takes the crop water use and the effective rainfall over a period, both as a depth in inches, subtracts rainfall from ET to get the net irrigation depth, then converts that depth over the field area into gallons using 27,154 gallons per acre-inch. All three inputs are editable so you can model any crop, weather and field size. Knowing the irrigation requirement helps you schedule watering, size a pump or system, and budget water use. The US Geological Survey is the federal authority on water resources and publishes water-use data and the conversion factors involved. Effective rainfall is the portion of rain the crop actually uses, which is less than total rainfall after runoff and deep percolation. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator so you can follow each step yourself.
Irrigation need is crop water use minus rainfall, converted to gallons: gallons = (ET - rainfall) x acres x 27,154. With 1.5 in ET, 0.5 in rain over 5 acres, you need 135,770 gallons.
Irrigation requirement formula
Gallons = (ET - R) x A x 27,154
ET = crop water use (inches)
R = effective rainfall (inches)
A = field area (acres)
27,154 = gallons per acre-inch
One acre-inch, an inch of water over one acre, is 27,154 gallons. If rainfall meets or exceeds ET, no irrigation is needed for that period.
Worked example
A crop uses 1.5 inches of water over a period, receives 0.5 inches of effective rainfall, on a 5 acre field.
- Net depth = 1.5 - 0.5 = 1.0 inch
- Acre-inches = 1.0 x 5 = 5 acre-inches
- Gallons = 5 x 27,154 = 135,770 gallons
The field needs 135,770 gallons of irrigation. These are the calculator's default inputs, so the result above matches the widget exactly.
Irrigation Water Requirement Calculator: frequently asked questions
What is evapotranspiration?
Evapotranspiration (ET) is the combined water lost from a field by evaporation from the soil and transpiration from the plants. It represents a crop's water use over a period and is usually expressed as a depth in inches. Higher temperatures, wind and sunlight increase ET.
What is effective rainfall?
Effective rainfall is the portion of rain that actually reaches the crop's root zone and is available for use, after losses to runoff, evaporation and deep percolation. It is less than total rainfall. Using effective rather than total rainfall avoids overestimating how much rain offsets irrigation.
What is an acre-inch?
An acre-inch is the volume of water needed to cover one acre to a depth of one inch. It equals 27,154 gallons, or about 3,630 cubic feet. Multiplying the net irrigation depth in inches by the area in acres gives acre-inches, which convert to gallons.
What if rainfall exceeds crop use?
Then no irrigation is needed for that period, and the calculator shows zero net depth. Excess rain beyond the crop's use is lost to runoff or drainage and is not stored for later by this simple balance. Scheduling tools track soil moisture over time for a fuller picture.
Does this include system efficiency?
No. This is the net water the crop needs. Irrigation systems lose water to evaporation, wind drift and uneven coverage, so the gross water to apply is higher. Divide the net requirement by your system's application efficiency (for example 0.75) to get the gross amount to pump.
Official sources
- Water resources and water-use data: US Geological Survey (USGS). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.