Irrigation calculator

From reference evapotranspiration and your crop coefficient, find how many millimeters to apply, how long to run the system and how much water it takes.

Free, no sign-up

Before you use it

Works well for

  • Estimating crop water need from ETo and Kc
  • Irrigation time from the application rate
  • Water volume per irrigation

Not the right tool for

  • Irrigation scheduling based on soil moisture sensors
  • Designing an irrigation system (pipes, pressure, pumps)

The steps

  1. Crop water use: ETc = ETo × Kc. ETo comes from a nearby weather station; Kc from FAO-56 tables for your crop and growth stage.
  2. Net depth: ETc × days between irrigations − effective rain.
  3. Gross depth: net depth ÷ system efficiency (some water is always lost).
  4. Time: gross depth ÷ the system's application rate (mm/h).

Volume: 1 mm over 1 hectare is 10 m³ of water.

Application rate

For sprinklers: emitter flow (L/h) ÷ spacing area (m²) gives mm/h. For center pivots and drip, use the manufacturer's data or a catch-can test.

Frequently asked questions

Where do I find ETo?

From your national weather service, agricultural research stations, or a station on your farm. Many publish daily ETo.

Does it consider soil water capacity?

No. Make sure the gross depth doesn't exceed what your soil can hold in the root zone, or water will drain away.

What efficiency should I use?

Around 90% for well-maintained drip and 75–85% for sprinklers and pivots.