How to use the evapotranspiration calculator
Estimate reference evapotranspiration. Enter max temperature, min temperature, extraterrestrial radiation in the fields above; the et₀ is computed instantly and plotted on the chart so you can see how the answer changes.
Formula
Variables
- tmax — Max temperature (°C)
- tmin — Min temperature (°C)
- ra — Extraterrestrial radiation (mm/day)
Worked example
With max temperature = 30 °C, min temperature = 18 °C, extraterrestrial radiation = 15 mm/day, the calculator gives et₀ ≈ 4.995581 mm/day.
Frequently asked questions
What is the formula for the evapotranspiration calculation?
ET₀ (mm/day) = 0.0023 × ra × ((tmax+tmin) / 2+17.8) × √(tmax-tmin). Here tmax is max temperature (°C), tmin is min temperature (°C), ra is extraterrestrial radiation (mm/day).
What inputs do I need for the evapotranspiration (hargreaves) calculator?
You need max temperature in °C, min temperature in °C, extraterrestrial radiation in mm/day. Each field is pre-filled with a typical example so you can see how the result responds as you edit the numbers.
How do I use this calculator?
Type values into the input fields. Results and the chart update instantly. Use Reset to restore the example values, Copy results to paste them elsewhere, or Share to copy a link that preserves your inputs.
Is the result exact?
Calculations use standard formulas and double-precision arithmetic, then display about seven significant figures. Real-world measurements carry uncertainty, so treat results as estimates unless your inputs are exact.