Daily ETo Calculator (FAO-56 Penman-Monteith)
Estimate reference evapotranspiration (ETo) in mm/day for irrigation planning. Enter your local weather data below.
What is ETo?
ETo (reference evapotranspiration) is the amount of water that would evaporate and transpire from a well-watered reference grass surface. In practical terms, it tells you the atmosphere’s “water demand” for a given day and location.
Farmers, gardeners, and irrigation designers use ETo as the base value for irrigation scheduling. Once you know ETo, you can estimate crop water use by multiplying ETo by a crop coefficient (Kc), producing ETc.
How this eto calculator works
This tool uses the FAO-56 Penman-Monteith equation, widely considered the standard method for daily ETo. It combines temperature, humidity, wind speed, radiation, elevation, latitude, and day of year to produce a daily estimate in millimeters per day.
Inputs you need
- Latitude & day of year: used to estimate extraterrestrial radiation and day length effects.
- Elevation: affects atmospheric pressure and psychrometric constant.
- Tmax/Tmin: daily temperature range drives saturation vapor pressure and energy exchange.
- Mean RH: estimates actual vapor pressure and dryness of air.
- Wind speed: stronger wind usually increases evapotranspiration.
- Solar radiation (Rs): primary energy source for evaporation.
Understanding your result
The calculator returns ETo (mm/day). One mm/day means about one liter of water per square meter per day. If you also enter a crop coefficient and area, the tool estimates:
- ETc (mm/day) = ETo × Kc
- Water need (liters/day) = ETc × area (m²)
Quick interpretation guide
- Low ETo (1–3 mm/day): cool/cloudy periods, lower irrigation demand.
- Moderate ETo (3–6 mm/day): common in warm seasons.
- High ETo (6+ mm/day): hot, sunny, windy, and dry conditions.
Best practices for irrigation scheduling
- Use local weather station data whenever possible.
- Adjust Kc by crop stage (initial, development, mid-season, late-season).
- Account for effective rainfall before irrigating.
- Monitor soil moisture to verify model estimates.
- Recalculate daily or every few days in hot weather.
Formula summary
The implemented daily equation is:
ETo = [0.408Δ(Rn − G) + γ(900/(T + 273))u₂(es − ea)] / [Δ + γ(1 + 0.34u₂)]
where daily soil heat flux G is assumed to be 0. Net radiation is estimated from solar radiation and longwave radiation terms following FAO-56 guidance.
Important note
This eto calculator is intended for planning and education. Real-world irrigation decisions should also consider soil type, root depth, irrigation system efficiency, rainfall forecasts, and local agronomic recommendations.