Calculate reference evapotranspiration (ETo) using the FAO Penman-Monteith method for optimal irrigation planning and water resource management.
Start Calculating
This web application calculates reference evapotranspiration (ETo) using the FAO Penman-Monteith method, which is the standard for estimating crop water requirements in agricultural and irrigation planning.
Input climate data manually to calculate daily reference evapotranspiration values.
Use CalculatorImport EnergyPlus Weather (EPW) files to automatically calculate ETo for any location.
Import EPWGenerate heatmaps and visualizations of climate data and evapotranspiration patterns.
View HeatmapsConnect to weather APIs to calculate real-time evapotranspiration for your location.
Get Live DataThe FAO Penman-Monteith equation is the international standard for calculating reference evapotranspiration (ETo). It combines energy balance with mass transfer methods to provide accurate estimates of crop water requirements.
Our calculator implements the complete methodology as described in FAO Irrigation and Drainage Paper No. 56, accounting for all relevant climate variables:
Where ETo is reference evapotranspiration [mm day-1], Rn is net radiation, G is soil heat flux, T is temperature, u2 is wind speed, es is saturation vapor pressure, ea is actual vapor pressure, Δ is slope of vapor pressure curve, and γ is the psychrometric constant.
Flaha's ETo Calculator has become an essential tool for our irrigation planning. The accuracy and ease of use have significantly improved our water management practices.
Join agricultural professionals worldwide who use Flaha ETo Calculator for precision irrigation planning.
