Free evapotranspiration calculator. Calculate ETo using FAO Penman-Monteith, Hargreaves, or Blaney-Criddle methods. Get ETc for 27+ crops with Kc coefficients by growth stage.
FAO-56 standard. Most accurate, requires full weather data.
Field corn for grain production
Kc: 0.3 → 1.2 → 0.6 | Growing: 150 days
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Evapotranspiration (ET) is the foundation of precision irrigation. Our calculator uses the FAO Penman-Monteith equation—the international standard from FAO Irrigation and Drainage Paper 56—to calculate reference evapotranspiration (ETo), then applies crop-specific coefficients (Kc) to determine actual crop water needs (ETc).
Reference evapotranspiration (ETo) represents water loss from a standardized reference surface (well-watered grass). Crop evapotranspiration (ETc) adjusts ETo using crop coefficients (Kc) that vary by growth stage: initial (0.3-0.5), mid-season (0.9-1.2), and late (0.5-0.8). This two-step approach allows calculating water needs for any crop anywhere.
FAO Penman-Monteith Reference ET Formula
ETo = [0.408Δ(Rn-G) + γ(900/(T+273))u₂(es-ea)] / [Δ + γ(1+0.34u₂)]Replace guesswork with physics—calculate water loss from solar radiation, temperature, humidity, and wind.
FAO Penman-Monteith is adopted worldwide by agricultural agencies, researchers, and irrigation professionals.
Built-in Kc values for 27+ crops automatically adjust for canopy development and growth stages.
Proper ET-based scheduling can reduce irrigation water use by 15-30% while maintaining yields.
Calculate daily crop water demand to determine when and how much to irrigate.
Estimate seasonal water requirements for permits, reservoirs, and water rights.
Model how temperature and humidity changes affect agricultural water demand.
Compare Penman-Monteith, Hargreaves, and Blaney-Criddle methods for validation studies.
ETo (reference ET) is water loss from a standardized grass surface. ETc (crop ET) is actual water loss from a specific crop, calculated as ETc = ETo × Kc. The crop coefficient (Kc) adjusts for crop type and growth stage.