Free evapotranspiration calculator. Find ETo with FAO Penman-Monteith, Hargreaves, or Blaney-Criddle, plus ETc for 27+ crops by growth stage.
FAO-56 standard. Most accurate, requires full weather data.
Kc: 0.3 1.2 0.6 · 150-day season
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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
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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.
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.
Use Hargreaves when you only have temperature data (no humidity, wind, or radiation). It's less accurate but practical for data-limited situations. Studies show it performs reasonably in semi-arid and arid climates.
Young crops with small canopies transpire less (Kc 0.3-0.5). At full canopy (mid-season), transpiration peaks (Kc 0.9-1.2). As crops mature and senesce, leaves dry and Kc drops (0.5-0.8).
FAO Penman-Monteith is considered the gold standard with typical errors of 5-10% when using quality weather data. It outperforms empirical methods in most climates and is recommended by FAO, ASCE, and national agricultural agencies worldwide.