Evapotranspiration Rate Calculator
Reference evapotranspiration (ETo) is the amount of water that would evaporate from a well-watered grass reference surface under current weather conditions. It is the foundation of irrigation scheduling, crop water requirement calculations and hydrological modelling. This calculator uses the Hargreaves-Samani (1985) method, which FAO recommends when humidity and wind data are unavailable. Enter the daily maximum temperature, minimum temperature and extraterrestrial radiation (Ra) to get ETo in millimetres per day.
Hargreaves-Samani ETo formula (FAO Paper 56)
Tmean = (Tmax + Tmin) / 2
ETo (mm/day) = 0.0023 × (Tmean + 17.8) × (Tmax - Tmin)0.5 × Ra × 0.408
Where temperatures are in deg C and Ra is extraterrestrial radiation in MJ/m2/day. The factor 0.408 converts MJ/m2 to mm of water (1 MJ/m2 = 0.408 mm). Source: FAO Irrigation and Drainage Paper No. 56, Chapter 3 (Hargreaves and Samani, 1985).
Typical ETo values by climate
- Cool and humid (Pacific Northwest winter): 0.5 to 1.5 mm/day
- Temperate and moderate humidity (US Midwest summer): 3.0 to 5.0 mm/day
- Hot and dry (US Southwest summer): 6.0 to 9.0 mm/day
- Tropical humid (Florida summer): 4.0 to 6.0 mm/day
Evapotranspiration calculator: frequently asked questions
What is evapotranspiration (ET)?
Evapotranspiration is the combined water loss from soil evaporation and plant transpiration. It is the primary pathway by which water returns from land to the atmosphere. Reference evapotranspiration (ETo) is the ET from a hypothetical reference surface (well-watered short grass or alfalfa) and is used to calculate crop water requirements by multiplying by crop-specific coefficients (Kc). FAO defines the grass reference as the standard for ETo.
What is the Hargreaves-Samani formula for ETo?
The Hargreaves-Samani (1985) formula estimates daily ETo from temperature and extraterrestrial radiation: ETo = 0.0023 x (T_mean + 17.8) x (T_max - T_min)^0.5 x Ra, where T_mean, T_max and T_min are mean, maximum and minimum daily air temperatures in deg C, and Ra is extraterrestrial radiation in mm/day (or MJ/m2/day converted by dividing by 2.45). This formula is recommended by FAO when humidity and wind data are not available.
How does ETo differ from actual crop evapotranspiration (ETc)?
ETo is the potential demand from a standardised grass reference surface. Actual crop evapotranspiration (ETc) is calculated as ETc = Kc x ETo, where Kc is the crop coefficient that varies by crop type and growth stage. For example, mature corn (maize) has Kc values of 1.15 to 1.20 at mid-season. Kc values for hundreds of crops are published in FAO Irrigation and Drainage Paper 56.
What is extraterrestrial radiation (Ra) and how do I get it?
Extraterrestrial radiation (Ra) is the solar radiation received at the top of the atmosphere at a given latitude and time of year, expressed in MJ/m2/day or mm/day (1 MJ/m2/day = 0.408 mm/day). It depends only on latitude and day of year, not on local weather. FAO Paper 56 (Table 2.6) provides monthly Ra values for latitudes from 70N to 70S. The USDA NRCS also provides seasonal Ra tables for the contiguous US.
When should I use Hargreaves-Samani vs Penman-Monteith?
The FAO-56 Penman-Monteith (PM) method is the internationally recommended standard when complete weather data (temperature, humidity, wind speed, solar radiation) are available. Hargreaves-Samani is recommended when only temperature and radiation data are available, which is the case for many remote or data-sparse locations. FAO Paper 56 states that Hargreaves-Samani gives results within 10 to 15% of PM under most conditions.
Official sources
- FAO Irrigation and Drainage Paper No. 56, Penman-Monteith and Hargreaves-Samani: fao.org.
- USDA NRCS Irrigation Guide Chapter 2 ET tables: nrcs.usda.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.