What ETo and ETc tell you
Evapotranspiration is water lost to the atmosphere from soil evaporation and plant transpiration combined, measured as a depth per day, the same way rainfall is measured. Reference evapotranspiration (ETo) is that loss from a hypothetical, well-watered, short green grass surface, defined so that it depends only on the weather, not on what's actually planted. It's a common yardstick that lets you compare water demand across locations and dates on equal footing.
Crop evapotranspiration (ETc) takes that yardstick and applies it to an actual plant by multiplying ETo by a crop coefficient (Kc), a number that captures how a specific crop at a specific growth stage uses water compared to the grass reference. A newly planted crop with little leaf cover might have a Kc well under 1; a mature, dense crop in peak growth can exceed 1.
As a rough sense of scale, daily ETo commonly runs from under 1 mm (0.04 in) on a cool, overcast day up to 9–10 mm (roughly 0.35–0.4 in) on a hot, dry, windy one. Season, latitude and local climate matter far more than any single number, which is why this calculator asks for the day's actual conditions rather than offering a lookup table.
This page is the entry point: tell it what data you have, and it applies the matching method. If you'd rather work through crop growth stages, adjust for irrigation efficiency, or dig deeper into a single method's inputs, links to those specific calculators are throughout this page.
How this calculator works
The calculator applies one of three established methods, chosen by what you select above. All three estimate ETo in millimeters or inches per day; adding a crop coefficient converts that into ETc.
Method 1: Full weather data — FAO Penman-Monteith
With temperature, humidity, wind and solar radiation, the calculator uses the FAO-56 Penman-Monteith equation, the internationally standard reference-ET method. It balances an energy (radiation) term against an aerodynamic (wind and humidity) term.
Penman-Monteith reference ET
ETo = [0.408 Δ (Rn − G) + γ (900 / (T + 273)) u2 (es − ea)] / [Δ + γ (1 + 0.34 u2)]
Δ is the slope of the vapor pressure curve, γ is the psychrometric constant (from elevation), Rn is net radiation, G is soil heat flux (taken as zero for a daily estimate), u2 is wind speed at 2 m, es and ea are saturation and actual vapor pressure, and T is mean air temperature.
Net radiation Rn comes from the solar radiation you enter, the extraterrestrial radiation for your latitude and date, and outgoing longwave radiation, which depends on temperature, humidity and how clear the sky was. Elevation feeds in through atmospheric pressure, which sets γ.
Method 2: Temperatures only — Hargreaves-Samani
When only the daily high and low temperature are available, the calculator falls back to the Hargreaves-Samani equation, which estimates radiation from temperature swing and day length rather than requiring it as an input.
Hargreaves-Samani
ETo = 0.0023 × (Tmean + 17.8) × √(Tmax − Tmin) × Ra
Tmean is the average of the day's high and low, Tmax − Tmin is the daily temperature range, and Ra is extraterrestrial radiation for your latitude and date, converted to an equivalent water depth. It typically runs within about 10–15% of a full Penman-Monteith estimate for the same day.
Method 3: Pan evaporation
With a measured Class A pan reading, the calculator applies a pan coefficient to convert pan water loss, which runs faster than plant-driven ET, down to an ETo estimate.
Pan evaporation method
ETo = Kp × Epan
Epan is the measured daily pan evaporation and Kp is a pan coefficient reflecting the pan's surroundings, wind and humidity. This calculator offers typical Kp ranges rather than the full interpolated table, since the exact value depends on site specifics you're best placed to judge.
Adding ETc
Crop ET
ETc = ETo × Kc
When this calculator isn't the right tool
Use a different tool for:
- A deeper look at one method, with more control over its inputs (separate max/min humidity, measured sunshine hours instead of solar radiation, a non-standard wind measurement height). See the reference ET calculator, Hargreaves ET calculator, Blaney-Criddle calculator, or pan evaporation to ET calculator.
- Crop coefficients by growth stage rather than one seasonal Kc. See the crop evapotranspiration calculator.
- Turning ETo into an irrigation depth after subtracting effective rainfall. See the irrigation water requirement calculator.
Worked example
A site at 36°N and 100 m (328 ft) elevation on July 15: high 32°C (89.6°F), low 18°C (64.4°F), average humidity 40%, wind speed 2.0 m/s (4.47 mph), solar radiation 25 MJ/m²/day.
- Mean temperature: (32 + 18) ÷ 2 = 25°C.
- Vapor pressures: es ≈ 3.41 kPa (from Tmax and Tmin); ea = 0.40 × 3.41 ≈ 1.36 kPa.
- Slope and psychrometric constant: Δ ≈ 0.189 kPa/°C at 25°C; γ ≈ 0.0666 kPa/°C at 100 m elevation.
- Radiation terms: extraterrestrial radiation Ra ≈ 40.8 MJ/m²/day for this latitude and date; clear-sky radiation Rso ≈ 30.7 MJ/m²/day; net radiation Rn ≈ 14.1 MJ/m²/day after accounting for the entered solar radiation and outgoing longwave loss.
- ETo: combining both Penman-Monteith terms gives about 6.4 mm/day (0.25 in/day), split roughly 57% radiation-driven and 43% wind/humidity-driven.
The same day with only the temperatures (32°C/18°C, same latitude and date) gives a Hargreaves estimate of about 6.1 mm/day (0.24 in/day) — close to the full weather result, as expected for a method built to approximate it.
A site with a Class A pan reading 8 mm/day (0.31 in/day) and a typical pan coefficient of 0.7 gives ETo = 0.7 × 8 = 6.0 mm/day (0.24 in/day). Adding a crop coefficient of 0.8 to the first example's 6.4 mm/day ETo gives ETc ≈ 5.1 mm/day (0.20 in/day).
Getting good inputs
ETo is only as good as the weather data behind it. A nearby airport, state agricultural weather network, or personal weather station with solar radiation and wind sensors gives the most reliable full-weather estimate. Confirm your wind speed represents a 2-meter measurement height; a taller anemometer reads higher than a 2 m one for the same actual wind, and using it unadjusted will overstate ETo.
If you're missing solar radiation, wind or humidity but have reliable daily high/low temperatures from any thermometer or station, the temperatures-only method is a reasonable fallback rather than guessing at the missing values. Wind speed and solar radiation move the full Penman-Monteith result the most, so double-check their units specifically if a result looks off.
For the pan method, a Class A pan needs to be read at the same time each day, refilled to its standard depth, and screened from animals and debris. Its surroundings (grass or bare soil upwind) and typical wind and humidity for your site decide which Kp range fits best; if you've run a side-by-side comparison against a weather-based ETo before, use the Kp that reproduced it.
What changes evapotranspiration
The weather inputs you enter directly set the result, but it helps to know which ones matter most and why.
| Factor | Effect on ETo | What to check |
|---|---|---|
| Solar radiation | Usually the single largest driver; more energy input means more water loss. | Confirm units (MJ/m²/day, not W/m²) and that the value is a daily total, not an instantaneous reading. |
| Wind speed | Higher wind strips more moisture from the surface, especially in dry air. | Confirm the measurement height is 2 m, or close to it. |
| Humidity | Drier air pulls more moisture out; humid air suppresses ET even on hot days. | Use the day's average, not the daily minimum. |
| Temperature | Higher temperatures raise the air's capacity to hold moisture, increasing ET. | Match the unit system (°F vs °C) to what you selected above. |
| Elevation | Lower air pressure at altitude has a small, positive effect on ETo. | Only used by the full weather method. |
| Latitude and date | Set day length and sun angle, which drive the extraterrestrial radiation used by two of the three methods. | Use the date the weather was actually recorded, not today's date. |
Frequently asked questions
What's the difference between ETo and ETc?
ETo, reference evapotranspiration, is the water use of a hypothetical, well-watered grass surface under the weather conditions you enter. ETc, crop evapotranspiration, applies a crop coefficient (Kc) to ETo to estimate water use for a specific crop or plant at a specific growth stage. This calculator gives you ETo directly, and ETc as soon as you add a Kc value.
Which evapotranspiration method should I use?
Use whichever method matches the data you actually have. Full weather data (temperature, humidity, wind and solar radiation) gives the most reliable estimate through the Penman-Monteith equation. If you only have daily high and low temperatures, the Hargreaves method gives a reasonable approximation. If your site has a Class A evaporation pan, the pan method converts a measured pan reading directly into ETo.
Can this calculator give me ETc directly?
Yes. Enter a crop coefficient (Kc) in the optional field and the calculator multiplies it by ETo to show ETc. If you need a Kc value for a specific crop and growth stage rather than a single seasonal estimate, the crop evapotranspiration calculator works through those stages in more detail.
See the crop evapotranspiration calculator.
Where do I find solar radiation data for my site?
Many state and regional agricultural weather networks, such as CIMIS in California or a state mesonet, publish daily solar radiation in megajoules per square meter per day (MJ/m²/day) alongside temperature and wind data. If your source reports radiation in watts per square meter as a daily average, multiply by 0.0864 to convert to MJ/m²/day.
Why does my ETo estimate seem too high or too low compared to what I expected?
Small input errors have an outsized effect. Wind speed and solar radiation move the Penman-Monteith result the most, so double-check their units, and confirm your wind speed represents a 2-meter measurement height, which this calculator assumes. A humidity or temperature entered in the wrong unit system will also throw the result off substantially.
Does elevation affect evapotranspiration?
Yes, but only through the full weather method. Higher elevation means lower atmospheric pressure, which changes the psychrometric constant used in the Penman-Monteith equation and very slightly increases the reference ET for a given set of weather conditions. The Hargreaves and pan methods in this calculator don't use elevation directly.
What's the difference between reference ET and potential ET?
They describe closely related but not identical ideas. Reference ET (ETo), the figure this calculator produces, is specifically defined for a hypothetical grass reference surface with fixed height, albedo and resistance properties. Potential ET is an older, more general concept for maximum water loss from any well-watered surface, and different potential ET formulas can disagree with each other and with ETo by a meaningful margin.
Can I use pan evaporation data if my weather station doesn't measure solar radiation?
Yes, that's exactly the situation the pan evaporation method is built for. Enter your measured daily pan evaporation and a pan coefficient (Kp) that reflects your pan's surroundings, and the calculator multiplies the two to estimate ETo without needing radiation, wind or humidity data at all.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- All three formulas, the unit conversions and every worked example were verified by hand and traced through the calculator's code.
- Switching between US and metric units was confirmed to return the same physical result.
- Method selection follows FAO-56 and the original Hargreaves-Samani paper; no method or coefficient was invented.
Sources
- Allen, R.G., Pereira, L.S., Raes, D., Smith, M. Crop Evapotranspiration — Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper No. 56. Food and Agriculture Organization of the United Nations, 1998. [VERIFY URL]
- Hargreaves, G.H., Samani, Z.A. "Reference Crop Evapotranspiration from Temperature." Applied Engineering in Agriculture, 1(2): 96–99, 1985.
- California Department of Water Resources. California Irrigation Management Information System (CIMIS).
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 623, Chapter 2: Irrigation Water Requirements. [VERIFY URL]