Evapotranspiration Calculator
Estimate monthly potential evapotranspiration (PET) from average temperature, daylight hours, and days in the month using a simplified Thornthwaite-style formula. Returns millimetres of water lost per month.
Last updated: May 2026
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About this calculator
The formula is PET (mm/month) = 16 × (10 × avgTemp / 20) × (daylight / 12) × (daysInMonth / 30), which simplifies algebraically to PET = 8 × avgTemp × (daylight/12) × (daysInMonth/30). This is a heavily simplified Thornthwaite-style equation: the original Thornthwaite (1948) PET = 16 × N × (10T/I)^a uses a location-specific heat index I and exponent a computed from 12 monthly mean temperatures, and N is a daylight correction. This calculator drops the heat-index term and assumes a fixed exponent of 1, replacing (10T/I)^a with the linear (10T/20). Inputs: avgTemp is monthly mean air temperature in °C; daylight is mean daylight hours over the month; daysInMonth is calendar days. Edge cases: PET is set to zero or negative when avgTemp ≤ 0; the formula returns a negative number which should be clamped to 0 manually. The (10×T/20) term means PET = 0 when T = 0 °C and rises linearly — a poor approximation since real PET rises more steeply at higher temperatures (real Thornthwaite has an exponent typically 1.2–3.0). The formula gives potential ET (atmospheric demand for water assuming unlimited supply), not actual ET, which depends on soil moisture, vegetation, and rooting depth. For accurate agricultural or hydrological work, use FAO Penman-Monteith (the international standard) or Hargreaves-Samani equations.
How to use
Example 1 — temperate summer month. avgTemp 25 °C, daylight 14 hours, daysInMonth 31. Step 1: 10 × 25 / 20 = 12.5. Step 2: 14/12 ≈ 1.167. Step 3: 31/30 ≈ 1.033. Step 4: 16 × 12.5 × 1.167 × 1.033 ≈ 241 mm/month. Verify: typical PET in temperate-zone summer months is 100–180 mm/month depending on climate; the simplified formula here tends to overestimate at high temperatures because of the linear scaling. A FAO Penman-Monteith calculation for the same conditions would likely return 130–160 mm/month, so the result here is about 50% high — flag as a known limitation of the linearised Thornthwaite approach. Example 2 — winter month. avgTemp 5 °C, daylight 9 hours, daysInMonth 31. Step 1: 10 × 5 / 20 = 2.5. Step 2: 9/12 = 0.75. Step 3: 31/30 ≈ 1.033. Step 4: 16 × 2.5 × 0.75 × 1.033 ≈ 31 mm/month. Verify: winter PET in temperate zones is typically 10–40 mm/month, so 31 mm is in the realistic range ✓. The low temperature dampens PET appropriately. For freezing or below-freezing average temperatures the formula gives zero or negative results, which should be interpreted as 'essentially no evapotranspiration' since liquid water transitions to ice and frozen surfaces don't evaporate significantly.
Frequently asked questions
What is the difference between PET and actual evapotranspiration (AET)?
PET (potential evapotranspiration) is the maximum amount of water that could be evaporated and transpired from a well-watered reference surface under given atmospheric conditions — it represents atmospheric demand. AET (actual evapotranspiration) is the amount that actually occurs given real-world water availability, vegetation, and soil; it equals PET only when water is abundant and a reference crop is in full growth. In semi-arid and arid regions, AET is often much less than PET because water is the limiting factor. The ratio AET/PET ranges from near 1 in well-watered conditions to <0.3 in deserts. This calculator returns PET; to estimate AET you would need a soil-water-balance model and information about precipitation, soil-water-holding capacity, and vegetation. For irrigation planning, PET is the target — you want to supply enough water to bring AET close to PET — while for natural-system hydrology, AET is the more meaningful quantity.
Why is this Thornthwaite approximation considered less accurate than other methods?
Thornthwaite (1948) is the simplest of the widely-used PET methods because it requires only air temperature — making it valuable historically when other data was scarce. However, it ignores wind speed, humidity, and net radiation, which are all important drivers of evapotranspiration. The FAO Penman-Monteith equation (FAO Paper 56, 1998) is the international standard for reference ET because it incorporates all of these variables and aligns closely with lysimeter measurements (direct field measurements of ET). Hargreaves-Samani uses temperature plus daily temperature range as a proxy for radiation, performing better than Thornthwaite in arid regions. The further simplification in this calculator (dropping the heat-index and exponent terms) makes it even less accurate than the original Thornthwaite, especially at temperature extremes. For research or operational hydrology, use Penman-Monteith with weather-station data; for back-of-envelope estimates this calculator's coarse PET is acceptable when you understand its limitations.
How do I use PET for irrigation scheduling?
Compute reference PET (PET₀) using this calculator or a more accurate method, then multiply by a crop coefficient (Kc) that varies by crop type and growth stage to get crop ET (ETc). For example, mature tomatoes have Kc ≈ 1.15, mature maize Kc ≈ 1.20, and most crops follow a Kc curve over the growing season starting around 0.3–0.4 and peaking at 0.9–1.3. Net irrigation requirement = ETc − effective rainfall. Account for irrigation efficiency (drip 85–95%, sprinkler 70–85%, flood 40–60%) to get gross irrigation = net / efficiency. Most precision-irrigation systems schedule based on cumulative ETc since last irrigation versus available soil-water capacity, with thresholds depending on crop sensitivity to water stress. FAO Irrigation and Drainage Paper 56 is the canonical reference for Kc values and methodology. For commercial agriculture, weather-station-driven evapotranspiration models like CIMIS (California) provide daily ET₀ values that growers use directly.
What are the common mistakes when using PET calculations?
The biggest is treating PET as AET — in water-limited regions, actual water use is far below PET, and budgeting irrigation or water supply based on PET alone overestimates requirements. The second is unit confusion: PET is often expressed in mm/day (1 mm = 1 L/m²) or mm/month; converting between them requires knowing days per month. The third is applying this temperature-only formula in arid windy environments where temperature is a poor proxy for ET — windy desert areas have ET 30–50% higher than this formula suggests because wind strips moisture from surfaces faster than temperature alone predicts. People also use mean monthly temperature when daily values would be more accurate (especially in months with large diurnal variation); the formula linearises a non-linear process. Finally, ignoring the assumption that PET applies to a well-watered short grass reference — actual cropped surfaces, forests, lakes, and bare soil all have different effective ET, mediated through the crop coefficient or surface resistance.
When should I not use this calculator?
Do not use it for serious irrigation scheduling, agricultural decision-making, or water-balance modelling in semi-arid/arid regions — use FAO Penman-Monteith with full weather-station data instead. It is not appropriate for daily-resolution ET (only monthly) and cannot capture short-term weather effects like heat waves. Do not use it at high latitudes where temperature poorly correlates with PET due to long summer days but moderate temperatures; the formula systematically underestimates Arctic summer ET. It is unreliable at temperature extremes (below freezing or above ~35 °C) because the linear scaling breaks down — the true relationship is highly non-linear. Avoid using it for crops with Kc significantly different from 1 (forests, orchards, paddy rice) without applying appropriate crop coefficients to convert reference PET to crop ET. For climate-change impact studies, snowpack-melt modelling, or any context requiring radiative inputs, use a physically-based method like Penman-Monteith or Priestley-Taylor; this calculator's temperature-only approach misses key processes.