What is evapotranspiration?

Evapotranspiration (ET) is the combined water loss from soil evaporation and plant transpiration. It determines how much water your garden loses daily.

Why does ET matter for watering?

Knowing ET helps you water the right amount. Replace the water lost through ET and you maintain optimal soil moisture without over or under-watering.

What affects ET rate?

Temperature (higher = more ET), humidity (lower = more ET), wind speed (windier = more ET), and sun exposure (more sun = more ET).

What is a typical ET rate in the UK?

In summer, 3-5 mm/day. In spring/autumn, 1-3 mm/day. In winter, under 1 mm/day. UK evaporation in December is roughly a fifth of June, so watering need drops sharply outside the growing season. These are rough averages.

Does rainfall reduce my watering need?

Yes. Subtract recent rainfall (in mm) from the ET figure to get net water loss — 1 mm of rain on 1 m² delivers 1 litre, the same basis this calculator uses for ET. If rainfall over a period exceeds ET, the soil has a moisture surplus and you can skip watering.

How accurate is this estimate?

This is a quick temperature-led estimate, not a full Penman-Monteith (FAO-56) or Hargreaves-Samani calculation. It has no direct solar-radiation or day-length term, so two days at the same air temperature in June and October return the same ET even though real ET0 can differ roughly twofold. Treat it as a watering guide and check soil moisture before irrigating.

Evapotranspiration Calculator — Daily Water Loss

Estimate daily water loss from your garden based on weather conditions.

Temperature

Daytime average air temperature. Higher temperatures speed up evaporation and raise ET — a UK summer day is around 20-28°C.

Humidity

Relative humidity. Below 40% the dry-air boost kicks in (more ET); above 70% the humid-air reduction applies (less ET). UK summer days are often 50-70%.

WindWind carries away moist air over leaves and soil, so an exposed plot loses water faster than a sheltered walled garden (×0.8 calm to ×1.3 windy).

Sun ExposureHow much direct sun the bed gets. Solar energy drives evaporation, so full sun loses far more than a shaded border (×1.0 full to ×0.4 full shade).

Garden Area

Planted area you are watering, in square metres. Daily litres = ET (mm) × this area, since 1 mm over 1 m² equals 1 litre.

How We Calculate This

This is a rough, temperature-led estimate of evapotranspiration (ET) — it is not a full Penman-Monteith (FAO-56) or Hargreaves-Samani calculation and has no solar-radiation or day-length term, so it cannot tell seasons apart at equal temperature. Use it as a watering guide, not a precise figure.

Base ET = 0.5 + (Temperature − 10) × 0.15 mm/day. This is then scaled: ×1.30 in dry air (humidity below 40%), ×0.80 in humid air (humidity above 70%); by wind (calm 0.8, moderate 1.0, windy 1.3); and by sun (full 1.0, partial 0.7, shade 0.4). A floor of 0.5 mm/day is applied.

Daily water loss in litres = ET (mm) × Area (m²). Per FAO-56, 1 mm of depth over 1 m² = 1 litre (= 10 m³/ha), so the conversion is dimensionally exact. Weekly loss = daily × 7.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.