Evapotranspiration Calculator — Daily Water Loss

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

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.

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

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.

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.

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.