GardenCalcs.uk
Evapotranspiration Calculator — Daily Water Loss
Estimate daily water loss from your garden based on weather conditions.
Daytime average air temperature. Higher temperatures speed up evaporation and raise ET — a UK summer day is around 20-28°C.
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%.
Wind 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).
How 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).
Planted area you are watering, in square metres. Daily litres = ET (mm) × this area, since 1 mm over 1 m² equals 1 litre.
Extra ET added when humidity is below 40%. Dry air pulls moisture from soil and leaves faster, so ET rises — 20-40% is typical for a low-humidity day.
ET cut when humidity is above 70%. Moist air slows evaporation and transpiration, so ET falls — around 20% on a muggy day.
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
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.
Related Calculators
Hose Flow Rate Calculator
Estimate water flow rate based on hose diameter and pressure.
Water Butt Size Calculator
Calculate the ideal water butt size based on your roof area and rainfall.
Water Pressure Calculator
Calculate water pressure and flow rate for garden irrigation systems.
Watering Can Calculator
Calculate how many watering cans you need for your garden beds.
Rainwater Harvesting Calculator
Calculate annual rainwater collection potential from your roof.
Was this calculator helpful?
One tap. Votes are anonymous and carry the page address, your device type, screen size and country, the numbers in the form and the results on screen, so we can check the answer you saw.
Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.