What this calculator does
Evaporation from an open water surface, a swimming pool, a pond or a reservoir, depends mainly on how much drier the air is than the water surface itself, and how much wind is moving that drier air across it. Warmer water and lower humidity both push evaporation up, and still air holds evaporation back because the air right above the water becomes saturated and evaporation slows until that moist air is replaced.
This calculator uses a simplified version of Meyer's formula, a long-standing estimate for evaporation from open water that combines the difference in vapour pressure between the water surface and the air with a wind-speed adjustment. It is an estimate, not a precise physical model: it leaves out solar radiation, water agitation and a number of smaller effects, but it gives a reasonable order-of-magnitude figure from inputs most people can actually supply.
The formula
Saturation vapour pressure at a given temperature is estimated with the Tetens approximation, es = 6.108 × exp(17.27T / (T + 237.3)) in hectopascals. The air's actual vapour pressure is its own saturation value scaled down by relative humidity. Evaporation rate in millimetres per day is then a coefficient (0.5 for a small or shallow body of water, 0.36 for a large or deep one) multiplied by the vapour pressure difference and a wind-speed factor. Multiplying that rate by the surface area converts it to litres per day, since one millimetre of depth over one square metre is one litre.
| Term | Meaning |
|---|---|
| es | Saturation vapour pressure: the maximum vapour pressure air can hold at a given temperature, before condensation begins. |
| ea | Actual vapour pressure of the air, found from its saturation value and relative humidity. |
| K | A coefficient reflecting how exposed the water body is: higher for small, shallow water that responds quickly to conditions. |
The inputs explained
| Field | What to enter |
|---|---|
| Water surface temperature (°C) | The temperature of the water surface itself, in degrees Celsius, which is often a little different from the air temperature. |
| Air temperature (°C) | The air temperature above the water, in degrees Celsius. |
| Relative humidity (%) | The relative humidity of that air, as a percentage. |
| Wind speed (km/h) | Wind speed over the water surface, in kilometres per hour. |
| Surface area (m²) | The surface area of the water in square metres. |
| Water body size | Whether the water is a small, shallow body that reacts quickly to weather, or a large, deep one that responds more slowly. |
When to use it
Estimating pool top-up water
A backyard or lap pool loses a measurable volume to evaporation on a warm, dry, windy day; estimating that loss in litres helps explain a falling water level that is not necessarily a leak.
Sizing evaporation losses for irrigation or storage dams
Farm dams and irrigation storages lose water to evaporation across their whole surface area, and a rough daily or monthly estimate feeds into water-budget planning alongside inflow and usage.
Understanding why a heated pool costs more to run
Raising water temperature increases the vapour pressure difference driving evaporation, which is part of why a heated pool loses more water, and more heat, than an unheated one at the same size.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does wind speed change evaporation from a 32 m² pool?
The same pool and conditions, at a range of wind speeds.
| Wind speed | Evaporation rate | Water lost per day |
|---|---|---|
| 0 km/h | 9.35 mm/day | 299.1 L |
| 5 km/h | 12.27 mm/day | 392.6 L |
| 10 km/h | 15.19 mm/day | 486.1 L |
| 15 km/h | 18.11 mm/day | 579.5 L |
| 20 km/h | 21.03 mm/day | 673.0 L |
| 30 km/h | 26.87 mm/day | 860.0 L |
How does humidity change evaporation from the same pool?
The same pool and wind, across a range of relative humidity levels.
| Relative humidity | Evaporation rate | Water lost per day |
|---|---|---|
| 10% | 24.89 mm/day | 796.4 L |
| 30% | 20.04 mm/day | 641.2 L |
| 50% | 15.19 mm/day | 486.1 L |
| 70% | 10.34 mm/day | 330.9 L |
| 90% | 5.49 mm/day | 175.7 L |
Questions
How accurate is this evaporation estimate?
It is a simplified model intended to give a realistic order of magnitude, not a precise reading. Meyer's formula leaves out solar radiation, splashing or wave agitation, and local shading, all of which shift real evaporation up or down. Treat the result as a planning estimate rather than an exact figure.
Why does the calculator ask for water temperature separately from air temperature?
Evaporation is driven by the difference between the vapour pressure right at the water surface and the vapour pressure of the air above it, so the water's own temperature matters independently of the air. A sun-warmed pool surface, for instance, can sit several degrees above the surrounding air temperature.
Why choose "small/shallow" or "large/deep" for the water body?
Meyer's formula uses a different coefficient depending on exposure: a small or shallow body of water tracks changing conditions more closely and evaporates relatively faster for the same vapour pressure difference than a large, deep body with more thermal mass.
Can evaporation ever be estimated as negative, meaning condensation?
The underlying vapour pressure difference can come out negative when the air is more humid, relative to its own temperature, than the water surface, but this calculator reports evaporation as zero in that case rather than a negative loss, since the simplified formula is not intended to estimate condensation gain.
For the energy side of a heated pool or tank rather than the water lost to the air, see the specific heat capacity of water calculator. For a general unit conversion between water volume measures, see the volume conversion calculator.