Evaporation Rate Calculator

Imagine holding a wet sponge in a hot, dry, windy room versus a cool, humid, still one. The sponge dries out far faster in the first case — not because the water is hotter, but because the surrounding air is hungry for moisture. Evaporation is fundamentally a competition between the vapor pressure at the liquid surface and the vapor pressure in the air above it.

This calculator estimates that competition using a form of the vapor pressure deficit equation. The air temperature determines how much water vapor the air can hold at maximum (saturation vapor pressure). The relative humidity tells us how much it actually holds. The gap between those two values — the deficit — is what drives evaporation. A bigger gap means faster evaporation. Wind accelerates the process by continuously replacing the saturated air layer directly above the water with drier air from elsewhere.

The surface area simply scales the total volume lost. A pool twice as large loses twice as much water per day under identical conditions. The liquid type correction factor accounts for physical differences between liquids — acetone has higher vapor pressure than water at the same temperature, so it evaporates faster. Saltwater has lower vapor pressure than fresh water, so it evaporates more slowly. These factors are approximations, but they are close enough for practical planning.

Use this calculator when you need to plan water top-ups for pools, ponds, or ornamental water features, estimate storage losses from open tanks or reservoirs, assess solvent evaporation in a lab or industrial tray, or size ventilation for a space containing evaporating liquids. It is also useful for irrigation planning when an uncovered water source feeds a drip system — knowing daily losses helps size the tank correctly.

This calculator is not appropriate for pressurized systems, for flow-through channels where the water surface is constantly renewing, or for calculating evapotranspiration from soil and plant surfaces (which requires the Penman-Monteith method and additional inputs). It is also not suitable for cryogenic liquids, highly viscous liquids, or liquids with complex multicomponent vapor pressures like fuel blends.

If you are working on a regulatory or engineering submission, treat these results as a screening estimate only. Field measurement using an evaporation pan or lysimeter will always be more accurate for site-specific conditions. This tool gives you the right order of magnitude to make confident operational decisions.

The most common mistake is ignoring wind when estimating outdoor evaporation. Users often enter temperature and humidity accurately but leave wind at zero, which is almost never true outdoors. A calm-day estimate for a rooftop tank or irrigation reservoir can underestimate actual losses by 30-50%, leading to undersized top-up schedules or incorrect water budget calculations.

A second frequent error is confusing air temperature with water temperature. In practice, a pool in afternoon sun may have water that is 5-8 degrees cooler than the air above it. This tool uses a combined temperature as a working approximation. When precision matters, use the water surface temperature, which is the actual driver of vapor pressure at the evaporation interface. Using air temperature on a hot sunny day will overestimate evaporation.

A third mistake is applying this model to boiling or near-boiling liquids. The vapor pressure relationship changes dramatically above around 80 degrees Celsius, and at 100 degrees the liquid boils rather than evaporates. The formula breaks down at these temperatures. Similarly, applying it to frozen surfaces (sublimation from ice) requires an entirely different approach. Always check that your inputs sit within the valid range of 0-99 degrees Celsius.

The core calculation is built on the Magnus formula for saturation vapor pressure. For temperature T in Celsius, saturation vapor pressure eSat in hPa is computed as: eSat = 6.1078 times e raised to the power of (17.27 times T divided by T plus 237.3). Actual vapor pressure eAct equals eSat multiplied by relative humidity divided by 100. The vapor pressure deficit VPD equals eSat minus eAct.

Evaporation rate per unit area (in kg per square meter per day) equals an empirical mass transfer coefficient k multiplied by VPD multiplied by a wind enhancement factor and the liquid correction factor. The wind enhancement factor used here is (1 + 0.015 times wind speed in km/h), which models the linear increase in surface renewal as airflow increases. The coefficient k is calibrated at 0.0175 kg per square meter per day per hPa — an open-water value consistent with published empirical data for lakes and reservoirs.

Because 1 kg of water occupies approximately 1 liter, the mass rate converts directly to a volume rate in liters per square meter per day. Multiplying by surface area gives total daily volume loss. Depth loss per day (in millimeters) equals the volumetric rate per unit area, since 1 liter per square meter equals exactly 1 millimeter depth.

The mass transfer coefficient used here (0.0175 kg/m2/day/hPa) is a bulk empirical constant calibrated for open, quiescent water surfaces. It embeds assumptions about boundary layer thickness and turbulent mixing that break down at very high wind speeds, very small surface areas (where edge effects dominate), or when a surface film is present. The linear wind factor also understates the effect at high wind speeds — at 60 km/h and above, a power-law or logarithmic wind profile is more physically accurate. For tanks narrower than about 2 meters, edge ventilation effects can increase real evaporation by 10-20% above the planar estimate.