Evaporation Rate Calculator

Measured mass-loss method

This direct method is often the most defensible option when the liquid can be weighed accurately before and after a controlled observation period.

Measured volume-loss method

The volume reduction is converted to mass with the selected liquid density. Meniscus reading, thermal expansion, leakage, and liquid transfer can affect the result.

Predictive mass-transfer method

The calculator converts vapor partial pressures to concentrations using the ideal-gas relation. It then multiplies the concentration driving force by exposed area, an estimated or custom mass-transfer coefficient, and scenario adjustment factors.

Heat limitation

When enabled, the calculator uses the smaller of the mass-transfer estimate and the heat-limited rate. This prevents the predicted phase change from demanding more energy than the entered heat source can supply.

Select the most appropriate method

Use measured mass loss when reliable scale readings are available. Use measured volume loss when liquid-level or volume readings are more practical. Choose a predictive mode for water surfaces, pools, spills, or solvents when only physical and environmental conditions are available.

Define the liquid

Select a stored liquid or choose the custom option. For a custom liquid, enter density, molecular weight, latent heat, boiling point, and vapor pressure at the liquid temperature. Poor property data can dominate the uncertainty in the final result.

Enter the exposed geometry

Select direct area, rectangle, circle, ellipse, annulus, pool, or circular spill. Enter only the liquid surface exposed to air. The wetted walls and container bottom normally do not count as evaporating area unless the model specifically represents a thin film.

Add environmental conditions

Liquid temperature controls saturation vapor pressure. Air temperature affects vapor concentration conversion. Relative humidity is especially important for water. Air speed influences the estimated mass-transfer coefficient. Indoor, outdoor, and enclosed settings apply broad screening adjustments.

Review advanced factors carefully

Activity, occupancy, waves, ventilation, covers, mixture composition, safety factors, and heat limits can change results substantially. Keep neutral values near 1.0 unless you have a reason to use another value. A cover reduction is entered as a percentage from zero to one hundred.

Interpret the outputs

The main result is mass evaporated per hour. The page also reports volume loss, area-normalized flux, liquid-depth reduction, projected daily and annual losses, latent heat demand, vapor-pressure driving force, time to empty, and surface area needed for a target rate.

Evaporation occurs when energetic molecules leave a liquid surface and enter the gas phase. The rate is not controlled by one variable alone. Temperature changes vapor pressure, while surrounding vapor concentration changes the available driving force. Air movement can remove vapor from the boundary layer above the surface, increasing mass transfer. Surface area matters because a larger interface provides more space for molecules to escape.

A measured-loss calculation is conceptually simple, but experimental details still matter. A changing room temperature can alter scale readings or liquid density. Splashing, leaks, sampling, condensation, and container handling can appear as evaporation. Better experiments use a stable container, repeat measurements, consistent timing, and a protected balance. Multiple trials provide a clearer estimate than one short observation.

Predictive calculations are more uncertain. The air directly above a liquid may be calmer than a nearby weather station reports. A spill can spread, soak into a porous surface, cool as it evaporates, or become compositionally different over time. A pool can experience waves, swimmers, water features, covers, changing humidity, and mechanical ventilation. Chemical mixtures can deviate strongly from ideal behavior, so a single activity coefficient or mole-fraction correction may not capture the complete vapor-liquid equilibrium.

The calculator therefore separates stored physical properties from scenario adjustment factors. Vapor pressure supplies the thermodynamic tendency to evaporate. The mass-transfer coefficient approximates transport through the air. Area scales the total loss. Mixture, cover, activity, ventilation, wave, and safety factors modify the screening result. An optional heat limit represents the energy required for phase change.

Results should be treated as estimates rather than guaranteed operating values. For equipment sizing, occupational exposure, emergency response, environmental permitting, fire protection, or hazardous-material planning, use validated correlations, site measurements, and qualified professional review. The most useful role of this calculator is comparison: testing how area, temperature, humidity, airflow, covers, and liquid properties may change the expected rate under clearly stated assumptions.