The evaporation rate calculator above estimates how fast water leaves an open surface, using the mass-transfer correlation that pool and natatorium design has used for a century. It returns the loss per hour and per day, the rate per square metre, and the latent heat load that evaporation places on the space — which is usually the dominant load in a pool hall and the reason the calculation is done at all.
Arb Digital publishes free engineering calculators that state what they are estimating and how confidently. Evaporation from a real surface is genuinely hard to predict: the correlations in use scatter substantially against measurement, and the honest description of any answer here is "an engineering estimate, good to within tens of per cent", not a precise figure.
What This Evaporation Rate Calculator Does
It computes the humidity ratio of saturated air at the water surface temperature, the humidity ratio of the room air from its temperature and relative humidity, and the mass transfer driven by the difference between them at the air velocity you specify. It then scales that by the surface area and converts to a daily volume and a heat load.
It is worth separating this from the humidity tools that already exist. The psychrometric calculator, the relative humidity calculator and the absolute humidity calculator all describe the state of a body of air: what it is holding, and how close to saturation it is. None of them describes a rate. This page is about mass transfer across a surface over time, which needs an area, an air velocity and a transfer coefficient on top of the air properties.
How to Use It
- Enter the water surface area. The wetted deck and any splash-out are additional and are not included.
- Enter the air velocity just above the surface, in metres per second. This is the input the result is most sensitive to and the one hardest to know.
- Enter the water and air temperatures separately. They are rarely equal, and both matter for different reasons.
- Enter the relative humidity of the air, and the atmospheric pressure if you are working at altitude.
- Apply an activity factor if your design standard specifies one, and read the daily loss and the latent heat load.
The Formula: Vapour Pressure, Humidity Ratio and Mass Transfer
The chain has three steps. First, saturation vapour pressure at a temperature T in degrees Celsius comes from the Magnus form psat = 0.61094 exp(17.625T ÷ (T + 243.04)) in kilopascals, which is the Alduchov and Eskridge refinement of the classic relation and is accurate to a fraction of a per cent across ordinary temperatures.
Second, the humidity ratio — kilograms of water vapour per kilogram of dry air — follows from the partial pressures: x = 0.62198 pv ÷ (patm − pv). At the water surface the air is taken as saturated at the water temperature, so pv = psat(Twater). In the room, pv = RH × psat(Tair). The National Weather Service discussion on humidity sets out the relationship between these quantities in plain terms.
Third, the evaporation rate per unit area is the humidity ratio difference multiplied by a transfer coefficient that rises with air velocity: E = (25 + 19v)(xs − x) in kilograms per square metre per hour, with v in metres per second. This is the SI form of the Carrier-type correlation that pool design has used since 1918; the same relation appears in I-P units in the natatorium chapter of the ASHRAE Handbook, which is the reference to follow for design work.
The latent heat load uses hfg = 2,501 − 2.361T kJ/kg at the water temperature, the standard linear approximation to the latent heat of vaporisation.
Work the defaults through by hand. At 28 °C water, psat = 0.61094 exp(493.5 ÷ 271.04) = 3.773 kPa, so xs = 0.62198 × 3.773 ÷ (101.325 − 3.773) = 0.02406 kg/kg. At 30 °C air, psat = 4.236 kPa, and at 60 per cent relative humidity pv = 2.542 kPa, giving x = 0.01600 kg/kg. The difference is 0.00805 kg/kg. With v = 0.15 m/s the coefficient is 25 + 2.85 = 27.85, so the flux is 0.2243 kg per square metre per hour. Over 32 m² that is 7.18 kg/h, or about 172 litres a day, and at hfg = 2,435 kJ/kg the latent load is 4.85 kW.
Why Air Velocity Dominates, and Why That Is a Problem
Look at the coefficient: 25 + 19v. At still-air conditions of 0.1 m/s it is 26.9. At 1 m/s it is 44, and at 3 m/s it is 82 — three times the still-air value. Air movement over the surface is the single largest lever in the whole calculation.
That is also why estimates disagree so widely. Evaporation removes water vapour into a thin boundary layer above the surface; if nothing sweeps that layer away, it approaches saturation locally and evaporation slows dramatically. Air movement replaces it with drier air and the process continues. The trouble is that the velocity just above a water surface is rarely known, is not the same as the ventilation air change rate, and varies from one end of a pool to the other. If you have not measured it, treat any velocity you assume as the largest source of uncertainty in the answer.
What the Correlation Does Not Capture
Several real effects sit outside this model. Surface agitation from swimmers, jets or fountains increases the effective area and the local turbulence, which is what design activity factors exist to account for — and those factors are specific to a standard and a use case rather than universal, which is why this page takes one as an input instead of assuming it.
Outdoors, solar radiation warms the surface layer above the bulk water temperature, so the vapour pressure at the surface is higher than the measured water temperature suggests, and evaporation runs ahead of the prediction. Rain, splash-out and backwash are separate water losses that have nothing to do with evaporation but show up in the same meter reading. A pool cover suppresses evaporation dramatically, and no correlation of this form applies to a covered surface at all.
Finally, the correlation itself is empirical. Published comparisons of pool evaporation models against measurement show substantial scatter, particularly at low air velocities where the base correlation tends to over-predict for an undisturbed surface. Use the result to size equipment with sensible margin, not to settle a dispute about a leak.
Reading the Latent Heat Load
The heat figure is often the reason the calculation is done. Every kilogram of water that evaporates carries away roughly 2,430 kilojoules of latent heat, taken from the water itself. That is why an uncovered pool cools, why heating costs are dominated by evaporation rather than by conduction through the shell, and why covering a pool overnight saves so much energy.
For an indoor pool the same heat, plus all that moisture, ends up in the room, where the ventilation and dehumidification system has to remove it. That latent load usually exceeds the sensible load in a pool hall by a wide margin, and getting it wrong in either direction causes real problems: undersize the system and the building suffers condensation and corrosion; oversize it and it runs inefficiently and overdries the space.
Where This Sits Next to the Other Humidity Tools
The distinction is between state and rate. The psychrometric calculator tells you the full state of a body of moist air; the relative humidity calculator and the absolute humidity calculator give particular moisture measures for it; the dew point calculator gives the temperature at which that air would begin to condense. All four describe air as it is. This page uses those same properties as inputs and returns a mass flow across a surface over time, which none of them does.
The vapour pressure calculator handles the saturation pressure step on its own, and the vapour pressure deficit calculator gives the driving difference in the form horticulture uses. For the water volume itself, the pool volume calculator converts a daily loss into a percentage of the pool contents.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Using the room's ventilation velocity as the surface velocity — the air just above the water moves far more slowly, and the result is highly sensitive to this figure.
- Assuming water and air are at the same temperature — they rarely are, and both appear in the calculation for different reasons.
- Applying this to a covered surface — a cover changes the physics entirely and the correlation does not apply.
- Blaming evaporation for all water loss — splash-out, backwash and genuine leaks are separate and show up identically on a meter.
- Treating the output as precise — published evaporation correlations scatter widely against measurement, so design with margin.
Related Free Tools From Arb Digital
Pair this with the psychrometric calculator for the full air state, and the relative humidity calculator or the absolute humidity calculator for the moisture inputs. The dew point calculator covers condensation risk on cold surfaces, the vapour pressure calculator and the vapour pressure deficit calculator handle the driving pressures, and the pool volume calculator puts the daily loss in proportion. Everything Arb Digital publishes is on the free online tools hub.
Frequently Asked Questions
It is an engineering estimate, not a measurement. Published comparisons of pool evaporation correlations against experiment show substantial scatter, especially at low air velocity where the classic relation tends to over-predict for an undisturbed surface. Expect to be right to within tens of per cent, and design with margin rather than treating the number as exact.
Because evaporation saturates the thin layer of air immediately above the water. If that layer is not swept away, the local vapour pressure rises towards saturation and evaporation slows sharply. Moving air replaces it with drier air continuously, which is why the transfer coefficient roughly triples between still air and a brisk three metres per second.
Not by itself. What drives evaporation is the difference in humidity ratio between saturated air at the water surface and the air above it. Warm air can hold more moisture, so warm dry air evaporates a lot, but warm air already near saturation evaporates very little. Both temperature and humidity are needed to answer the question.
Because vaporising water takes roughly 2,430 kilojoules per kilogram, and that energy comes out of the water. For an indoor pool it then has to be removed from the room by the ventilation and dehumidification system. In a pool hall this latent load usually exceeds the sensible heating load by a wide margin.
Yes, and the effect is dramatic, which is why energy guidance emphasises it. A cover stops the exchange between the water surface and the room air almost entirely, so the correlation on this page no longer applies. Nothing here models a covered surface, and applying it to one will badly overstate the loss.
Because humidity ratio depends on the ratio of vapour pressure to the remaining atmospheric pressure. At lower total pressure the same vapour pressure corresponds to a higher humidity ratio and vapour diffuses away more readily, so evaporation is faster at altitude than at sea level under otherwise identical conditions.
With more caution. The formula applies, but outdoors the air velocity is far higher and far more variable, and solar radiation warms the surface layer above the bulk water temperature so the real driving vapour pressure is higher than the measured water temperature suggests. Outdoor estimates carry considerably more uncertainty.
Not necessarily. Splash-out, backwash, and leaks all reduce the level and are indistinguishable from evaporation on a meter or a mark on the tiles. Comparing your measured loss against a calculated evaporation rate is a reasonable first check, but the wide uncertainty in the calculation means only a large discrepancy is informative.
This tool is provided for educational and preliminary engineering use. It applies an empirical mass-transfer correlation that scatters substantially against measurement, does not model surface agitation, solar gain, covers, splash-out or leakage, and is not a substitute for design to the applicable standard. Size ventilation, dehumidification and pool plant with a qualified engineer and the relevant published design guidance.
