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Weather & Environment

Evaporation Rate Calculator

Estimate open-water evaporation from air temperature, water temperature, humidity, wind speed, and surface area, with volume and depth-loss context.

Published

Volume loss rate
Volume loss rate
0.00 L/hour
Daily volume loss
0.00 gallons/day
Depth loss rate
0.00 mm/day
Relative humidity
50%
Surface area
50 m²

Based on the given conditions, the water surface will lose approximately 0.00 mm of depth per day.

Ambient air temperature.
°C
Water surface temperature.
°C
%
Wind speed at 2m height.
m/s
Water surface area.
m²

Results update as you type.

Evaporation Rate Calculator

An evaporation rate calculator is a reality check for water disappearing from a pool, pond, tank, or reservoir. Hot dry air, warm water, and wind can remove surprising amounts of water, while humid calm weather can make losses barely noticeable. This page explains the exact compute formula behind the estimate, where it fits physically, and why the result should be used as a screening tool rather than a definitive leak diagnosis.

How it works

Evaporation needs energy and a vapor-pressure gradient. Warm water has molecules that can escape into the air. If the air above the surface is already humid, fewer molecules can be accepted before the air approaches saturation. Wind matters because it sweeps that humid boundary layer away and replaces it with air that can accept more water vapor. The calculator uses those three ideas: saturation vapor pressure at the water temperature, actual vapor pressure estimated from air temperature and relative humidity, and a wind factor.

The formula is not the FAO Penman-Monteith reference evapotranspiration method used in agriculture, and it does not model net radiation, heat storage, water salinity, pool covers, shading, splash, or inflows. For irrigation and crop-water planning, FAO methods are more appropriate. For household context, compare humidity with the relative humidity calculator and dew-point comfort with the dew point calculator. If wind exposure is the concern, the wind power calculator illustrates how strongly moving air changes transfer rates.

Formula

The calculation uses constants for water density, latent heat of vaporization, a wind factor, and a minimum wind speed:

ρ=997\rho=997 λ=2.26×106\lambda=2.26\times10^6 Ue=max⁡(U,0.1)U_e=\max(U,0.1)

Saturation vapor pressures are calculated with an exponential temperature relationship:

ew=610.7×e17.27Tw237.3+Twe_w=610.7\times e^{\frac{17.27T_w}{237.3+T_w}} ea=610.7×e17.27Ta237.3+Ta×RH100e_a=610.7\times e^{\frac{17.27T_a}{237.3+T_a}}\times\frac{RH}{100}

Then the vapor pressure deficit, wind term, and volume rate are:

VPD=ew−eaVPD=e_w-e_a F=0.0018×Ue×(1+0.15(Tw−Ta))F=0.0018\times U_e\times(1+0.15(T_w-T_a)) Q=F×VPD×Aλ×ρQ=\frac{F\times VPD\times A}{\lambda\times\rho}

The displayed liters per hour are Q times 3,600, rounded to two decimals and floored at zero. Gallons per day are liters per hour times 24 times 0.264172. Displayed millimeters per day are liters per hour times 24 divided by surface area times 1,000, then rounded and floored.

Worked example matching compute

Use a large 1,000 square meter pond, air temperature 25 degrees Celsius, water temperature 30 degrees Celsius, relative humidity 30 percent, and wind speed 5 meters per second. The water saturation vapor pressure is about 4,242.37 pascals. The air saturation vapor pressure is about 3,167.26 pascals, and actual vapor pressure is 30 percent of that, or 950.18 pascals. The vapor pressure deficit is therefore 3,292.19 pascals.

The effective wind speed is 5. The temperature difference is 5, so the wind term is 0.0018 times 5 times 1.75, which equals 0.01575. The raw Q value is 0.01575 times 3,292.19 times 1,000 divided by 2,260,000 divided by 997, or about 0.0000230124 in the calculator’s units. Multiplying by 3,600 gives 0.0828, displayed as 0.08 liters per hour. Daily gallons are 0.08 times 24 times 0.264172, or 0.51 gallons per day. The displayed depth loss rounds to 0.00 mm per day.

This example exposes a calculation issue: the depth-loss conversion divides liters by area times 1,000, which makes millimeters 1,000 times smaller than the usual liter-per-square-meter relationship.

Interpreting water loss

For pools, the best use is comparison. Run the calculator for a hot windy afternoon and for a humid calm day. If measured water loss only appears during swimming, splash-out may dominate. If the loss continues under a cover or in calm humid weather, a leak may be more plausible. Marking a water level on steps or using a bucket comparison can help separate evaporation from leakage, but this calculator does not perform that test.

For ponds and small reservoirs, surface area matters more than volume. A broad shallow pond can lose more water than a narrow deep tank with the same stored volume because evaporation occurs at the surface. Wind breaks, floating covers, shade cloth, and reducing heated water exposure can reduce loss. In gardens, remember that plant transpiration and soil evaporation are separate pathways from open-water evaporation.

Edge cases, limitations, and common mistakes

The wind term includes 1 plus 0.15 times the water-air temperature difference. If water is much cooler than the air, that factor can shrink or even become negative, after which the calculator floors the result at zero. The vapor pressure deficit can also be negative in saturated air. Those cases do not mean water physics stopped; they mean the simplified formula is outside a comfortable range or net condensation may be possible.

Do not use this as a regulatory water-rights calculation, engineered cooling-pond model, or agricultural evapotranspiration estimate. It ignores solar radiation, cloud cover, atmospheric stability, water mixing, salinity, waves, fetch, pool covers, and rainfall. The calculation also produces very small volume rates for ordinary backyard-pool inputs because of its unit structure. Treat those rates with caution.

Sources

  • FAO, Crop evapotranspiration guidelines — authoritative background on evaporation, vapor pressure, and weather-driven water loss.
  • FAO, Meteorological data chapter — humidity, wind, and weather variables used in evaporation and evapotranspiration estimates.
  • EPA WaterSense, Outdoors — practical water-efficiency context for outdoor water use.

Frequently asked questions

What does the Evaporation Rate Calculator estimate?
It estimates open-water loss from air temperature, water temperature, relative humidity, wind speed, and surface area. The output includes liters per hour, gallons per day, and an indicated depth loss. It is a weather-based estimate, not a leak test or water-balance audit.
Why does humidity reduce evaporation?
Evaporation slows when the air already contains a lot of water vapor. The calculator estimates saturation vapor pressure at the water surface, estimates actual vapor pressure from air temperature and relative humidity, and uses the difference as the driving force for evaporation.
Why does wind speed increase water loss?
Wind removes humid air sitting just above the water and replaces it with drier air. That maintains a vapor pressure difference at the surface. The calculator therefore multiplies the vapor pressure deficit by an effective wind speed term, with a small minimum wind speed.
Can the result be zero?
Yes. If the formula produces a negative or tiny value after rounding, the displayed evaporation values are clamped to zero. That can happen in saturated air, when water is cooler than air, or because very small rates are rounded to two decimals.
Can I use this for a swimming pool?
Yes, as a rough weather estimate. Pool evaporation also depends on covers, heating schedules, splash-out, wind shelter, solar gain, night cooling, and humidity close to the water. If measured water loss greatly exceeds the estimate, leaks or plumbing losses may need investigation.
Does the calculator use the Penman or FAO reference ET method?
No. It uses a simplified vapor-pressure and wind relationship for an open water surface. FAO reference evapotranspiration methods include radiation, crop reference surfaces, aerodynamic terms, and standardized weather measurements, so they are more appropriate for irrigation scheduling and agricultural water management.

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