Water Evaporation Rate Calculator

Introduction: what an evaporation rate actually measures

Water leaves an open surface every hour of every day. A backyard pool, an irrigation pond, a fire-water tank, a wastewater lagoon and a municipal reservoir all give up depth to the atmosphere, and the only question is how fast. This calculator answers that question in two units at once: millimetres of depth per day, which is independent of how big the water body is, and litres per day, which is the volume you actually have to replace.

Physically, evaporation is a diffusion problem. Molecules escape the liquid where the vapour pressure at the surface exceeds the vapour pressure of the air just above it, and the wind then carries that moist boundary layer away so the process can continue. Three things therefore control the rate: how warm the water is (which sets the surface vapour pressure), how warm and how humid the air is (which sets the air-side vapour pressure), and how fast the air moves across the surface (which sets how quickly the boundary layer is refreshed). Surface area does not change the rate in millimetres per day at all; it only scales the volume.

How to use the water evaporation rate calculator

Enter the six inputs and press Estimate. Each one has a job:

• Surface area — the open water area in square metres or square feet. Use the wetted plan area, not the volume and not the wall area.
• Water surface temperature — measured in the top few centimetres, not at the bottom of a deep pond. A cheap floating thermometer is enough. Heated pools sit 4 to 8 °C above unheated ones and evaporate correspondingly faster.
• Air temperature — the shade air temperature over the water. Combined with relative humidity it sets the air-side vapour pressure.
• Relative humidity — a percentage from 0 to 100 measured at the same time and place as the air temperature. Use a mid-afternoon value if you want a daytime-dominated estimate.
• Wind speed and its measurement height — see the note below, because this is the input people get wrong most often.
• Surface activity — how disturbed the surface is. A still pond or an unoccupied pool is the quiet baseline; splashing swimmers, spas and water slides throw droplets into the air and multiply the loss.

Plain-text formula: pSatKpa(tC) = 0.61121 * exp((18.678 - tC / 234.5) * (tC / (257.14 + tC))); pW = pSatKpa(waterTempC); pA = (relativeHumidityPct / 100) * pSatKpa(airTempC); latentHeatKjPerKg = 2501 - 2.361 * waterTempC; surfaceWindMps = gaugeWindMps * heightFactor; massFluxKgPerSecM2 = activityFactor * (pW - pA) * (0.089 + 0.0782 * surfaceWindMps) / latentHeatKjPerKg; evaporationMmPerDay = max(0, massFluxKgPerSecM2 * 86400); litersPerDay = evaporationMmPerDay * areaM2; gallonsPerDay = litersPerDay * 0.264172; inchesPerDay = evaporationMmPerDay * 0.0393701. Units: temperatures in degrees Celsius, pressures in kilopascals, wind in metres per second, area in square metres.

Planning limitation: this is a screening estimate. It is not hydrologic engineering for reservoirs, pools, or industrial systems without calibration against local measurements.

The ASHRAE mass-transfer formula behind the estimate

The engine is the evaporation equation published in the ASHRAE Handbook — HVAC Applications for natatoriums, which descends from Carrier's 1918 mass-transfer work. In SI units the mass loss rate is

Formula: w_p = (A ⁢ F_a p_w − p_a 0.089 + 0.0782 ⁢ V) / Y

where is the evaporation rate in kilograms per second, is the water surface area in square metres, is the saturation vapour pressure at the water surface temperature in kilopascals, is the actual vapour pressure of the air in kilopascals, is the air velocity over the surface in metres per second, is the latent heat of vaporisation in kilojoules per kilogram, and is the activity factor.

The two vapour pressures come from the Buck (1981, revised 1996) saturation curve over liquid water, which is accurate to better than 0.1 percent between −30 °C and +60 °C:

Formula: p_sat t = 0.61121 ⁢ e^18.678−t/234.5⁢t/(257.14+t)

The water side uses the water temperature and the air side is scaled by relative humidity:

Formula: p_w = p_sat T_w, p_a = H / 100 ⁢ p_sat T_a

Latent heat falls slightly as the water warms, and the linear fit used here is accurate to within a few parts per thousand across the range this page allows:

Formula: Y = 2501 − 2.361 ⁢ T_w

Because one kilogram of water spread over one square metre is exactly one millimetre of depth, dividing the mass rate by the area and multiplying by 86 400 seconds converts the result straight into a depth rate:

Formula: E = 86400 ⁢ w_p / A

and the volume you must replace each day is simply the depth rate times the area, , in litres when is in millimetres and is in square metres.

Wind speed: the input that ruins most estimates

The velocity in the ASHRAE equation is the air speed at the water surface. National weather services report wind at a standard 10 m mast height, and over open water that reading is substantially faster than the air scraping across the pool. Choosing the "10 m weather-station reading" option applies the logarithmic wind profile with the World Meteorological Organization's open-water roughness length of 0.0002 m, evaluated at a 0.3 m working height:

Formula: V_0.3 / V_10 = (ln 0.3 / 0.0002) / (ln 10 / 0.0002) ≈ 0.676

Entering a forecast wind of 4 m/s without that correction inflates the surface velocity by roughly 48 percent, and because the wind term is close to linear the evaporation estimate inflates by a similar amount. If you measured with a handheld anemometer held just above the water, choose the surface option instead and no correction is applied.

Reading the mm/day and litres/day figures

The calculator reports two numbers that answer two different questions. The millimetre-per-day figure is a depth: how far the surface would drop in 24 hours if nothing refilled it, independent of how big the pond or pool is. The litre-per-day figure multiplies that depth by your surface area, so it tells you the actual volume you would have to replace. A 4 mm/day rate sounds small until you spread it over a 50 m² pool and discover that is 200 litres walking out the top every day.

For quick sanity checks: rates under about 2 mm/day are typical of mild, humid, still conditions; 4 to 8 mm/day is a warm breezy summer afternoon on a quiet surface; and anything above 12 mm/day points to hot, dry, windy desert-style weather or a busy, splashy pool. If your number lands far outside those bands, recheck the units on wind speed and confirm you entered water temperature rather than air temperature. The table below runs four representative days through the formula on a still surface (activity factor 0.5) so you can see how much each lever moves the result:

Notice the third row: the air is freezing compared with the water, the humidity is high, and the pool still loses more than the warm afternoon in row two. That is the vapour-pressure deficit at work, and it is invisible to any formula that only looks at air temperature.

Worked example: a 32 m² backyard pool in a heatwave

Take a 32 m² unoccupied residential pool during a heatwave. The water sits at 28 °C, the shade air temperature is 34 °C, relative humidity is 25 percent, and a handheld anemometer at the water surface reads 2.0 m/s. The activity factor for a still, unoccupied pool is 0.5.

Step 1, the water-side vapour pressure: p_sat(28) = 3.7814 kPa. Step 2, the air-side vapour pressure: p_sat(34) = 5.3229 kPa, and 25 percent of that is 1.3307 kPa. The deficit is therefore 3.7814 − 1.3307 = 2.4507 kPa. Step 3, the wind term: 0.089 + 0.0782 × 2.0 = 0.2454. Step 4, latent heat: 2501 − 2.361 × 28 = 2434.9 kJ/kg.

Putting them together, the mass flux is 0.5 × 2.4507 × 0.2454 ÷ 2434.9 = 1.2350 × 10⁻⁴ kg per second per square metre. Multiplying by 86 400 seconds gives 10.67 mm/day, or 0.42 in/day. Across the 32 m² surface that is 341 litres per day — about 90 US gallons, or nearly 2,400 litres a week. A cover that suppresses roughly 70 percent of that loss saves about 1,700 litres a week during the worst stretch of summer, which is usually enough to pay for itself inside one season.

Who reaches for an evaporation number

Irrigators are the classic users. If a field loses ten millimetres a day to the atmosphere, that is ten millimetres the crop is not getting from the soil, so the number feeds directly into how much to put back through the sprinklers. Pool owners run the same maths in reverse to decide whether a weekly top-off keeps up or whether it is time to buy a cover. Reservoir and pond managers care about the cumulative volume: a few millimetres a day across several hectares becomes a serious slice of the annual budget during a long summer, which is why open-water storage is often the first thing rationed in a drought. Even a rough figure is enough to tell these decisions apart from guesswork.

Beyond quantity, evaporation also influences water quality. As water evaporates, dissolved minerals and contaminants become more concentrated. Understanding the rate helps maintain balanced chemistry, especially in pools or artificial ponds where mineral buildup causes scaling or feeds algae. It also explains a common pool-owner puzzle: a pool that "loses chlorine fast" is often simply a pool that loses a lot of water and gets diluted by fresh top-up on a daily basis.

Factors the four inputs do not capture

Temperature, humidity, and wind are the most visible drivers of evaporation, but the story goes deeper. Sunlight supplies the energy that warms the surface layer, so solar radiation matters enormously — though in this model it enters indirectly through the water temperature you measure, which is exactly why the water temperature input exists. Dense cloud cover or shade from buildings and vegetation lowers the energy available and slows the process by cooling the water over hours to days.

Salinity also influences evaporation: seawater has a vapour pressure roughly 2 percent lower than fresh water at the same temperature, which reduces the rate by a similar fraction. At higher altitudes, reduced atmospheric pressure increases the diffusion coefficient of vapour in air, so mountain lakes can lose water briskly despite cooler temperatures; the ASHRAE correlation was fitted near sea level and does not include a pressure term. Finally, the equation assumes a well-mixed air stream over an essentially flat surface, so it says nothing useful about spray, fountains, cascades, or aerated features, where droplet surface area dominates everything else.

Measuring evaporation in practice

Scientists and water managers use several techniques to gauge actual evaporation. A common tool is the Class A evaporation pan, a standardised metal basin filled with water. By measuring how much the water level drops each day, researchers estimate evaporation rates for nearby bodies of water. Pan readings must be adjusted with pan coefficients — typically around 0.7 for a Class A pan — because the pan's small size and metal walls respond to weather differently than lakes or reservoirs.

Another approach is to install staff gauges or floating level sensors and track level changes over time. This method captures evaporation directly for the specific pond or tank, but it requires still conditions and a correction for rainfall and leakage. The simplest domestic version is the bucket test: float a bucket of pool water on the surface, mark both levels, and compare the drop after 24 rainless hours. If the pool drops more than the bucket, the difference is a leak rather than evaporation, and that is a distinction this calculator can help you make.

Strategies to reduce evaporation

Once you understand how much water is escaping, the next step is often to reduce the loss. Physical covers provide the most dramatic effect: published field studies and utility conservation programmes generally credit solid or bubble pool covers with 50 to 95 percent suppression, with 70 percent a fair planning figure. Covers work on two fronts at once — they cut the wind term to nearly nothing and they let the trapped air under the cover saturate, collapsing the vapour-pressure deficit.

Windbreaks made of fences, hedges, or shade sails lower the surface velocity across small ponds and pools. Because the wind term is 0.089 + 0.0782 V, dropping surface wind from 2.0 to 0.8 m/s takes the term from 0.245 to 0.152, a 38 percent cut in evaporation for zero running cost. Chemical suppressants, sometimes called monolayer covers, spread a molecular film across the surface and are common on agricultural reservoirs where physical covers are impractical; field results are much more variable than solid covers because wind and waves break the film up. Lowering the set-point of a heated pool by 2 °C is another quiet win, worth roughly 12 to 15 percent of the evaporative loss.

Seasonal and climate considerations

Evaporation rates fluctuate dramatically with the seasons. Summer brings higher water temperatures and lower humidity, often causing spikes in water loss. In colder months evaporation slows, but it does not stop: wind over a heated pool or a warm wastewater lagoon in winter can produce very high rates, because the vapour-pressure deficit between 28 °C water and 2 °C air is enormous. This is the same physics that makes steam rise off a heated pool on a frosty morning.

Climate trends add another layer. Rising air temperatures increase potential evaporation, and shifting precipitation patterns leave water bodies more exposed during droughts. Regions in prolonged drought increasingly require pool covers or restrict topping up, so a defensible estimate of your own losses is becoming a practical compliance document rather than a curiosity.

Limitations and assumptions you should carry with the number

This calculator streamlines a genuinely complex process, and the limitations are worth stating plainly. First, the ASHRAE correlation was developed and validated for indoor natatoriums at low air velocities, roughly 0.05 to 0.15 m/s; Shah's comparative reviews found that no published correlation is reliably accurate outside the data it was fitted to, and errors of 20 to 30 percent are normal even for the best of them. Applying it outdoors at several metres per second is an extrapolation that this page makes deliberately and that you should treat as indicative only.

Second, every input is a snapshot. Real weather varies hour by hour, so a single set of readings produces a single-day potential rate, not a reliable monthly total. Average several readings across the hot part of the day, or run the calculator two or three times for morning, afternoon, and night conditions and weight the results. Third, the model has no radiation term, no atmospheric pressure term, no salinity term, and no allowance for splash-out, leaks, backwash, or filter cleaning — all of which show up as "evaporation" in a pool owner's water bill. Finally, the activity factor is a coarse published lookup, not a measurement; occupancy in a real pool is bursty, and Shah's occupied-pool work shows agitation effects that a single multiplier cannot represent well. Calibrate against your own level records whenever you can, and treat the calculator as a way to compare scenarios rather than as a source of engineering-grade numbers.

Evaporation questions pool and pond owners ask

Sources and further reading

The evaporation engine, the vapour-pressure curve, the wind-profile correction, and the activity factors on this page come from the following published sources:

• M. M. Shah, "Methods for Calculation of Evaporation from Swimming Pools and Other Water Surfaces," ASHRAE Transactions 120(2), 2014 — states the ASHRAE/Carrier equation in SI form, and compares the published correlations against measured data.
• Measurement and Analysis of Evaporation in Indoor Swimming Pools: Comparison with the ASHRAE Activity Factor, E3S Web of Conferences 396, 2022 — reproduces the ASHRAE equation and the published activity-factor table.
• A. L. Buck, "New Equations for Computing Vapor Pressure and Enhancement Factor," Journal of Applied Meteorology 20, 1981 — the saturation vapour pressure curve used for both p_w and p_a.
• FAO Irrigation and Drainage Paper 56: Crop Evapotranspiration — the Penman-Monteith reference method, plus the logarithmic wind-profile correction and pan-coefficient guidance.
• US Department of Energy, Swimming Pool Covers — published evaporation-suppression ranges for solid and bubble pool covers.