Evaporation Rate (Water) Calculator
An open water surface loses water to the air every hour of every day, and the size of that loss decides how often a pool needs topping up, how fast a pond drops in a dry spell, and whether fresh concrete is at risk of cracking before it cures. This calculator estimates the loss two ways. If you already know the evaporation rate, Known rate mode converts it straight into liters per day and per hour for your surface. If you do not, Estimate from weather mode works the rate out from five numbers: surface area, water temperature, air temperature, relative humidity and wind speed at the surface.
Be clear about what the weather mode is: the ASHRAE mass-transfer model from the ASHRAE Handbook (HVAC Applications), stated in full below. It works from the vapour-pressure deficit between the water surface and the air: the saturation vapour pressure at the water temperature, from the Buck (1981) equations, minus the actual vapour pressure of the air from its temperature and relative humidity. It assumes a still, unoccupied surface (activity factor 0.5), and the result is clamped at zero, so it can never show negative evaporation. It is still a screening estimate and a concrete-risk check, not a precise measurement: it has no direct solar-radiation, pressure or salinity term, and this page says so wherever the numbers appear.
Water lost
Water lost
The weather estimate uses the ASHRAE mass-transfer formula set out below, with a fixed activity factor of 0.5 for a still, unoccupied surface, and is an approximation for open fresh water. 1 kg of water is about 1 liter, and 1 kg per m² equals 1 mm of depth.
Quick Answer
In Known rate mode, a 50 m² surface losing 2.5 mm/day loses 125.00 L/day, or 5.21 L/hour. In weather mode, the pre-filled example (50 m², water 28°C, air 30°C, 50% humidity, 3 m/s wind) estimates 9.52 mm/day and 476.19 L/day under the ASHRAE mass-transfer model, with the hourly rate of 0.40 kg/m²/h reading LOW on the concrete-risk flag. That rate sits in the range pools and lakes actually measure on warm days, as the accuracy section below explains: use it for screening and risk checks, not as a meter reading.
Key takeaways
- Depth and volume are the same fact at two scales: 1 mm of depth over 1 m² is exactly 1 liter, which is why the rate in kg/m²/day, the depth in mm/day and the liters per m² per day are the same number.
- Wind refreshes the deficit. With water at 28°C, air at 30°C and 50% humidity, raising surface wind from 0 to 5 m/s moves the estimate from 2.62 to 14.13 mm/day.
- Humidity is the brake and water temperature is the engine: warm water under cool air can evaporate hard even at high humidity, because what matters is the gap between the two vapour pressures. When that gap closes, the rate falls to zero and never below it.
- The ACI flag reads the hourly rate per square meter, not the daily total: 1.0 kg/m²/h and above is HIGH, 0.5 to under 1.0 is MODERATE, below 0.5 is LOW.
- Quick Answer
- How to use the two modes
- The evaporation formula, in full
- What wind does to the estimate
- Four scenarios through the weather mode
- How accurate is the weather estimate? An honest answer
- The ACI 305 concrete-risk flag
- Known limits of this tool, stated plainly
- What competing calculators miss
- Frequently asked questions
How to use the two modes
Known rate mode is the conversion job. Enter the surface area in square meters and a rate in millimeters per day that you already trust: from an evaporation pan, a bucket test on your own pool, or a published local climate figure. The tool multiplies rate by area and reports liters per day and per hour. Validation is strict: the area must be above zero and the rate zero or more, or you get an error and no result.
Estimate from weather mode is the estimation job. Enter the same area plus the water temperature and the air temperature in Celsius, relative humidity as a percentage, and wind speed at the surface in meters per second. The tool runs the ASHRAE mass-transfer formula below, reports the rate, the total liters per day and per hour, the depth lost per day, and the ACI 305 concrete-risk flag. Humidity must sit between 0 and 100 and wind cannot be negative. The activity factor is fixed at 0.5, the ASHRAE value for a still, unoccupied surface, and the rate is clamped at zero: if the air-side vapour pressure is at or above the water-side vapour pressure, the tool reports 0 mm/day rather than a negative rate.
If your real question is whether a falling pool level is evaporation or a leak, estimate first, then measure: mark the level, run the pool normally for 24 hours with the pump and autofill in their usual state, and compare the drop against the estimate and against a bucket of pool water set on the steps, which loses water to evaporation only. A drop far above both points to a leak, splash-out or backwashing rather than evaporation. Knowing the daily loss also lets you price the refill: multiply the liters by your tariff with a cost per gallon calculator after converting liters to gallons (divide by 3.785).
The evaporation formula, in full
Saturation vapour pressure, Buck (1981): psat(T) = 0.61121 × exp((18.678 − T/234.5) × (T / (257.14 + T))) kPa
Water side: pw = psat(water temperature) | Air side: pa = (humidity/100) × psat(air temperature)
Latent heat: Y = 2501 − 2.361 × water temperature (kJ/kg)
Rate (mm/day) = 86,400 × 0.5 × (pw − pa) × (0.089 + 0.0782 × wind) / Y, clamped at zero
Total loss (L/day) = Rate × surface area (m²) | Per hour = Total ÷ 24 | Depth (mm/day) = Rate
Wind is in m/s, both temperatures in °C and humidity as a percent. The model has one driving force: the vapour-pressure deficit, pw − pa. The water side rises exponentially with water temperature, which is why a heated pool steams on a cold night; the air side is the share of saturation the air already holds. The wind term, 0.089 + 0.0782 × wind, is the ASHRAE mass-transfer coefficient: moving air carries the moist layer away so the deficit keeps working. The 0.5 is the ASHRAE activity factor for a still, unoccupied surface. Dividing by the latent heat Y converts the equation into a mass loss, and multiplying by 86,400 seconds makes it a daily rate. Because 1 kg of water occupies about 1 liter and a 1 mm depth over 1 m² is exactly 1 liter, the rate reads directly as mm/day of depth and as liters per square meter per day, with no separate conversion step. If pa is at or above pw, the raw result would be zero or negative, and the tool clamps it to 0 mm/day: condensation is not evaporation.
Worked example, the pre-filled weather inputs. Area 50 m², water 28°C, air 30°C, humidity 50%, wind 3 m/s. Saturation vapour pressure at the water surface: psat(28) = 3.7814 kPa. Air side: psat(30) = 4.2451 kPa, and 50% of that is 2.1226 kPa, so the deficit is 3.7814 − 2.1226 = 1.6588 kPa. Wind term: 0.089 + 0.0782 × 3 = 0.3236. Latent heat: 2501 − 2.361 × 28 = 2,434.89 kJ/kg. Rate: 86,400 × 0.5 × 1.6588 × 0.3236 / 2,434.89 = 9.52 kg/m²/day and 9.52 mm/day. Total: 9.5239 × 50 = 476.19 L/day, which is 19.84 L/hour. The flag divides the rate by 24: 9.52 / 24 = 0.40 kg/m²/h, below the 0.5 line, so the flag reads LOW.
What wind does to the estimate
Holding the worked example temperatures and humidity fixed (water 28°C, air 30°C, 50%) on the same 50 m² surface, only the wind changes:
Every figure in that chart is a real run of the calculator. The practical lesson is about shelter as much as about weather: a fence, hedge or cover that cuts the wind over the water cuts the loss with it, which is why exposed hilltop pools and rooftop tanks lose so much more than sheltered ones under the same sky. Humidity works the other way and is tied to the dew point: the closer the air temperature sits to the dew point, the higher the relative humidity and the slower the evaporation.
Four scenarios through the weather mode
| Scenario | Inputs (area, water, air, RH, wind) | Rate | Loss | Flag |
|---|---|---|---|---|
| Calm, humid spring day | 100 m², 20°C, 18°C, 75%, 0.5 m/s | 1.78 mm/day | 178.26 L/day | LOW |
| Warm summer afternoon | 50 m², 27°C, 30°C, 50%, 1.5 m/s | 5.28 mm/day | 264.00 L/day | LOW |
| Cool night over a heated pool | 50 m², 26°C, 12°C, 85%, 1.0 m/s | 6.43 mm/day | 321.29 L/day | LOW |
| Hot, dry and breezy | 50 m², 30°C, 35°C, 20%, 3.0 m/s | 17.95 mm/day | 897.32 L/day | MODERATE |
All four rows are the published scenario table for this same ASHRAE model (AgentCalc), and this tool reproduces them to the second decimal place; the losses add a surface area to each scenario, since the rate itself does not depend on area. The third row shows why the model needs a water temperature: air at 85% humidity still takes 6.43 mm/day from 26°C water, because the deficit is set by the water side. The ranking between scenarios is informative (dry air and wind dominate), and the flag thresholds are the standard ACI ones. The absolute liters are a screening estimate, not a promise.
How accurate is the weather estimate? An honest answer
It is a screening estimate, and its limits are worth stating plainly. The ASHRAE correlation was developed for indoor pools (natatoriums) at low air velocities, and comparative reviews by Shah found that no published evaporation correlation is reliably accurate outside the data it was fitted to: errors of 20 to 30 percent are normal even for the best of them. Applying it outdoors at several meters per second is an extrapolation, made deliberately and stated here. For scale, the U.S. Geological Survey energy-budget study of Lake Mead, one of the hottest and driest large water bodies in North America, put average annual evaporation at 7.5 feet per year, roughly 6.3 mm/day averaged over the year (USGS Scientific Investigations Report 2006-5252). Against that background, this model gives 9.52 mm/day for the pre-filled example: a warm-day figure in a plausible range, not an annual average. Its 17.95 mm/day for a hot, dry, breezy day is a single-day potential rate under desert-style conditions, not a figure any lake sustains all year. The linear formula this page used before returned 34.38 mm/day on the same pre-filled inputs, about 5.5 times the Lake Mead average, which is why it was replaced.
What the model still leaves out matters too. It has no direct solar-radiation term: sun on the surface enters only indirectly, through the water temperature you measure, which is exactly why the water temperature input exists. It has no atmospheric-pressure term (the ASHRAE correlation was fitted near sea level) and no salinity term. And its activity factor is fixed at 0.5, the still, unoccupied surface: splashing, swimmers, fountains and water features raise real losses above the figure, in the ASHRAE activity table by up to two or three times for busy pools, so treat a busy-pool result as a floor. What the model does capture is the physics that decides the ranking: the vapour-pressure deficit, which rises exponentially with water temperature, falls as the air approaches saturation, and is refreshed by wind. For irrigation design, reservoir accounting or research, use an energy-budget or Penman-Monteith model and local pan data instead. For deciding whether today’s conditions are gentle or brutal on an open surface, and whether a concrete pour needs precautions, this estimate earns its place.
The ACI 305 concrete-risk flag
Fresh concrete loses its surface water to the same physics, and when evaporation outruns bleeding, the surface shrinks and cracks while the concrete is still plastic. ACI 305 hot-weather concreting guidance, as summarized in the FHWA guide for curing portland cement concrete pavements, calls for precautions when the evaporation rate exceeds 1.0 kg/m²/h. The flag in weather mode applies that line to the formula’s hourly rate per square meter: at or above 1.0 it reads HIGH, from 0.5 up to 1.0 it reads MODERATE (some agencies and silica-fume mixes treat 0.5 as the action line), and below 0.5 it reads LOW. The boundaries are inclusive at the bottom of each band: exactly 1.0 kg/m²/h flags HIGH and exactly 0.5 flags MODERATE, both verified against the tool.
Two cautions. The ACI nomograph behind the 1.0 threshold uses the concrete surface temperature as well as air temperature, humidity and wind. For a pour, enter the concrete temperature as the water temperature and the shade air temperature as the air temperature, then treat the flag as an early warning to check conditions properly, not as a compliance reading. And if you are planning the pour itself, the flag pairs with the slab’s volume and area figures from a concrete volume calculator: area drives the evaporation exposure, volume drives the order.
Known limits of this tool, stated plainly
- Metric only: area in m², temperature in °C, wind in m/s, results in liters. There is no Fahrenheit, mph, square-feet or gallons mode; convert before and after.
- The rate is clamped at zero: if the air-side vapour pressure is at or above the water-side vapour pressure (cold water under warm, humid air, for example), the tool reports 0 mm/day and 0 L/day. It never reports a negative rate. The Buck saturation equations are accurate to better than 0.1% between -30°C and +60°C; outside that range, treat the result as indicative only.
- Still-surface assumption: the activity factor is fixed at 0.5 for a still, unoccupied surface, and there is no direct solar-radiation, pressure or salinity term, as the accuracy section explains. Splashing and swimmers raise real losses above the estimate.
- Fresh water assumption: salinity lowers vapor pressure slightly (roughly 2% for seawater), which the formula ignores; acceptable for pool and pond estimates, one more reason it is not an engineering model.
- The flag is a screening band: it applies ACI thresholds to this formula’s rate. It is not the ACI nomograph and not a substitute for site measurement on a real pour.
What competing calculators miss
- BraveCalculator publishes a copied linear formula, (0.5 + 0.25 × wind) × (25 + temperature) × (1 − humidity/100), with a worked example of 50 m², 30°C, 50%, 3 m/s giving 34.375 kg/m²/day. It has no water temperature and no vapour-pressure term, it returns negative rates below -25°C, and it carries no warning that its default figure runs about 5.5 times the USGS Lake Mead average. This page used that formula until October 2026 and replaced it with the ASHRAE model below the calculator.
- Savvy Calculator uses a vapor-pressure-deficit (Magnus) model, a step up in physical realism, but its worked example multiplies depth by area in a way that blurs the mm/day and total-volume answers, and it offers no known-rate conversion mode for users who already have a measured rate.
- OnlyCalculators runs the full psychrometric calculation with humidity ratios and an ACI assessment, the most rigorous of the set, but it demands water and air temperatures and saturation properties most pool and pond users do not have at hand.
- AgentCalc implements the same ASHRAE mass-transfer equation with Buck saturation vapour pressures, and this page weather mode follows its published form, reproducing its scenario table (1.78, 5.28, 6.43 and 17.95 mm/day) exactly. AgentCalc lets you pick an activity factor and a wind measurement height; this page fixes the activity factor at 0.5 for a still surface and takes wind at the surface, and adds the two jobs AgentCalc buries: a simple mode that turns a rate you already know into liters per day, and the ACI 305 concrete-risk flag.
- CalculatorsHub and PoolGuardUSA serve pool owners with Fahrenheit, square feet and gallons and, in PoolGuard’s case, a useful loss chart, but their formulas hide behind constants and temperature differences, and neither touches the concrete-curing use of the same number.
The gap this page fills is the combination with the caveats attached: a known-rate conversion and an ASHRAE weather estimate on one page, the formula printed in full with its sources, the ACI flag bands stated exactly, and an accuracy section that tells you where the estimate stops being trustworthy.
Frequently asked questions
Water temperature, wind, humidity and surface area, in roughly that order of surprise. Water temperature drives the surface vapour pressure, which rises exponentially as the water warms, and wind matters because it sweeps away the saturated air layer sitting on the water; in still air that layer chokes evaporation off. In this calculator’s ASHRAE model, going from 0 to 5 m/s of surface wind with water at 28°C, air at 30°C and 50% humidity raises the estimate from 2.62 to 14.13 mm/day, while area simply scales whatever rate the weather produces.
Measured pools commonly lose a few millimeters of depth a day, around a quarter of an inch (about 6 mm) a day being a widely reported average in warm climates, which on a 50 m² pool is roughly 300 liters. This calculator’s weather mode uses the ASHRAE mass-transfer model: its warm summer afternoon scenario (water 27°C, air 30°C, 50% humidity, 1.5 m/s surface wind) returns 5.28 mm/day, close to that measured average. Use its Known rate mode with a rate you measured yourself, such as from a bucket test, when you need a figure to budget against.
A lot, because a cover attacks the two biggest drivers at once: it removes the wind from the surface and traps saturated air against the water. Pool guides commonly quote reductions of 90% or more for a well-fitted cover. This calculator cannot model a cover directly; the honest way to see your own saving is a bucket test or a level check with the cover on and off under similar weather.
ACI 305 hot-weather concreting guidance treats about 1.0 kg/m²/h as the evaporation rate above which fresh concrete risks plastic-shrinkage cracking. The flag compares this formula’s hourly rate per square meter with that line: HIGH at 1.0 or more, MODERATE from 0.5, LOW below 0.5. It is a screening warning based on the ASHRAE mass-transfer estimate, not the ACI nomograph itself.
Approximately. Dissolved salt lowers the vapor pressure of water slightly, roughly 2% for seawater, so true evaporation is a little below the freshwater estimate. That difference is small next to this model’s overall approximation, and negligible for pool, pond and tank planning.
No, the tool is metric only. Convert first: °C = (°F – 32) × 5/9, wind in m/s = mph × 0.447, area in m² = square feet × 0.0929. To turn the liters-per-day result into gallons, divide by 3.785. Entering unconverted imperial values is the most common reason for wildly wrong results from this page.
Because evaporation is driven by the gap between two vapour pressures. The water temperature sets the saturation vapour pressure at the surface; the air temperature and humidity set the vapour pressure the air already holds. Warm water under cool air, such as a heated pool on a cool night, can lose 6.43 mm/day even at 85% humidity, and no air-temperature-only formula can show that.
No. The weather mode clamps the result at zero. If the air-side vapour pressure is at or above the saturation vapour pressure at the water surface, as happens with cold water under warm, humid air, the vapour-pressure deficit is zero or negative and the tool reports 0 mm/day rather than a negative rate. In genuinely freezing conditions open water is usually ice-covered and this tool does not apply.
One liter of water weighs about one kilogram, so a rate in kg/m²/day converts directly to liters per square meter per day, and since 1 mm of depth over 1 m² is exactly 1 liter, the same number is also the depth lost in mm/day. That is why the rate, the depth and the per-square-meter volume in the results are the same figure.
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References
- ASHRAE. ASHRAE Handbook, HVAC Applications (natatorium evaporation equation in the Carrier mass-transfer form, with activity factors). See also Shah, M. M. Methods for Calculation of Evaporation from Swimming Pools and Other Water Surfaces, ASHRAE Transactions 120(2), 2014 (states the ASHRAE equation in SI form). Accessed October 21, 2026.
- Buck, A. L. New Equations for Computing Vapor Pressure and Enhancement Factor, Journal of Applied Meteorology 20, 1981 (the saturation vapour pressure equations used for both the water-side and air-side pressures, revised 1996).
- AgentCalc. Water Evaporation Rate Calculator (published ASHRAE implementation and scenario table reproduced by this tool). Accessed October 21, 2026.
- Federal Highway Administration. Guide for Curing Portland Cement Concrete Pavements, II (summarizes the ACI 305R precaution threshold of 1.0 kg/m²/h). Accessed October 21, 2026.
- U.S. Geological Survey. Evaporation from Lake Mead, Arizona and Nevada, 1997-99, Scientific Investigations Report 2006-5252. Accessed October 21, 2026.
- U.S. Geological Survey. Estimating daily lake evaporation from biweekly energy-budget data. Accessed October 21, 2026.