Water Evaporation Rate & Loss Calculator

Estimate evaporation from an exposed water surface

Water evaporation can be easy to miss because the level usually falls gradually. A pool may look unchanged from one afternoon to the next, a pond can appear stable for several days, and exposed soil may only look a little drier. But warm air, low humidity, and wind can turn that small daily drop into substantial water loss. This calculator provides a quick evaporation planning estimate for refill-water budgeting, routine maintenance, weather comparisons, and deciding whether observed loss appears consistent with evaporation.

Evaporation occurs at the exposed surface where water meets air. Water molecules are continually moving, and some escape into the atmosphere. Warm, dry air can accept more water vapor, while wind replaces the humid air just above the surface with drier air. More exposed square footage gives that transfer more room to occur. For that reason, a broad shallow pool or pond can lose a meaningful volume even when its measured water-level change is slight.

This water-loss calculator is deliberately a quick estimator, not a full hydrology or energy-balance model. It uses the main conditions that are relatively easy to observe or obtain from a forecast: exposed area, surface type, air temperature, relative humidity, and wind speed. Its results are most useful for practical comparisons and initial troubleshooting rather than precise site-specific measurement.

Water evaporation inputs and what they represent

Water Surface Area directly controls evaporation volume. With the same estimated evaporation depth and weather, doubling exposed square footage approximately doubles the number of gallons lost. Enter only the surface actually exposed to air, rather than nearby deck, landscaping, or the full property area.

Surface Type applies a practical adjustment to the evaporation estimate. A swimming pool, open pond, exposed soil patch, lake edge, and small container do not present identical exposure conditions. The page uses a surface-type factor to adjust the quick estimate up or down. This is not a substitute for measured local loss, but it recognizes that different kinds of exposed surfaces can respond differently under the same weather.

Air Temperature, Relative Humidity, and Wind Speed are the direct weather drivers in this evaporation model. Higher air temperature increases the temperature factor. Lower relative humidity leaves more capacity in the air for vapor. Wind increases the wind factor by moving humid boundary-layer air away from the surface. A dry, breezy day can therefore produce more estimated loss than a calm, humid day at a similar temperature.

Water Temperature is relevant to real evaporation physics, but this page records it as scenario context rather than using it as a direct multiplier. Keeping it in the form can help document and compare conditions, especially when the water is notably warmer or cooler than the air. Calculate Evaporation Over converts the daily gallons estimate into a daily, weekly, or monthly total.

How this water evaporation calculator estimates loss

This calculator begins with a baseline evaporation depth of 0.1 inches per day. It multiplies that baseline by temperature, humidity, wind, and surface-type factors. The temperature factor increases above 70°F, the humidity factor decreases as relative humidity rises, and the wind factor increases with miles per hour of wind.

In the current page logic, the simplified evaporation depth is:

Here, E is daily evaporation depth in inches per day, T is air temperature in °F, RH is relative humidity in percent, W is wind speed in miles per hour, and S is the selected surface-type factor. The clamp limits the result to 0.01 through 1.0 inches per day, preventing extreme permitted entries from producing an implausible result in this simplified model.

The calculator then converts evaporation depth to water volume. One inch of water over one square foot is approximately 0.623 gallons, which the page approximates by dividing by 1.6. The gallons-per-day calculation is:

In this expression, A is exposed surface area in square feet and G is estimated gallons lost per day. Selecting weekly multiplies daily gallons by 7; selecting monthly multiplies daily gallons by 30. Consequently, area changes the gallons result directly, higher humidity reduces the estimated rate, and stronger wind increases it. If a trial result moves contrary to those relationships, review the entry units and selected surface type.

Pool evaporation example using the page defaults

For this pool evaporation example, use a 400-square-foot swimming pool with 85°F air temperature, 50% relative humidity, 5 mph wind, and 80°F water temperature. Water temperature describes the run but does not enter the direct rate equation; the pool surface factor is 1.0.

The temperature factor is 1 + (85 − 70) / 100 = 1.15. The humidity factor is 1 − 0.8 × 0.50 = 0.60. The wind factor is 1 + 0.3 × (5 / 10) = 1.15. Multiplying these with the 0.1-inch daily base rate gives 0.1 × 1.15 × 0.60 × 1.15 × 1.0 = 0.07935 inches per day. Although this is a modest depth, applying it across 400 square feet makes the volume noticeable.

Converting the estimated depth to gallons gives 400 × 0.07935 / 1.6 = about 19.8 gallons per day. Rounded to whole gallons, the calculator displays roughly 20 gallons per day. Weekly loss is about 139 gallons over 7 days, while monthly loss is about 595 gallons over 30 days. If the same conditions persisted every day, the annualized value would be about 7,241 gallons.

This pool example is a weather snapshot, not a claim that every day of the year behaves alike. A calm, humid week would generally calculate lower loss, while a hot, dry, windy stretch would calculate higher loss. Comparing several plausible weather cases is usually more informative than treating one output as a fixed forecast.

Weather scenarios for estimated pool evaporation

These water-evaporation scenarios keep the 400-square-foot pool area and pool factor unchanged while varying weather conditions. They illustrate how the calculator combines temperature, humidity, and wind rather than attributing the result to only one condition.

The change from roughly 12 to 29 gallons per day demonstrates how the model’s weather factors reinforce one another. Heat, dry air, and wind all increase estimated evaporation under this equation. In practical water conservation, limiting exposure to one or more of those influences—such as with a cover, shade, or a windbreak—may reduce actual water loss.

Interpreting an estimated evaporation result

The calculator reports evaporation depth in inches per day, gallons per day, and gallons over the selected daily, weekly, or monthly period. Use the depth value when comparing the estimate to a measured water-level drop. Use the gallons values when planning makeup water, chemical balancing, irrigation scheduling, or operating expenses. The annual total is a rough annualization of the current daily estimate, not a season-by-season prediction.

If measured water loss is far larger than the estimated evaporation, evaporation may not be the only explanation. Leaks, splash-out, filter backwashing, irrigation overspray, drainage, and inaccurate level readings can all resemble evaporation. The calculation is useful as a benchmark: a measured loss close to the estimate may be weather-related, whereas a much larger loss deserves further inspection.

For a useful evaporation comparison, change one condition at a time. Raising humidity should reduce the result, raising wind speed should increase it, and enlarging surface area should raise gallons nearly proportionally. This approach identifies which listed input has the largest effect on the estimate for a particular pool, pond, soil surface, or container.

Limits of this evaporation-loss estimate

This water evaporation calculator is simplified. It does not directly account for solar radiation, cloud cover, atmospheric pressure, overnight cooling, salinity, water agitation, cover use, surrounding vegetation, or local microclimate effects such as reflected heat from paving. Those omitted conditions can matter substantially. For example, an uncovered pool in direct sun can behave differently from a shaded pond even at the same recorded air temperature.

That limitation defines the proper use of the calculator: water-loss planning, scenario comparison, and first-pass troubleshooting. It can help answer questions such as whether a reported loss is plausible for current weather, how a humidity change affects the estimate, how much refill water may be needed in a month, or whether reducing exposure could matter.

For engineering, compliance, or scientific analysis, use local observations and a model that includes the conditions relevant to the site. For day-to-day operation, this quick calculation turns area and weather inputs into a transparent estimate that can be tested and compared instead of judged only by appearance.

Finally, water temperature remains context rather than a direct rate factor in the current page logic. It can still help explain why observed conditions appear unusual and can indicate when a more detailed evaporation model may be appropriate.