Water Evaporation Rate & Loss Calculator

Estimate how quickly water disappears into the air

Evaporation is easy to underestimate because it happens quietly. A pool can look nearly unchanged from one afternoon to the next, a pond can seem stable for days, and exposed soil can appear only slightly drier than it did the day before. Yet when warm air, low humidity, and moving wind work together, the missing water adds up fast. This calculator gives you a quick planning estimate of that loss so you can judge whether you are seeing ordinary evaporation, budgeting for refill water, sizing maintenance routines, or comparing one set of weather conditions with another.

The most useful way to think about evaporation is as a boundary process at the water surface. Water molecules are always moving. When the surrounding air is warm and dry enough, and when wind keeps sweeping away the damp air sitting just above the surface, more of those molecules escape into the atmosphere. A larger exposed area means more opportunity for that escape. That is why a shallow but wide pool can lose meaningful volume even when the visible drop in water level seems small.

This page is intentionally practical rather than academic. It does not attempt a full energy-balance or Penman-style hydrology model. Instead, it uses a simplified rate estimate built from the main drivers most people can actually observe or look up: exposed surface area, surface type, air temperature, relative humidity, and wind speed. The result is most valuable when you need a fast estimate for everyday planning, comparison, and troubleshooting.

What each input means in plain language

Water Surface Area is the most direct input. If all other conditions stay the same, doubling the exposed area roughly doubles the evaporation volume. This is why wide pools, decorative ponds, irrigation reservoirs, and damp soil beds can lose substantial water even when the evaporation depth in inches per day is modest. Measure the area that is actually exposed to air, not the total property area around it.

Surface Type acts as a practical adjustment for how exposed or sheltered the surface is likely to be. A swimming pool, open pond, exposed soil patch, large lake edge, and small container do not all behave exactly the same way. The calculator uses a type factor to nudge the estimate up or down. That does not replace site measurement, but it helps capture the idea that some surfaces evaporate a little more aggressively while others lose moisture a bit more slowly under the same weather.

Air Temperature, Relative Humidity, and Wind Speed work together. Higher air temperature generally supports faster evaporation because warmer air can hold more water vapor. Lower relative humidity means the air is farther from saturation, so there is more room for additional vapor. Wind strips away the thin humid layer just above the surface, which keeps the gradient active. If you have ever noticed a breezy dry day emptying a pool faster than a still muggy day, you have already seen these variables in action.

Water Temperature matters in real evaporation physics, but this page’s quick estimator treats it as contextual information rather than as a direct multiplier in the current formula. That may sound unusual, but it is better to be explicit than to pretend the model is doing more than it really is. Keeping water temperature in the form still helps you document the scenario, compare runs, and decide whether a warmer-than-air surface might explain why actual evaporation feels strong. Finally, Calculate Evaporation Over simply rescales the daily estimate into a daily, weekly, or monthly total so the output matches the decision you are trying to make.

How the calculator turns conditions into a water-loss estimate

The page starts from a baseline evaporation depth of 0.1 inches per day and then adjusts that baseline with three environmental multipliers plus a surface-type factor. The temperature multiplier rises as air temperature climbs above 70°F. The humidity multiplier falls as relative humidity increases, reflecting the idea that humid air slows additional vapor transfer. The wind multiplier rises with wind speed because moving air tends to carry moisture away from the surface boundary layer.

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

Here, E is the 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 surface-type factor. The clamp keeps the result between 0.01 and 1.0 inches per day so extreme inputs do not produce obviously unrealistic values in this simplified model.

After that, the calculator converts depth into volume. One inch of water spread over one square foot is about 0.623 gallons, which this page approximates as division by 1.6. That makes the gallons-per-day estimate:

where A is surface area in square feet and G is gallons lost per day. Weekly and monthly values are then just the daily gallons multiplied by 7 or 30. At a higher level, this still fits the familiar pattern that a result is a function of several inputs and, in many calculators, a weighted combination of terms:

Those general formulas are useful because they remind you what to expect when you test scenarios: if you increase area, the result should scale upward; if you increase humidity, the result should usually ease downward; if you raise wind speed, the result should increase. When an output moves in the opposite direction from physical intuition, it is usually a sign that one input or unit needs another look.

Worked example using the default values

Suppose you are estimating evaporation from a 400 square foot swimming pool on a warm afternoon. You enter 400 square feet of area, choose Swimming Pool as the surface type, set air temperature to 85°F, relative humidity to 50%, wind speed to 5 mph, and water temperature to 80°F. The water temperature helps describe the scenario, but the direct rate calculation uses the other environmental factors plus the pool surface factor of 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 per day base rate gives 0.1 × 1.15 × 0.60 × 1.15 × 1.0 = 0.07935 inches per day. That is a modest depth, but because it applies across 400 square feet, it becomes a noticeable volume.

Now convert that depth into gallons: 400 × 0.07935 / 1.6 = about 19.8 gallons per day. Rounded to whole gallons, the calculator reports roughly 20 gallons per day. If you switch the time period to weekly, that same daily estimate becomes about 139 gallons over 7 days. If you choose monthly, the estimate becomes about 595 gallons over 30 days. The annualized figure based on the same daily conditions is about 7,241 gallons per year.

This example also shows why the result should be interpreted as a weather snapshot rather than a literal prediction for every day of the year. A calm humid week will usually fall below the estimate. A hot dry windy spell can easily exceed it. That is why it is smart to run a few comparison cases instead of trusting a single number too literally.

Scenario comparison: how sensitive the estimate is

Below is a simple comparison using the same 400 square foot pool area. Only the weather conditions change. This makes it easy to see how strongly evaporation responds to the atmosphere around the water.

The jump from 12 gallons per day to 29 gallons per day is the main lesson. Evaporation is rarely about one variable acting alone. Heat, dryness, and wind reinforce one another. If you are trying to cut loss, reducing exposure to any one of those drivers can help, but combining methods such as covers, shade, and wind protection usually works much better.

How to interpret the result without over-trusting it

The output gives you three different ways to read the estimate: evaporation depth in inches per day, gallons per day, and the selected total for a daily, weekly, or monthly period. The depth figure is helpful if you are comparing this estimate with measured water-level drop. The gallons figure is better if you are planning makeup water, chemical balancing, irrigation scheduling, or operating cost. The annual estimate is most useful for rough long-range budgeting, not for forecasting every season with precision.

If the calculator predicts a small daily evaporation loss but your real system is losing much more water than that, evaporation may not be the whole story. Leaks, splash-out, filter backwashing, irrigation overspray, drainage, and measurement error can all masquerade as evaporation. That is one of the calculator’s best real-world uses: it gives you a benchmark. If observed loss is close to the estimate, normal weather may explain it. If observed loss is far larger, the problem may be mechanical rather than environmental.

A good quick check is to change only one input at a time. Increase humidity and the result should drop. Increase wind speed and the result should rise. Increase surface area and the gallons should rise almost proportionally. That kind of scenario testing is often more valuable than the single baseline result because it shows which variable is driving your site the most.

Assumptions, limits, and practical judgment

This calculator is a simplified estimator. It does not directly include solar radiation, cloud cover, atmospheric pressure, overnight cooling, salinity, water agitation, cover use, surrounding vegetation, or microclimate effects such as reflected heat from paving. In some locations those omitted factors matter a lot. An uncovered dark pool in full sun behaves differently from a shaded pond surrounded by trees, even if the air temperature reading is the same.

That limitation does not make the tool unhelpful. It simply defines the right job for it. Use it for planning, comparison, and first-pass troubleshooting. It is especially useful when you want to answer practical questions such as: “Is this amount of water loss plausible for this weather?”, “What happens if humidity rises tomorrow?”, “How much refill water should I expect over a month?”, or “Would a cover likely save enough water to matter?”

For higher-stakes engineering, compliance, or scientific work, use a more detailed model and local measurements. For everyday operation, though, the quick estimate is often exactly what you need. It converts weather and area into a number you can discuss, test, and compare. That is much more informative than guessing based on appearance alone.

One final note: because the current page logic records water temperature but does not directly multiply it into the evaporation-rate equation, think of that field as valuable context. It helps you document whether the water body itself is unusually warm or cool, which may matter when you compare runs or decide whether you need a more advanced model.