Evaporation Rate Calculator
Estimate how fast water evaporates from a pool or open surface using the Carrier equation. Enter the surface area (ft²), water and air temperature (°F or °C), relative humidity, and wind speed — the tool returns loss in gallons per day, inches of depth per day, and pounds per hour.
Example: with Surface area (ft²) 400 · Water temperature 82 · Air temperature 80 · Relative humidity (%) 60 · Wind speed (mph) 2 → Evaporation rate: 50.6 gal/day.
- Depth lost per day0.203 in/day
- Mass loss rate17.59 lb/hr
Computed by the calculator below using its default values. Change any input to see your own numbers.
Evaporation is driven by the gap between the water surface's vapor pressure and the air's. Warm water, dry air, and wind widen that gap and speed the loss.
What drives evaporation
Water leaves an open surface whenever the vapor pressure right at the water is higher than the vapor pressure of the air above it. Warm water pushes its surface vapor pressure up; dry air keeps the air-side pressure down; and wind sweeps away the humid boundary layer so fresh dry air keeps arriving. The Carrier equation, long used for pool design, rolls these into one estimate proportional to the vapor-pressure difference and to a wind term.
For a typical backyard pool this works out to a loss on the order of a quarter-inch of depth a day, more in hot, dry, breezy weather and much less when the air is already humid. The single biggest lever a pool owner controls is a cover: cutting off the surface stops the vapor exchange and can reduce evaporation by the majority. Treat any single number here as a planning estimate, since real losses swing widely with weather.
How it’s calculated
Carrier equation: mass loss (lb/h) = A·(95 + 0.425·V)·(Pw − Pa)/1050, with A in ft², wind V in mph, and vapor pressures in inHg. Pw is the saturation vapor pressure at the water temperature; Pa = (RH/100) × saturation at the air temperature, from the Magnus curve. Gallons/day = lb/h × 24 × 0.119826; depth in/day = gal/day ÷ (7.48052·A) × 12.
A screening estimate. It assumes a calm, unshaded surface and steady conditions; real evaporation swings with sun, splashing, and gusts, and swimmer activity raises it. Not a substitute for metered make-up water data.
Pool evaporation drivers
| Condition | Effect on loss | Why |
|---|---|---|
| Warmer water | Higher | Raises surface vapor pressure |
| Lower humidity | Higher | Widens the vapor-pressure gap |
| Higher wind | Higher | Clears the humid boundary layer |
| Pool cover | Much lower | Blocks vapor exchange |
| Cooler nights | Higher | Warm water into cool, dry air |
Directional effects from the Carrier evaporation equation.
Common mistakes
- Entering water and air temperatures in different units, or leaving the unit set to the wrong scale.
- Expecting one figure to hold all season — evaporation can triple between a humid calm day and a hot dry wind.
- Blaming a leak for normal evaporation; a quarter-inch a day is typical before any leak.
- Ignoring wind and humidity, which together can move the result by a factor of two or more.
Frequently asked questions
How is pool evaporation calculated?
With the Carrier equation: loss depends on surface area, wind speed, and the difference between the vapor pressure at the water surface and in the air. This tool reports it in gallons per day, inches per day, and pounds per hour.
How much water does a pool lose to evaporation?
Often around a quarter-inch of depth per day, but it ranges widely. Hot, dry, windy weather can push it well past half an inch, while humid calm days lose much less.
What reduces evaporation the most?
A pool cover. By cutting off the water-to-air vapor exchange it can reduce evaporation by more than half, saving both water and heat.
Is my pool leaking or just evaporating?
Compare your loss with this estimate. Losses near a quarter-inch a day are usually evaporation; consistently much higher, especially in humid weather, points to a possible leak worth investigating.