Pool Evaporation Calculator
Estimate how many gallons your pool loses to evaporation each day and week, and whether that mysterious water drop is normal or a leak.
Based on the Carrier evaporation equation used in HVAC and pool engineering. Real-world loss varies with sun, night-time cooling, and swimmer activity. Losing much more than the estimate? Run a bucket test to rule out a leak.
The physics: why pools lose water
Evaporation is driven by the difference in vapor pressure between the warm water surface and the air above it, amplified by wind sweeping saturated air away. This calculator uses the Carrier equation, the same relationship HVAC engineers use to size dehumidification for indoor pools:
water loss (lbs/hr) = heat loss x area / 1050
P_water is the saturation vapor pressure at your water temperature and P_air is the actual vapor pressure of the air (saturation pressure at air temperature times relative humidity). The 1050 divisor is the latent heat of vaporization, the energy each pound of water takes with it as it leaves. That stolen energy is also why evaporation dominates your heating bill, which you can see priced out in the pool heating cost calculator.
Worked example: summer evening in Texas
A 512 sq ft pool at 84°F water, 90°F air, 50 percent humidity, 5 mph breeze:
- Vapor pressure gap: 0.576 − (0.50 x 0.697) ≈ 0.23 psia
- Wind factor: 95 + 0.425 x 5 = 97
- Heat loss: 97 x 0.23 ≈ 22 BTU/hr per sq ft → about 11,300 BTU/hr total
- Water loss: 11,300 / 1050 ≈ 10.8 lbs/hr ≈ 31 gallons/day... rising sharply if the night turns dry and windy
Run your own numbers above, the result is extremely sensitive to humidity and wind, which is exactly why neighbors with identical pools report different water loss.
What normal looks like
- Mild conditions: 1/8 to 1/4 inch of level per day
- Hot, dry, or windy: 1/4 to 1/2 inch per day
- More than 1/2 inch per day sustained: run the bucket test for a leak
Water loss also silently concentrates everything dissolved in your pool: refill water adds calcium and the evaporated water leaves its minerals behind. If you are topping up constantly, keep an eye on calcium hardness and salt levels, the salt calculator handles the dilution math when levels creep too high.
Frequently asked questions
How much water does a pool lose to evaporation per day?
A typical 16x32 pool (512 sq ft) in warm summer conditions loses roughly 30 to 120 gallons per day, an eighth to a quarter inch of water level. Hot, dry, windy conditions can push well past that; cool humid nights can halve it.
How do I tell evaporation from a leak? (the bucket test)
Fill a bucket with pool water, set it on a pool step so it shares the water's temperature, and mark both the bucket level and the pool level. After 24 to 48 hours with the pump off, compare: if the pool dropped noticeably more than the bucket, you likely have a leak, because both surfaces experience the same evaporation.
Does a pool cover really stop evaporation?
Yes, dramatically. A simple solar cover blocks 90 to 95 percent of evaporation, which is also why covers cut heating bills so much: evaporation is the largest heat-loss path for an outdoor pool.
Why does my pool lose more water at night?
On clear nights the water is often warmer than the air and humidity drops, both of which increase the vapor pressure gap driving evaporation. An uncovered heated pool on a cool dry night is close to the worst case.