Pool evaporation calculator
Is your water level dropping from the weather, or from a leak? Enter your conditions to estimate daily evaporation and compare it with what you measured.
Estimated evaporation
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Uses the ASHRAE (Carrier) evaporation equation with wind at the water surface taken as half the forecast speed. Splash-out from swimmers and backwashing aren’t included. Treat this as an estimate within about ±30%.
The bucket test, step by step
- Fill the pool to its normal level. Put a bucket on the first or second step and fill it with pool water to the same level as the pool.
- Mark the water line inside the bucket and on the pool tile or skimmer.
- Run the pump as usual for 24 hours. Don’t swim or add water.
- Compare: if the pool dropped more than the bucket, you’re losing water beyond evaporation. Repeat with the pump off to narrow down plumbing vs. shell.
What drives evaporation
Evaporation is driven by the difference between the vapor pressure at the water surface and in the air. Warm water, dry air and wind all widen that gap. That’s why a heated pool on a cool, dry, breezy evening can lose more water than on a humid, still, hot afternoon.
Questions pool owners ask
How much water does a pool lose to evaporation per day?
Commonly between about 1/8 and 1/2 inch a day, depending on sun, wind, humidity and how much warmer the water is than the air. Hot, dry, windy days at the top of the range; a covered pool loses very little.
How do I tell a leak from evaporation?
Do the bucket test: set a bucket of pool water on a pool step so its water level matches the pool’s, mark both levels, and wait 24 hours with normal pump operation. If the pool drops noticeably more than the bucket, you likely have a leak. Repeat with the pump off to tell a plumbing leak from a shell leak.
Does evaporation raise my salt and calcium?
Yes. Only pure water evaporates, so dissolved salt, calcium, stabilizer and other solids stay behind and concentrate as you top up with fill water. That’s why hard-water areas see calcium creep up over a season.
How much does a pool cover reduce evaporation?
A well-fitting cover stops most evaporation while it is on; covers are also the most effective way to cut pool heating costs, since evaporation is where most pool heat goes.