What makes a pool evaporate faster
Water evaporates when the air above it can hold more moisture than it already has. The bigger that gap, the faster the pool drops. Four things set it:
- Water temperature. Warm water gives off vapour faster, so heated pools lose the most.
- Humidity. Dry air soaks up water quickly. In humid weather, evaporation slows right down.
- Wind across the surface. A breeze carries moist air away and keeps the gap open. Fences, hedges and screens make a big difference.
- Surface area. Evaporation happens at the surface, so a shallow pool and a deep pool of the same size lose the same amount.
Air temperature matters mostly through humidity. Warm water under cool, dry night air is a common reason heated pools lose more than expected.
The formula
Evaporation (lb per ft² per hour) = (70.3 + 0.314 × V) × (pw − pa) ÷ Y
V is air speed about a foot above the water in feet per minute. pw is the vapour pressure of saturated air at the water temperature and pa the vapour pressure in the air, both in inches of mercury. Y is the heat needed to evaporate a pound of water, about 1,048 Btu at 80°F. Smith, Löf and Jones fitted this to measurements on an outdoor pool; its coefficients are 74% of the ASHRAE equation for indoor pools.We work out both vapour pressures from your temperatures and humidity, multiply by the area and convert pounds of water to gallons and inches. A cover is assumed to cut evaporation by 90% while it’s on, at the low end of the EPA’s figures for solid, mesh and hybrid covers.
Worked example
A 16 × 32 ft pool on a typical summer day
- Water 82°F, air 80°F at 50% humidity, a 2 mph breeze at the surface
- Vapour pressure at the water: 1.102 inHg; in the air: 0.516 inHg
- Evaporation = (70.3 + 0.314 × 176) × 0.586 ÷ 1,047 = 0.0703 lb per ft² per hour
- Over 512 ft² and 24 hours: 104 gallons a day, or 0.32 inches
- With a cover on for 12 hours a night: 57 gallons a day
How the weather changes it
| 16 × 32 ft pool, uncovered | Inches a day | Gallons a week |
|---|---|---|
| Warm and humid: water 82°F, air 85°F, 75% humidity | 0.08 | 185 |
| Typical summer day: water 82°F, air 80°F, 50% humidity | 0.32 | 725 |
| Hot and dry: water 84°F, air 95°F, 20% humidity | 0.57 | 1,274 |
| Heated pool on a cool evening: water 84°F, air 60°F, 50% | 0.51 | 1,133 |
Daily evaporation and breeze at the surface
Show the numbers
| Breeze (mph) | 30% RH | 50% RH | 70% RH |
|---|---|---|---|
| 0 | 0.25 | 0.18 | 0.12 |
| 1 | 0.34 | 0.25 | 0.16 |
| 2 | 0.44 | 0.32 | 0.21 |
| 3 | 0.54 | 0.4 | 0.26 |
| 4 | 0.63 | 0.47 | 0.3 |
| 5 | 0.73 | 0.54 | 0.35 |
| 6 | 0.82 | 0.61 | 0.39 |
Evaporation or a leak?
The EPA’s WaterSense pool guide treats a drop of more than 2 inches a week, or 3 inches in hot, dry areas, as a sign of a possible leak. If you lose more than this calculator predicts, run the bucket test the EPA describes:
- On a calm day, turn off the pump, cleaners and any auto-fill for 24 hours.
- Fill a bucket to about an inch from the top and stand it on a pool step so it’s two-thirds submerged.
- Mark the water level inside the bucket and on the pool wall.
- After 24 hours, compare. Both lose the same to evaporation, so if the pool dropped more than the bucket, water is going somewhere else.
Repeat with the pump running. A bigger drop with the pump on points to the pressure side of the plumbing.
Evaporation is also heat loss
Every pound of 80°F water that evaporates takes about 1,048 Btu of heat with it, according to the U.S. Department of Energy, which makes evaporation the largest heat loss for most pools. That’s why a cover saves so much on heating: the DOE and EPA both put the savings at 50 to 70%. The pool heating cost calculator shows what that’s worth for your heater.
Frequently asked questions
How much water does a pool lose to evaporation per day?
On a typical warm day, around a quarter of an inch. The example above works out to 0.32 inches for a 16 × 32 ft pool. Hot, dry, windy weather can double that, and humid, still days can cut it to almost nothing.
How much water loss is normal for a pool per week?
Roughly 1 to 2 inches in warm weather. The EPA treats more than 2 inches a week, or 3 inches in hot, dry climates, as a sign you may have a leak.
Does a pool cover stop evaporation?
Mostly. The EPA says solid covers cut evaporation by more than 90% while they’re on, and lists up to 95% for solid, mesh and hybrid covers. Solar bubble covers also help.
Why does my heated pool lose so much water?
Warm water evaporates faster, especially under cool, dry night air. A heated pool can lose several times what an unheated pool loses in the same weather. A cover at night is the fix.
Do waterfalls and fountains increase evaporation?
Yes. Spraying water into the air exposes far more surface to the air than the flat pool does. Turn water features off when you don’t need them.
How do I measure the breeze at the water surface?
A cheap handheld anemometer held about a foot above the water works. Without one, assume 1 to 2 mph for a sheltered backyard and 3 to 5 mph for an open, windy site.
Sources
- Methods for Calculation of Evaporation from Swimming Pools and Other Water Surfaces (reviews the Smith et al. outdoor-pool equation). M. M. Shah, ASHRAE Transactions, 2014.
- Pool Water Efficiency (WaterSense guide EPA-832-F-22-015), September 2022. U.S. Environmental Protection Agency.
- Gas Pool Heaters. U.S. Department of Energy, Energy Saver.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.