Pool Evaporation Calculator
Usually much lower than the weather report.
Water lost per day
99 gallons/day
Under these conditions the pool loses about 99 gallons a day to evaporation, a drop of 0.31 in in water level.
- Level drop per day
- 0.31 in
- Per week
- 695 gallons
- Per 30 days
- 2,977 gallons ($19.77 of water)
- Heat carried away
- 867,394 BTU/day
How it's calculated
The calculator uses the evaporation equation in the ASHRAE Handbook (HVAC Applications, natatorium design), originally developed by Carrier:
- A: water surface area, sq ft.
- pw: saturation vapor pressure at the water temperature, inches of mercury (Magnus formula, Alduchov & Eskridge 1996).
- pa: vapor pressure of the air = relative humidity × saturation pressure at the air temperature.
- V: air speed over the water in ft/min (1 mph = 88 ft/min).
- Y: latent heat of vaporization, about 1,048 BTU per lb.
One inch of water over one square foot weighs 5.2 lb and a gallon weighs 8.34 lb, which turns pounds per hour into inches and gallons per day. The equation was built for indoor pools; outdoors it is best read as an upper estimate, and wind measured at the water surface is usually far below the forecast wind speed.
Worked example
A 16 × 32 ft pool (512 sq ft), water 82 °F, air 80 °F at 55% humidity, 1 mph breeze:
- pw at 82 °F = 1.1 in Hg; pa = 0.55 × 1.031 = 0.567 in Hg.
- V = 88 ft/min, so 95 + 0.425 × 88 = 132.4.
- Evaporation = 512 × 0.533 × 132.4 ÷ 1,048 = 34.5 lb/h.
- Per day: 828 lb = 99 gallons, a drop of 0.31 inches.
- Over 30 days that is 2,977 gallons, about $19.77 at $6.64 per 1,000 gallons.
Daily evaporation for a 512 sq ft pool
| Conditions | Water / air | Gallons/day | Inches/day |
|---|---|---|---|
| Warm, humid, calm | 82 / 85 °F, 70% | 40 | 0.13 |
| Warm, average | 82 / 80 °F, 55% | 99 | 0.31 |
| Hot, dry, breezy | 84 / 95 °F, 25% | 221 | 0.69 |
| Heated pool, cool night | 84 / 60 °F, 60% | 206 | 0.64 |
Frequently asked questions
How much water does a pool lose to evaporation per day?
Typically a fraction of an inch. With 82 °F water, 80 °F air, 55% humidity and a light breeze, a 16 × 32 ft pool loses about 0.31 inches (99 gallons) a day by this estimate. Hot, dry, windy days and heated pools on cool nights lose much more.
How do I know if my pool is leaking or just evaporating?
Do a bucket test: set a bucket on the pool step, fill it to the pool water level, and mark both levels. After 24 hours with the pump on its normal schedule, compare. If the pool drops noticeably more than the bucket, you likely have a leak.
Does a pool lose more water at night?
A heated pool often does. Evaporation depends on the gap between the water vapor pressure at the surface and in the air; warm water under cool, dry night air evaporates fast. That is why covering a heated pool overnight saves so much.
How can I reduce pool evaporation?
Use a cover whenever the pool is not in use; covers block most evaporation while they are on. Windbreaks such as fences and hedges help, and lowering the water temperature of a heated pool reduces losses sharply.
Does evaporation change my pool chemistry?
Evaporation removes only water, so salt, calcium and stabilizer stay behind and become more concentrated, and the top-up water adds more minerals. Over a season this slowly raises calcium hardness and TDS.
Related calculators
See all Pool Heating Calculators.