Pool Evaporation Calculator

A 512 square foot backyard pool (32 by 16 ft) in warm humid weather, with 84 °F water, 85 °F air, 60 % humidity and a light breeze, loses about 1.4 inches a week to evaporation: 450 gallons a week or roughly 1,955 gallons a month. In the dry Southwest the same pool loses about twice that, and a cover on for 12 hours a day cuts the loss by nearly half. The calculator lets you set the weather for your region, then shows the loss with and without a cover.

How evaporation is calculated

The physics is the ASHRAE pool evaporation equation, as printed by the Geo-Heat Center at Oregon Tech. It says the water lost depends on how much more vapor the water surface can hold than the air already does, boosted by wind.

Rate (kg/s per m²) = (pw − pa) × (0.089 + 0.0782 × V) ÷ Y

Here pw is the saturation vapor pressure at the water temperature, pa is the vapor pressure in the air (saturation at the air temperature times the relative humidity), V is the air speed over the water in m/s and Y is the latent heat, about 2,433 kJ/kg. That is a design formula for indoor pools, and outdoors it overshoots. A measured test on an outdoor pool in Colorado found evaporation was 72 % of the ASHRAE value in still air and 82 % at 2.2 m/s. The calculator multiplies by that ratio.

Worked example: water at 84 °F (28.9 °C) has pw = 3.98 kPa. Air at 85 °F and 60 % gives pa = 2.46 kPa. With 0.5 m/s the formula gives 6.93 mm a day, times 0.74 is 5.15 mm, or 0.20 in a day and 1.41 in a week. Over 512 sq ft that is 512 × 1.41 × 0.6234 = 450 gal.

Evaporation by region

Weekly loss for a 512 sq ft pool in the summer conditions the region menu uses (our round-number assumptions for July and August, not a weather record):

Wind is the lever you control. The open case (2.2 m/s, about 5 mph over the water) more than doubles the typical loss, so a fence, hedge or wall around the pool saves water before you buy a cover. The Cal Poly study for the National Plasterers Council puts California open-water evaporation at 50 to 80 inches a year, which averages 1 to 1.5 inches a week over the year and more in summer, in line with the Southwest rows above.

Cover savings

The same Cal Poly study measured six covers on identical test pools. Solid track, foam and bubble covers cut evaporation by 93.9 %, 95.9 % and 94.9 % while they stayed on the water all the time, solar disks by 50.1 %, and two liquid covers by about 15 %. The authors call those maximums, since real covers come off for swimming. The US Department of Energy figure quoted in the report is 30 to 50 % for solid covers in practice. The calculator scales the 94.9 % by the hours a day the cover is on: 12 hours gives 47 %, so the 450 gallon week drops to 236 gallons, saving about 214 gallons a week.

Evaporation and heating

Evaporation is the largest heat loss of an outdoor pool. The Geo-Heat Center cites 50 to 60 % of the total heat loss in most cases, and the outdoor test above found 56 % by evaporation, 26 % by radiation and 18 % by convection overnight. Our example pool sends about 23,400 BTU an hour away with the vapor. That is why a cover is a heating tool too: see the pool heater calculator for what it costs to make up.

What the calculator leaves out

Sun on the water, splash-out, backwash and swimmers all add to the total, and this formula covers none of them. If your pool drops more than these numbers by a wide margin, check for a leak before you blame the weather.

Keeping the water up

Replace what you lose with the pool fill time calculator, which also prices the water. Each top-up dilutes the chemicals but not the dissolved stabilizer and salt that stay behind when water evaporates, so retest with the pool chemical calculator and watch cyanuric acid with the pool stabilizer calculator. Get your surface area from the pool volume calculator if the shape is not a rectangle.

Every pool tool, with your saved gallons, is on the pool calculators page.