Pool Evaporation Calculator

Evaporation is driven by one thing above all others: the gap between how much water vapour the air right above the surface could hold, and how much it actually holds. Wind matters because it clears that saturated layer away and replaces it with drier air. Surface area just scales the whole thing.

The Carrier equation, which is what this calculator runs, is the standard in pool and natatorium engineering:

E = (95 + 0.425 × V) × (Pw − Pa) ÷ Y

• E = evaporation, in pounds of water per hour per square foot of surface
• V = air velocity across the water, in feet per minute
• Pw = saturation vapour pressure at the water temperature, in inches of mercury
• Pa = actual vapour pressure of the air, which is relative humidity × saturation pressure at the air temperature
• Y = latent heat of vaporisation, about 1,050 Btu per pound

Water weighs 62.4 lb per cubic foot, so pounds per square foot per day divides straight into a depth. One inch of depth over one square foot is 0.623 gallons, which is where that constant comes from.

Water temperature matters more than air temperature. This is the part most people get backwards. Pw is set by the water, not the air, and vapour pressure climbs steeply with temperature: raising a pool from 78°F to 86°F lifts the saturation pressure about 30% and the evaporation roughly in step. A heated pool in cool autumn air evaporates faster than an unheated one in summer. It is also why a spa at 104°F loses water at a rate that shocks people the first time they measure it.

Typical results, for a sanity check:

If you are losing more than about half an inch a day in mild weather, suspect a leak rather than evaporation. The bucket test settles it: float a bucket of pool water on the top step, mark both water levels, and compare after 24 hours. Evaporation hits both equally, a leak only hits the pool.

Worked example. A 24 ft by 25 ft pool, so 600 sq ft. Water at 82°F, air at 90°F, 40% humidity, moderate wind. Saturation pressure at the water surface is 1.100 in Hg, the air holds 0.568 in Hg, so the deficit is 0.532. At 250 ft/min that gives 0.102 lb/hr/sq ft, which works out to 0.47 inches a day, about 176 gallons. Over a 30 day month that is roughly 5,300 gallons, and every gallon takes dissolved chlorine and stabiliser with it.

A cover is the only thing that really moves the needle. A solid cover cuts evaporation by 90 to 95%, liquid covers by perhaps 15 to 40% in still air and much less when it is windy.