Pool Evaporation & Water Loss Calculator
Estimate how much water your pool loses to evaporation each day and week. Enter your pool dimensions, water and air temperatures, relative humidity, and wind conditions to get an accurate evaporation estimate using the industry-standard Penman evaporation model adapted for swimming pools.
Enter your pool details above and click Calculate.
Formula
Shah (1981) Pool Evaporation Model (as referenced in ASHRAE Fundamentals):
E = A × (Pw − Pa) × (0.089 + 0.0782 × V)
- E = Evaporation rate (kg/hr)
- A = Pool surface area (m²)
- Pw = Saturation vapor pressure at water surface temperature (kPa), calculated via the Buck equation:
P = 0.61121 × exp[(18.678 − T/234.5) × T/(257.14 + T)] - Pa = Actual vapor pressure of ambient air = Psat(Tair) × RH/100 (kPa)
- V = Wind speed at pool surface (m/s)
- 0.089 = Still-air evaporation coefficient
- 0.0782 = Wind-driven evaporation coefficient
Daily loss accounts for cover usage: Edaily = E × 16 hrs (open) + E × cover_factor × 8 hrs (covered)
Depth loss (inches): D = (Gallons ÷ Surface_ft²) × 0.1337 × 12