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
Assumptions
- Pool is outdoors and exposed to ambient air conditions entered above.
- Wind speed is measured at pool deck level (~1 m above water surface).
- When a cover is used, it is applied for approximately 8 hours per night.
- Water density is taken as 1 kg/L (1 kg ≈ 0.264 US gallons) at typical pool temperatures.
- The Shah (1981) model is validated for water temperatures between 15°C–35°C (59°F–95°F) and wind speeds up to ~3 m/s (6.7 mph); results outside these ranges are extrapolated.
- Evaporation rate is assumed constant throughout the day (no diurnal variation modeled).
- Heated pools in cold climates will experience significantly higher evaporation than estimated here due to increased temperature differential.
- Pool usage (swimmers) can increase evaporation by 10–20%; this is not included in the base calculation.
- Water cost estimate uses a U.S. national average of approximately $0.004 per gallon; check your local utility rate for accuracy.
- Kidney/freeform shape factor of 0.85 is an approximation; actual area may vary.