Pool Evaporation Calculator

Water evaporation rate is the speed at which liquid water converts to vapor and leaves a surface. It depends on temperature, humidity, wind speed, and surface area. The most practical engineering formula is the Penman equation (simplified) or the mass transfer method.

Simplified evaporation rate formula: E = K × A × (Ps − Pa)

Where:

• E = evaporation rate (kg/hr or lbs/hr)
• K = mass transfer coefficient (typically 0.0022 + 0.00144 × V, where V = wind speed in m/s)
• A = surface area of water (m²)
• Ps = saturated vapor pressure at water surface temperature (kPa)
• Pa = actual vapor pressure of surrounding air = Relative Humidity × Ps_air (kPa)

Daily evaporation estimate (simplified for outdoor pools/tanks): Daily Loss (gallons) = Surface Area (sq ft) × Evaporation Rate (inches/day) × 0.623

Where 0.623 converts inch-feet to gallons.

Reference: average pan evaporation rates:

• Still, humid air (RH 80%+, no wind): 0.1–0.2 inches/day
• Moderate conditions (RH 50%, light wind): 0.2–0.4 inches/day
• Hot, dry, windy (RH 20–30%, 15+ mph wind): 0.5–1.2 inches/day
• Arizona summer desert conditions: 1.0–1.5 inches/day

What each variable means:

• Vapor pressure deficit (Ps − Pa) — the “drying power” of the air; the larger the gap between saturation and actual humidity, the faster evaporation occurs
• Wind speed effect — moving air replaces saturated air near the surface with drier air, accelerating evaporation non-linearly
• Temperature — warmer water has higher vapor pressure; every 10°C rise roughly doubles the vapor pressure

Worked example: Outdoor pool: 600 sq ft surface. Conditions: 95°F air, 40% RH, 10 mph wind. Estimated evaporation rate: 0.5 inches/day.

• Daily volume loss = 600 × 0.5 × 0.623 = 186.9 gallons/day
• Weekly loss = 186.9 × 7 = 1,308 gallons
• Monthly loss = ≈ 5,607 gallons

At $0.005/gallon water cost, this pool loses $28/month in water alone, before considering chemical replacement lost with the evaporated water.