Pool Evaporation Calculator
Estimates how many gallons your pool loses to evaporation each day and what that water costs to replace. Useful for pool owners budgeting water bills or deciding whether a pool cover is worth the investment.
Last updated: September 2026
Formula below · 1 source (Wikipedia) · Updated Sep 2026
Compare with similar
About this calculator
Pool evaporation is driven by the vapor-pressure difference between the water surface and the air, the air speed across the surface, and the surface area. This calculator uses the Carrier evaporation equation given in the ASHRAE Handbook: evaporation (lb/h) = A × (95 + 0.425 × V) × (pw − pa) / 1000, where A is the surface area in sq ft, V is the air speed in feet per minute, pw is the saturation vapor pressure at the water temperature and pa is the vapor pressure of the air (both in inches of mercury), and 1000 Btu/lb is the latent heat of evaporation. To keep the inputs simple, the water surface is assumed to be at the air temperature, so pw − pa = pSat(T) × (1 − humidity/100), with pSat from the Magnus formula. The wind options use the middle of each range (2.5, 7.5, 12.5 and 17.5 mph, i.e. 220, 660, 1,100 and 1,540 ft/min). Pounds per hour × 24 / 8.34 gives gallons per day, which is multiplied by your water cost. A heated pool warmer than the air evaporates faster than this estimate, and an outdoor wind sensor overstates the air speed right at the water surface, so treat the result as an estimate and confirm with a bucket test.
How to use
Pool surface = 400 sq ft; temperature = 85°F; humidity = 50%; wind = calm (220 ft/min); water cost = $0.005/gallon. Step 1 — Saturation vapor pressure at 85°F (29.4°C): 6.112 × e^(17.62 × 29.4 / 272.5) ≈ 41.0 hPa ≈ 1.211 inHg. Step 2 — Vapor-pressure difference: 1.211 × (1 − 0.50) ≈ 0.605 inHg. Step 3 — Air-speed term: 95 + 0.425 × 220 = 188.5. Step 4 — Evaporation: 400 × 188.5 × 0.605 / 1000 ≈ 45.7 lb/h, or 45.7 × 24 / 8.34 ≈ 131 gallons/day (about 0.53 inch). Step 5 — Cost: 131 × $0.005 ≈ $0.66/day. On a hot, dry, windy day costs rise sharply—a pool cover can cut evaporation by up to 95%.
Frequently asked questions
How much water does a typical swimming pool lose to evaporation per day?
An average 15×30-foot residential pool (450 sq ft) in a warm, dry climate can lose 1–2 inches of water per week, equating to roughly 280–560 gallons. In hot, low-humidity, windy conditions that rate can double. This is the single largest source of water loss in most pools, exceeding leaks and splash-out combined under normal conditions. Using a pool cover when the pool is not in use is the most effective way to reduce this loss.
Does wind speed really make a significant difference in pool evaporation?
Yes—wind continuously replaces the thin layer of saturated air hovering above the water surface with drier ambient air, removing the natural humidity barrier that would otherwise slow evaporation. Studies show that even a light breeze of 7 mph can double the evaporation rate compared to still-air conditions. Pool enclosures and windbreak landscaping around the pool perimeter are practical ways to mitigate wind-driven water loss. In this calculator the evaporation rate scales with (95 + 0.425 × air speed in ft/min), so the Windy option evaporates about four times as fast as Calm.
Is it cheaper to buy a pool cover or pay the higher water bill from evaporation?
A quality solar pool cover costs $100–$300 and can reduce evaporation by 90–95%, often paying for itself within a single summer season in hot climates. Beyond water savings, covers also reduce chemical consumption by 35–60% and retain heat, cutting heating costs. The exact payback period depends on local water rates, climate, and pool size—the evaporation calculator gives you the daily cost baseline needed to make that comparison accurately. For most pool owners in warm regions, a cover is the single best return-on-investment maintenance purchase available.