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.
About this calculator
Pool evaporation is driven by temperature, humidity, wind, and surface area. The formula used here is: Daily Cost ($) = Pool Surface Area (sq ft) × ((Temperature°F − 32) × 0.556 + 15) × (1 − Humidity/100) × (Wind Factor) × 0.024 × 7.48 × Water Cost ($/gallon). The temperature conversion term estimates vapor pressure at the water surface. The humidity factor (1 − H/100) represents the atmosphere's remaining capacity to absorb moisture—dry air evaporates water faster. The wind factor (windSpeed + 1) accounts for the boundary layer of saturated air being swept away by moving air. Multiplying by 0.024 converts the evaporation depth to daily inches, and 7.48 converts cubic feet to gallons. Together these factors produce a realistic daily water-replacement cost.
How to use
Pool surface = 400 sq ft; temperature = 85°F; humidity = 50%; wind = calm (factor = 1); water cost = $0.005/gallon. Step 1 — Temp term: (85 − 32) × 0.556 + 15 = 53 × 0.556 + 15 = 29.47 + 15 = 44.47. Step 2 — Humidity factor: 1 − 50/100 = 0.5. Step 3 — Wind factor: 1 + 1 = 2. Step 4 — Combine: 400 × 44.47 × 0.5 × 2 × 0.024 × 7.48 × 0.005 = 400 × 44.47 × 0.5 × 2 × 0.024 × 7.48 × 0.005 ≈ $0.80/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 500–1,000 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. The wind factor in this calculator multiplies the base rate by (windSpeed + 1), so calm conditions (factor = 1) represent the minimum evaporation scenario.
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.