Pool Water Evaporation Calculator
Where Does the Water Actually Go?
A pool that loses an inch of water a week isn't necessarily leaking — evaporation alone can account for that much, especially in hot, dry, or windy conditions. Distinguishing normal evaporative loss from an actual leak starts with knowing what evaporation should look like for your climate and pool size, which is exactly what this calculator estimates using surface area and a condition-based evaporation rate.
The Formula
The 0.623 constant converts inches of depth lost across the surface area into gallons (derived from 7.48 gallons per cubic foot ÷ 12 inches per foot). Evaporation rate itself depends heavily on conditions:
| Condition | Rate (inches/day) |
|---|---|
| Mild (cool, calm, high humidity) | 0.10 |
| Moderate (typical summer conditions) | 0.20 |
| Hot & Dry (low humidity) | 0.30 |
| Hot, Dry & Windy | 0.40 |
Daily and Monthly Loss by Condition
| Condition | Loss per day | Loss over 30 days |
|---|---|---|
| Mild | 24.9 gal | 748 gal |
| Moderate | 49.8 gal | 1,495 gal |
| Hot & Dry | 74.8 gal | 2,243 gal |
| Hot, Dry & Windy | 99.7 gal | 2,990 gal |
The gap between mild and hot-windy conditions is roughly 4× — a pool that seems to be losing water unusually fast in calm, humid weather is a better candidate for a leak check than one losing the same amount during a hot, windy stretch.
Where This Calculation Matters
- The bucket test baseline — comparing measured water loss against the expected evaporation figure for current conditions is the standard way to determine whether a leak actually exists.
- Planning refill needs — knowing expected daily loss helps schedule top-offs, particularly for automatic fill systems or during vacation periods with no one monitoring the pool.
- Evaluating a pool cover's payoff — since covers can cut evaporative loss dramatically, comparing covered vs. uncovered loss over a season quantifies the water (and chemical) savings.
How to Use This Calculator
- Enter your pool's surface area in square feet.
- Select the condition category that best matches your climate and weather.
- Enter the number of days to project (defaults to 1 day).
- Select Calculate to see total expected water loss in gallons.
Related Calculations
Don't have surface area handy? Start with the Pool Surface Area Calculator, or use the Pool Fill Time Calculator to see how long a top-off will take once you know how much water needs replacing.