Pool & Spa Pool Water Evaporation Calculator
Created by: Sophia Bennett
Last updated:
Estimate daily and monthly evaporation water loss from weather conditions and cover use, then convert projected loss into refill cost.
Pool & Spa Pool Water Evaporation Calculator
PoolEstimate refill water demand and evaporation-related cost
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What is a Pool & Spa Water Evaporation Calculator?
A Pool & Spa Water Evaporation Calculator estimates daily and monthly refill demand from weather conditions, surface area, and cover status.
Use this with the Pool Cost Calculator and Pool Heater BTU Calculator for complete operating-cost planning.
Evaporation Formulas
Estimated Evaporation (in/day) ≈ ((Water Temp - Air Temp) × 0.02 + Wind × 0.01 + (100 - RH) × 0.005) Gallons Lost/Day = (Evaporation in/day ÷ 12) × Surface Area (ft²) × 7.48052 Monthly Cost = (Gallons Lost/Month ÷ 1000) × Water Cost per 1000 gal
How to Calculate Evaporation: Example Calculations
Surface 500 ft², Water 84°F, Air 74°F, RH 45%, Wind 6 mph Evaporation ≈ 0.48 in/day Gallons/day ≈ (0.48 ÷ 12) × 500 × 7.48052 ≈ 149.6 gal
Common Applications
- Monthly refill water budget planning.
- Cover ROI and evaporation control comparisons.
- Leak suspicion baseline checks.
- Seasonal operating cost forecasting.
Tips for Accurate Results
- Use average local weather for monthly estimates.
- Track real refill volume to calibrate assumptions.
- Account for splash-out and backwash separately.
- Use covered mode when cover is consistently deployed.
Frequently Asked Questions
How much water evaporates from a pool each day?
Daily evaporation varies by water temperature, air temperature, humidity, wind, and whether a cover is used. In many climates, uncovered pools can lose noticeable water each day, especially during warm, dry, and windy periods. Tracking estimated daily loss helps plan refill volume and operating cost.
Does a pool cover reduce evaporation?
Yes. A well-fitted cover can substantially reduce evaporation compared with uncovered operation. Reduced evaporation lowers refill demand, chemical drift, and heat loss. For many owners, regular cover use is one of the most effective ways to reduce recurring water and heating cost.
How do humidity and wind affect evaporation?
Lower humidity increases the vapor-pressure gradient and usually raises evaporation. Higher wind speed removes moist air near the surface and can accelerate loss further. Warm water relative to air temperature also increases evaporation potential, especially in exposed pool locations.
When should I suspect a leak instead of evaporation?
If observed loss consistently exceeds expected evaporation for your weather and cover conditions, a leak check may be warranted. Large overnight drops, localized wet spots, or unexplained refill spikes can be warning signs. A bucket test is a common first diagnostic step.
Why track evaporation cost?
Evaporation cost is not only water replacement. It can also increase chemical demand and heating load. Estimating monthly refill gallons helps build realistic operating budgets and supports decisions about covers, windbreaks, and runtime optimization.
Sources and References
- ASHRAE evaporation and latent heat reference methods.
- PHTA best practices for pool covers and water management.
- Local utility rate structures for refill water estimation.