💧 Pool Water Evaporation Loss Estimator
Estimate how much water your pool loses to evaporation each day based on surface area and evaporation rate (commonly around 1/4 inch per day, more in hot/dry/windy conditions).
Inches off the surface, converted to gallons
One inch of water over one square foot is 0.623 gallons - a cubic foot of 7.48 gallons divided by twelve - so the whole calculation is:
gallons per day = surface area (sq ft) × rate (in/day) × 0.623The defaults, 450 sq ft losing a quarter inch a day, give 70.1 gallons a day and 491 a week. Only the surface matters; a 450 sq ft pool 4 ft deep and one 8 ft deep evaporate at the same rate.
Picking a rate, and checking it
| Conditions | Rate | 450 sq ft pool |
|---|---|---|
| Cool, humid, sheltered | 0.125 in/day | 35 gal/day |
| Typical summer | 0.25 in/day | 70 gal/day |
| Hot, dry, windy | 0.5 in/day | 140 gal/day |
| Heated water on cold nights | 0.75 in/day | 210 gal/day |
Wind matters as much as heat, because moving air carries away the humid layer sitting on the water. That is also why evaporation is the main heat loss from a heated pool, and why a cover pays for itself quickly.
Frequently asked questions
How much water loss is normal for a pool?
A quarter to half an inch a day in summer, so 70 to 140 gallons on a 450 sq ft pool. Losing an inch a day in mild weather is worth investigating.
How do I tell evaporation from a leak?
Do a bucket test. Sit a bucket of pool water on a step so it is part submerged, mark the level inside and outside, leave the pump on its normal schedule and compare after 24 hours. Both drop by evaporation; if the pool drops further, you have a leak.
What does the lost water cost to replace?
At 491 gallons a week and $5.50 per 1,000 gallons, about $2.70 a week, or roughly $35 across a thirteen week summer. The bigger bill is the heat that leaves with it.