Enter your pool's surface size and how much the water dropped — get gallons lost per day and a verdict on whether it looks like evaporation or a leak.
The evaporation math
gallons per inch = length × width × 0.623
daily loss = total drop ÷ days · normal ≈ 0.25 in/day, high ≈ 0.5
Evaporation scales with surface area, water temperature, and how dry and windy the air is — not with pool depth. That's why two pools on the same street can lose water at visibly different rates: a warm, uncovered pool in the wind evaporates like a pot on low simmer.
A worked example
A 32×16 ft pool drops 1.5 inches over 3 days: 0.5 in/day — the high end of plausible evaporation in hot weather. That's 32 × 16 × 0.623 × 0.5 ≈ 160 gallons a day. If the same rate persists during a cool, humid week, evaporation can't explain it — run the bucket test.
Where pools actually leak
The usual suspects, cheapest first: the multiport valve sending water down the backwash line, skimmer-to-shell seams (leak stops when you let the level drop just below the skimmer), light niches, and returns. Structural shell leaks are the rarest. Losing water only when the pump runs points at the pressure side plumbing; only when it's off, the suction side or shell.
Frequently asked questions
How much pool water loss is normal?
Roughly ¼ inch per day (about 2 inches a week) is typical summer evaporation, and hot, dry, windy weather can push it toward ½ inch a day. Consistently more than that — or loss that continues in cool, humid weather — points to a leak.
How do I tell if my pool is leaking or evaporating?
The bucket test: set a bucket of water on a pool step, mark the water line inside and outside the bucket, and wait 24–48 hours with the pump on. Evaporation lowers both equally; if the pool line drops more than the bucket line, water is leaving somewhere else.
How many gallons is an inch of pool water?
Surface area in square feet × 0.623 gallons per inch. A 32×16 ft pool loses about 320 gallons for every inch of depth — which is why a 'small' leak shows up fast on the water bill.
Does a pool cover really reduce evaporation?
Dramatically — evaporation is the pool's main water and heat loss, and a cover cuts it by 90%+ when on. If your make-up water or heating bills sting, the cover pays for itself in a season.