Pool Water Loss Calculator
Measure how far the water dropped, and this tool converts it to gallons lost and a daily rate — then flags whether it looks like normal evaporation or a possible leak. Confirm with a bucket test.
Calculator
A 2.0 in drop over 10 day(s) on 512 ft² is about 638 gallons (0.20 in/day). Compare to a bucket test to separate evaporation from a leak.
Every outdoor pool loses water to evaporation, and it is normal to top up regularly through the season. The question owners really want answered is whether the water they are adding is ordinary evaporation or the sign of a leak. This calculator converts a measured drop in the water level into the gallons lost and an inches-per-day rate, then compares that rate to a rule-of-thumb threshold so you can tell the two apart.
The gallons lost depend on the surface area, because a pool loses water off its whole surface: a one-inch drop over a large surface is far more water than the same drop over a small one. Enter the surface area — the surface-area calculator gives it from your shape and dimensions — along with how far the level fell and over how many days. Evaporation of up to roughly a quarter-inch per day is typical for many climates; consistently more than that, especially in cool or humid weather, points toward a leak worth investigating.
Formula
Convert the inch drop to feet, multiply by surface area for cubic feet, then by 7.48 to get gallons:
gallons lost = area (ft²) × (drop in inches ÷ 12) × 7.48
Dividing by the number of days gives the loss per day in both gallons and inches. A daily rate above about ¼ inch is the common flag for “more than evaporation — check for a leak.”
Worked example
A pool with 512 ft² of surface that dropped 2 inches:
512 × (2 ÷ 12) × 7.48 ≈ 638 gallons
If that 2-inch drop happened over 10 days, the rate is 0.20 in/day — within the evaporation range. If it happened over just 4 days (0.5 in/day), that is well above the quarter-inch threshold and suggests a leak. The total water lost is the same 638 gallons either way; the rate is what distinguishes the cause.
Evaporation or a leak? Use the bucket test
The definitive test is the bucket test, and this tool is meant to be used alongside it. Fill a bucket with pool water, set it on a step so the water inside sits at the same level as the pool, mark both levels, and leave the pump running normally for a few days. Evaporation affects the bucket and the pool equally, so if the pool drops noticeably more than the bucket, the difference is a leak rather than evaporation. Do the test in stable weather and avoid rain, which throws off both readings.
Evaporation itself rises with heat, wind, low humidity and pool use, and a heated pool or one with lots of splashing loses more. A pool cover cuts evaporation dramatically and is the single most effective way to reduce both water loss and heating cost. If the numbers and the bucket test both point to a leak, common culprits are the equipment pad fittings, the skimmer throat, and light-niche conduits; a professional leak-detection service can pinpoint it. Once you know the gallons lost, the fill-cost tool prices the top-up at your own water rate.