Pool Evaporation Calculator
Turn a measured pool water-level drop over a number of days, adjusted for rain and top-ups, into inches and gallons lost per day, month and year, and refill cost.
Interactive calculator · Method POOL-EVAPORATION-1.0 · Last reviewed: 2026-11-04
Calculate your scenario
Gallons per inch of level = surface area (sq ft) ÷ 12 × 7.48052 (1 ft³ = 1,728 ÷ 231 US gal).
Your inputs
Default worked example. Edit the inputs to calculate your scenario.
Default worked example
(1.5 in drop + 0 in rain + 0 gal ÷ 319.1688 gal/in) ÷ 7 days = 0.2143 in/day × 319.1688 gal per inch over 512 sq ft = 68.39 gal/day.
How to use the calculator
Mark the water line (tape or a skimmer mark), wait several days without swimming or backwashing if you can, and measure the drop in inches. Enter the days, any rainfall from a local rain gauge and any water you added, then the pool’s surface size and your water price.
What the result means
A 32 × 16 ft pool holds about 319 gallons per inch of depth. Losing 1.5 in over 7 days is about 0.214 in/day, roughly 68.39 gal/day or 24,981 gallons a year, $149.88 at an example $6 per 1,000 gallons.
Formula and units
Gallons per inch of level = surface area (sq ft) ÷ 12 × 7.48052 (1 ft³ = 1,728 ÷ 231 US gal). Total loss in inches = observed drop + rainfall + water added ÷ gallons per inch. Loss per day = total ÷ days; gallons per day = inches per day × gallons per inch. Monthly = daily × 30.4375; yearly = daily × 365.25. Refill cost = gallons ÷ 1,000 × your price per 1,000 gallons.
Worked examples
32 × 16 ft pool losing 1.5 in in a week: Pool length: 32 ft; Pool width: 16 ft; Surface area (if entering area): 512 sq ft; Observed water-level drop: 1.5 in; Over this many days: 7 days; Rainfall during the period: 0 in; Water added during the period: 0 gal; Water price per 1,000 gallons (example): 6 $ per 1,000 gal; Surface area from: dims.
Example result: Net level loss per day: 0.2143 in/day; Water lost per day: 68.39 gal/day; Monthly loss: 2,082 gal; Yearly loss: 24,981 gal; Monthly refill cost: $12.49; Yearly refill cost: $149.88.
450 sq ft pool with rain and a 200 gal top-up: Pool length: 32 ft; Pool width: 16 ft; Surface area (if entering area): 450 sq ft; Observed water-level drop: 0.75 in; Over this many days: 3 days; Rainfall during the period: 0.5 in; Water added during the period: 200 gal; Water price per 1,000 gallons (example): 9 $ per 1,000 gal; Surface area from: area.
Example result: Net level loss per day: 0.6543 in/day; Water lost per day: 183.55 gal/day; Monthly loss: 5,587 gal; Yearly loss: 67,042 gal; Monthly refill cost: $50.28; Yearly refill cost: $603.37.
Limitations and common mistakes
Based only on the level loss you measured; it does not predict evaporation from weather, temperature, wind, humidity or a cover, and the period you measured may not represent the whole year. Total loss includes evaporation, splash-out, backwashing and any leak together; this does not diagnose a leak. A bucket test compares the pool with a bucket of pool water set on a step: if the pool drops noticeably more than the bucket over the same days, with the pump on the same schedule, ask a pool professional to check for leaks. Sloped or irregular surfaces: enter the surface area directly.
Ignoring water added from an auto-fill or a hose top-up during the period, which hides most of the loss.
Sources and assumptions
Method POOL-EVAPORATION-1.0; last formula review 2026-11-04. Sources and their supported claims. Calculation methodology.