Pool Evaporation Calculator
See how much water your pool should lose to evaporation each day in your weather, compare it with what you measured, and find out whether you have a leak.
A pool loses water every day just to the air, and how much depends on the water temperature, the air, the humidity and the wind. This works out what your weather should be taking and compares it with what you actually measured, so you know whether the dropping line is normal or a leak.
Pool surface square footage
Length × width of the water, not the deck. A 16 × 32 pool is about 500 sq ft; a 20 × 40 is 800; a 24-ft round above-ground is about 450.
Water temperature
From the pool thermometer. Unheated summer pools run 78–86°F; a heated pool or spa spillover runs warmer, and warmer water evaporates much faster.
The weather on the day you measured
A typical afternoon high for the day you measured.
How humid is it?
Wind at the water
What the surface actually sees, not the forecast. A fenced yard with trees or a house on the windward side is sheltered even on a breezy day.
How much the water dropped in 24 hours
Mark the level on the tile or skimmer, turn the pump and any autofill off, keep swimmers out, and measure the drop a day later. A quarter inch is typical summer evaporation.
Evaporation explains it
about 0.46 inch a day
Your pool should lose 0.46 inch a day in this weather; you measured 0.25
Where a quarter inch a day goes
Wind and warm water do most of it: the same pool loses about five times as much on a windy day as a sheltered one, and every ten degrees of water temperature adds roughly a third. A heated pool in dry wind can pass three-quarters of an inch a day and still be nothing but evaporation. If the drop ever outruns the weather, or shows up only after the pump has been running, that's plumbing rather than air; a solid cover is the one thing that cuts evaporation and the chemicals and heat that leave with it.
Evaporation uses the ASHRAE-published equation fitted to a measured outdoor pool: a calm term plus a wind term, times the vapor-pressure gap between your water and the air. Wind is what the water surface sees, and a fenced yard sees far less than the forecast. A tape mark reads to about an eighth of an inch. 2026 figures.
💡About this calculator▼
Every outdoor pool loses water to the air, and the amount swings more than people expect: the same pool can lose an eighth of an inch on a still, humid day and three-quarters of an inch on a hot, dry, windy one. That's why "the water's dropping" is such a hard call to make by eye, and why leak companies get so many calls in August that turn out to be weather.
This does the weather's half of the bucket test. Enter the water surface, the water and air temperatures, how humid and how windy it was, and how far the level dropped in 24 hours with the pump off. The calculator works out what evaporation alone should have taken, using the equation engineers use for outdoor pools, and shows the loss the weather doesn't explain.
Up to an eighth of an inch over is nothing. That's the resolution of a tape mark on the tile. An eighth to a quarter is worth a second reading and a bucket test. More than a quarter inch a day beyond evaporation is the line leak companies use for "call us," and the calculator says so in gallons a day and a month.
Start with how thirsty the air is. Water evaporates because the air above it holds less vapor than the water surface pushes out. The calculator looks up the vapor pressure of water at your pool's temperature and the vapor pressure of the air from its temperature and humidity; the gap between them drives evaporation. Warm water into hot dry air is a big gap; cool water into humid air is almost none.
Multiply by the wind. Still air over a pool saturates and slows evaporation; wind sweeps it away. The equation, fitted to measurements on a real outdoor pool and published by ASHRAE, has a calm term plus a wind term: evaporation in pounds of water per hour per square foot equals (70.3 + 0.314 × wind in feet per minute) × the vapor-pressure gap in inches of mercury ÷ 1,048, the heat it takes to evaporate a pound of water. The wind that matters is at the water surface, which in a fenced yard is far below the forecast, so the calculator asks for sheltered, breezy or windy rather than a number.
Convert to inches and gallons. Pounds per hour per square foot × 24 ÷ 62.4 pounds per cubic foot × 12 gives inches a day; times the surface area ÷ 12 × 7.48 gives gallons.
Compare with what you measured. The measured drop minus the expected drop is the loss the weather doesn't explain. Up to an eighth of an inch is within what a tape mark can read; an eighth to a quarter is borderline; more than a quarter inch a day is past the line leak companies use for "call us."
Worked example. A 500 sq ft pool at 82°F, on an 85°F day at 50 percent humidity with a 5 mph breeze at the water: the vapor-pressure gap is 0.48 inches of mercury; (70.3 + 0.314 × 440) × 0.48 ÷ 1,048 = 0.0955 pounds per hour per square foot; × 24 ÷ 62.4 × 12 = 0.46 inch a day, about 142 gallons. A measured quarter inch is under that: evaporation. A measured 1.2 inches leaves 0.74 inch, about 232 gallons a day and 6,960 a month, that the weather doesn't explain: a leak.
📐How it's calculated▼
Evaporation (lb per hour per sq ft) = (70.3 + 0.314 × wind in ft/min) × (vapor pressure of the water − vapor pressure of the air, in inches of mercury) ÷ 1,048. Wind cards: sheltered 0, breezy 440, windy 880 ft/min (0, 5 and 10 mph). Vapor pressure of the water is the steam-table value at the water temperature; of the air, the humidity times the value at the air temperature. Humidity cards: dry 30%, moderate 50%, humid 70%.
Inches a day = lb per hour per sq ft × 24 ÷ 62.4 × 12.
Gallons a day = surface sq ft × inches ÷ 12 × 7.48.
Unexplained loss = measured drop − expected drop. Up to 0.125 inch: evaporation. 0.125 to 0.25: borderline. Over 0.25 inch: leak.
Default (500 sq ft, 82°F water, 85°F air, moderate, breezy): 0.46 inch and 142 gallons a day. Sheltered instead: 0.15 inch. Windy instead: 0.76 inch.
Hot and dry (88°F water, 105°F air, dry, breezy): 0.61 inch a day, so a measured inch leaves 0.39 unexplained: leak.
Humid (86°F water, 92°F air, humid, breezy): 0.18 inch a day, so a measured half inch leaves 0.32 unexplained: leak.
Heated pool in fall (90°F water, 60°F air, dry, windy): 1.93 inches a day, all of it evaporation.
📎Sources:Shah, M. M. — Methods for Calculation of Evaporation from Swimming Pools and Other Water Surfaces (ASHRAE Transactions, 2014),Engineering Toolbox — Water saturation pressure vs. temperature (vapor pressure table),Latham Pool — My Swimming Pool is Losing Water: Is It Evaporation or a Leak? (quarter-inch average, bucket test),Leak-Tech — The Bucket Test (steps, pump on vs off, the quarter-inch call line)
🔍Finding your inputs▼
Pool surface square footage: Length times width of the water itself, not the deck. A 16 × 32 pool is about 500 sq ft, a 20 × 40 is 800, a 24-foot round above-ground about 450. Only the gallons depend on this; the inches per day don't.
Water temperature: From the pool thermometer. Unheated summer pools run 78 to 86°F; a heated pool or one with a spa spillover runs warmer, and every 10°F of water temperature adds roughly a third to evaporation.
Daytime air temperature: A typical afternoon high for the day you measured. Air warmer than the water does not add evaporation by itself; it's the humidity that matters more.
How humid is it: Dry is the desert Southwest, around 30 percent; moderate is most of the country in summer, around 50; humid is the Gulf Coast and Florida, around 70. Humid air takes far less water off the pool.
Wind at the water: What the surface actually sees. A fenced yard with trees or the house on the windward side is sheltered even when the forecast says breezy; an open lot on a ridge is windy. This is the input with the biggest swing, five to one between sheltered and windy, so if you're unsure pick sheltered, which errs toward flagging a leak.
How much the water dropped in 24 hours: Mark the level on the tile or the skimmer faceplate, turn off the pump and any autofill, keep swimmers out, and measure the drop a day later to the nearest eighth of an inch. A quarter inch is typical summer evaporation.
⚠️Special situations▼
The pool drops half an inch a day in August. Everyone says a quarter is normal. Do I have a leak?
Not necessarily. A quarter inch is an average, not a limit. An 82°F pool on an 85°F day with a breeze at the water loses about 0.46 inch by the equation, and a windy day takes three-quarters. Put in your actual day and read the verdict; if the loss the weather doesn't explain is under an eighth of an inch, it's evaporation. If it's over a quarter, do the bucket test and call.
It's humid here on the Gulf Coast and I'm losing half an inch. That can't be evaporation, can it?
Probably not. Humid air is the one thing that shuts evaporation down: an 86°F pool on a 92°F day at 70 percent humidity with a breeze loses only about 0.18 inch a day, so half an inch leaves about a third of an inch, roughly 100 gallons a day on a 500 sq ft pool, that the weather doesn't explain. That's a bucket test, then a leak company.
The bucket test showed the pool and the bucket dropped the same. Do I still need this?
No, the bucket test is the real answer and this calculator is the weather half of it. What this adds is a number when you haven't run the test yet, or when the bucket was in the sun and the pool in the shade, or when you want to know whether the loss is big enough to be worth the trouble. It also tells you whether a heated pool in October losing an inch a day is plausible, which it is.
The pool only loses water when the pump is running. What does that mean?
A leak in the plumbing on the pressure side, after the pump. With the pump off the pipes aren't pressurized and the level holds; with it on, water is forced out through the crack or a fitting and often shows up as a soggy spot in the yard. Run the 24-hour reading both ways: loss with the pump off is the shell, liner, skimmer or light niche; extra loss with it on is the plumbing or the equipment pad.
❓Common questions▼
How much water should a pool lose to evaporation per day?
Most pools lose an eighth to a half inch a day in summer, and the leak industry's rule of thumb is about a quarter inch on average. The number depends on the water temperature, the humidity and above all the wind at the surface: a typical 82°F pool on an 85°F day loses about 0.15 inch sheltered, 0.46 inch with a breeze and 0.76 inch when it's windy. A heated pool in cool, dry, windy fall weather can lose well over an inch a day and still be evaporation.
How do I tell if my pool is leaking or evaporating?
The bucket test. Fill a five-gallon bucket with pool water to an inch from the top, set it on the first step, mark the water level inside the bucket and the pool level on the outside, and compare after 24 hours with the pump off. The inside drop is evaporation; any extra on the outside is a leak. This calculator gives you the same comparison from the weather, and the industry line is that more than a quarter inch a day beyond evaporation means call a leak-detection company.
How many gallons does my pool lose to evaporation?
Surface area in square feet times the inches lost, divided by 12, times 7.48. A 500 sq ft pool losing a quarter inch is 78 gallons a day, and at 0.46 inch on a breezy day it's about 142 gallons, roughly 4,000 gallons a month. That is why a solid cover pays: it stops most of the evaporation and the heat and chemicals that leave with the water.
Does a heated pool evaporate faster?
Much faster. Evaporation is driven by the gap between the vapor pressure of the water and of the air, and warm water pushes hard: every 10°F of water temperature adds roughly a third. A 90°F pool on a 60°F dry, windy fall day loses nearly two inches a day, which is why heated pools without a cover cost so much to run and why an autumn water-level drop on a heated pool is usually not a leak.
Why does the calculator ask for wind at the water instead of the forecast?
Because the equation was fitted to wind measured three feet above the pool, and a fenced backyard with trees or a house upwind sees a fraction of what the airport reports. Wind is also the biggest lever in the result, about five to one between sheltered and windy, so guessing high would hide a real leak behind imaginary evaporation. If you're not sure, pick sheltered: it errs toward flagging a loss so you run the bucket test.
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