Sun and heat
Solar energy and warmer temperatures support faster evaporation from exposed water.
Water-loss context
Pool evaporation chart: when the drop stops looking normal.
Evaporation changes with sun, temperature, humidity, wind, and water temperature. Use the chart as a decision guide—not as proof that a suspicious pool is fine.
Daily water loss
A practical pool evaporation chart
These are decision ranges, not universal laws. Local weather, pool exposure, heating, and use can move a pool outside the typical range.
| Pool drop | How to treat it | Best next step |
|---|---|---|
| Up to about 1/4 inch | Often plausible evaporation, especially with sun, wind, warm water, or drier air. | Monitor the pattern and check for hidden autofill use. |
| More than 1/4 inch | A gray area. Weather may contribute, but a small leak becomes more plausible. | Run a controlled 24-hour bucket test. |
| Around 1/2 inch or more | Too much recurring loss to dismiss with a chart alone. | Document the drop and consider professional detection. |
| Around 1 inch | Severe water loss. Protect the operating water level. | Request leak detection promptly. |
Measure from the same marked point at the same time of day. Account for rain, backwashing, splash-out, draining, overflow, and water-feature operation.
What the drop represents
A small-looking change can be hundreds of gallons.
One inch of water over one square foot is approximately 0.623 gallon. The estimates below show water volume—not whether the cause is evaporation or a leak.
| Pool surface | 1/4 inch | 1/2 inch | 1 inch |
|---|---|---|---|
| 400 sq. ft. | 62 gal. | 125 gal. | 249 gal. |
| 500 sq. ft. | 78 gal. | 156 gal. | 312 gal. |
| 600 sq. ft. | 93 gal. | 187 gal. | 374 gal. |
Why the rate changes
Four conditions move evaporation up or down
Air movement
Wind removes moist air near the surface and replaces it with air that can accept more vapor.
Humidity
Drier air generally accepts water vapor more readily. High humidity can slow evaporation.
Warm water and activity
Heated water, spillovers, fountains, swimmers, and splash-out can increase measured water loss.
Turn the chart into a decision
Use comparison, not comfort.
Measure the same 24 hours
Mark the level and avoid estimating from memory.
Compare the pool with a bucket
The same weather affects both water surfaces during the test.
Act when the pool loses more
The chart supplies context. Professional detection locates the source.
Scientific context: evaporation is the change of liquid water to water vapor. Temperature, relative humidity, sunlight, and wind influence its rate. Sources: U.S. Geological Survey and Penn State.
Evaporation questions
Interpret the chart carefully
Is one-quarter inch of pool water loss per day normal?
It can fall within a plausible evaporation range under warm, sunny, or windy conditions, but no fixed number proves that every pool is normal. Compare the pool with a controlled bucket test and watch for repeating symptoms.
Can a pool lose one-half inch per day from evaporation?
Unusually hot, dry, windy conditions and warm pool water can increase evaporation, but a repeated half-inch daily loss deserves investigation rather than reassurance from a chart alone.
Why does wind increase evaporation?
Moving air carries moisture away from the water surface and replaces it with air that can accept more water vapor. Sun, warmer temperatures, and lower humidity can also increase evaporation.
Does rain make the evaporation chart unreliable?
Rain adds water while evaporation and a possible leak continue removing it. Use a marked waterline or bucket test during a controlled period instead of estimating from an unmeasured rainy day.
The pool keeps outrunning the weather?
Stop estimating and locate the source.
Tell us the daily drop, bucket-test result, and pool location.