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Pool Evaporation Calculator

A pool evaporation calculator helps you tell normal water loss from a leak. Enter your surface area, water temperature, air temperature, humidity and wind speed. The tool returns gallons lost per day, the drop in inches, and what that costs you over a month.

Pool evaporation calculator illustration of pool water evaporating in the sun with a dropping water level

Run the numbers

Formula used

Loss (lb/hr) = 0.1 x Area x (Pw - Pa) x Activity Factor

Your result

Daily water loss
146 gallons per day
Water level drop
0.46 inches per day
Loss per month
4,370 gallons
Monthly water cost
$21.85

Results update as you type. Round up when you buy materials.

How the pool evaporation calculator works

Evaporation happens when water vapor pressure at the surface is higher than vapor pressure in the air above it. Warm water and dry air widen that gap and speed up the loss.

Wind strips the humid layer sitting on the water, which is why a breezy day can double the loss even when the temperature does not change.

The formula, step by step

Loss (lb/hr) = 0.1 x Area x (Pw - Pa) x Activity Factor

  • Pw is the vapor pressure of the water at its surface temperature, in inches of mercury.
  • Pa is the vapor pressure of the air, which is saturation pressure times relative humidity.
  • Multiply the difference by the surface area and by 0.1.
  • A wind factor scales the result up as air speed rises.
  • Divide pounds per hour by 8.34 and multiply by 24 for gallons per day.

Worked example

A 512 square foot pool at 84 degrees, with 80 degree air, 50 percent humidity and a 5 mph breeze.

The vapor pressure gap is roughly 0.6 inches of mercury.

That works out near 150 gallons a day, or about a third of an inch off the water line. Normal loss for a hot, dry week.

Watch it done

This short video walks through the same job in real life, so you can match what you see on screen to what you do at the pool.

Video guide: Does My Pool Have a Leak, or Is It Just Evaporation? by Inyo Pools. Video hosted on YouTube.

Tips before you start

Anything over a quarter inch per day beyond what this tool predicts points at a leak. Run a bucket test to confirm before you call a leak detection service.

A solar cover cuts evaporation by 85 to 95 percent while it is on. It also holds heat, so the same cover trims your water bill and your gas bill together.

Pool Evaporation Calculator FAQs

How much water does a pool lose to evaporation?

A quarter inch to half an inch a day is normal in summer. That is 100 to 300 gallons for a typical residential pool.

How do I tell evaporation from a leak?

Do a bucket test. Set a filled bucket on a step and mark both water levels. If the pool drops faster than the bucket over 24 hours, you have a leak.

Does a heated pool lose more water?

Yes. Warmer water pushes more vapor into the air, so a heated pool can lose twice as much as an unheated one on the same night.

Does wind really matter?

A lot. Wind clears the humid layer above the water and keeps the evaporation rate at full speed all day.