Pool Evaporation Calculator

What Controls Pool Evaporation?

Evaporation occurs at the exposed water surface. Pool depth and total gallons do not directly control the rate; surface area and weather do.

The major drivers are:

• Water temperature: warmer water has more vapor pressure.
• Air temperature: warm water under colder air tends to lose more heat and water.
• Humidity: dry air can accept more moisture than humid air.
• Wind: moving air removes the humid layer immediately above the water.
• Cover use: a physical barrier interrupts vapor movement.
• Water features: waterfalls, spillovers, fountains, and jets expose more water to air.

Sunlight, clouds, rain, overnight cooling, landscaping, and the shape of nearby structures also matter. That is why the calculator reports a range rather than an exact daily loss.

How to Convert Inches of Water to Gallons

One inch of water over one square foot equals about 0.623 gallons.

Gallons lost = surface area in ft² × level loss in inches × 0.623

A 16-by-32-foot pool has 512 square feet of surface. A one-inch drop represents:

512 × 1 × 0.623 = about 319 gallons

For a rectangle, surface area is length times width. For round pools, use π times radius squared. Freeform pools can be divided into simple shapes or measured from the plan.

If you know gallons but not time, the pool fill time calculator converts top-up volume into hose hours and utility cost.

How Pool Covers Reduce Water Loss

A fitted solid, automatic, foam, or bubble cover can sharply reduce evaporation while it is on the water. Actual performance depends on fit, damage, wind, and how often the pool is opened.

The calculator scales cover benefit by hours of daily use. A highly effective cover used only overnight cannot save as much as the same cover left in place whenever nobody is swimming.

Partial systems such as solar disks leave exposed water between pieces. Liquid evaporation reducers can help under calm conditions but do not create the physical barrier of a fitted cover.

A cover also reduces heat loss, chemical consumption from splash-out and refill, and humidity around indoor pools. It does not replace barriers or other safety measures unless specifically designed and approved for that purpose.

Evaporation vs. a Pool Leak

Weather-based estimates cannot prove that plumbing or the shell is watertight. Use a controlled bucket test:

1. Fill a bucket with pool water and place it on a pool step.
2. Adjust it so the water inside and outside experience similar conditions.
3. Mark both water levels.
4. Compare the drops after about 24 hours without rain or added water.
5. Follow a reliable leak-test procedure for pump-on and pump-off comparisons.

If the pool falls materially faster than the bucket, investigate. Also watch for wet ground, cracks, air entering the pump, unexplained chemical dilution, autofill operation, or water stopping at a repeatable level.

Turn off autofill during any controlled test. Do not let the pool level fall below safe skimmer or equipment operating limits.

Why Evaporation Changes Pool Chemistry

Pure water leaves during evaporation; most dissolved material remains. Replacing evaporated water can therefore add more calcium, alkalinity, salt, and total dissolved solids from the source water while the original dissolved material stays behind.

Over time:

• Calcium hardness can rise.
• Salt and TDS can accumulate.
• Scale risk can increase.
• Autofill can hide a leak and continuously dilute CYA or chemicals if water is also escaping.

Test the fill water and watch trends. The pool calcium hardness calculator and LSI calculator can help assess the resulting balance.

Pool Evaporation Calculator FAQ

Is one inch of pool-water loss per week normal?

It can be under some conditions, but “normal” varies widely. Dry air, warm water, wind, and water features can cause more; cool humid weather and consistent cover use can cause much less.

How many gallons does my pool lose when the level drops one inch?

Multiply surface area in square feet by 0.623. A 512-square-foot pool loses about 319 gallons per inch.

Does pool depth affect evaporation?

Not directly. Evaporation occurs at the surface, so exposed square footage and conditions above the water matter more than average depth.

Do waterfalls increase evaporation?

Yes. They increase exposed surface, turbulence, and air-water contact. Wind can also carry spray outside the pool.

Can an automatic filler hide a leak?

Yes. It can maintain the visible waterline while consumption rises. Monitor the water meter or isolate the autofill during a controlled leak test.

Does evaporation lower salt or CYA?

No. Salt and CYA remain when water evaporates. Their concentration may rise slightly until fresh water is added. A leak followed by refill, backwashing, splash-out, or draining removes dissolved chemicals and can lower them.