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Evaporation Math: How Much Water Is Your Pool Actually Losing This Month?

August 10, 2026 | Your Happy Dolphin Pool & Spa Team

Evaporation shimmer rising off a backyard pool at golden hour
 

Images are AI generated and do not depict an actual pool or Dolphin Pool & Spa location.

Your pool loses water every day. You don't see it go. No splash, no drip, no sound. But by the end of August, your pool could lose hundreds of gallons and you'd never spot a single drop leaving the water. Let's do the math and find out where it's going.

1 The Rule of Thumb

Heat pulls moisture off the surface of your pool and sends it into the air. On a calm, mild Minnesota day, most pools lose about a 1/4 inch of water. On a hot, dry, windy day, that number can climb to 1/2 inch or more.

A quarter inch sounds small. Spread across your whole pool surface, it isn't.

2 Do the Math for Your Pool

Here's the formula. Take your pool's surface area in square feet. Multiply by the inches of depth you lost. Divide by 12 to turn inches into feet. Multiply by 7.48, the number of gallons in a cubic foot.

Gallons lost = ((surface area (sq ft) × inches lost) ÷ 12) × 7.48

Try it on a standard 16 by 32 foot pool. That's 512 square feet of surface.

On a normal day, at a quarter inch: ((512 × 0.25) ÷ 12) × 7.48 ≈ 80 gallons.
On a hot, windy day, at a half inch: ((512 × 0.5) ÷ 12) × 7.48 ≈ 160 gallons.

Up to 160 gallons, gone in a single day. And your pool never looked one bit different.

Pool Size Surface Area Normal Day Hot & Windy Day
Small (12' x 24') ~288 sq ft ~45 gal ~90 gal
Medium (16' x 32') ~512 sq ft ~80 gal ~160 gal
Large (20' x 40') ~800 sq ft ~125 gal ~250 gal
24-ft Round Above Ground ~452 sq ft ~70 gal ~140 gal

3 What Speeds It Up, What Slows It Down

Speeds it up

  • Hot, dry air
  • Steady wind
  • Direct full sun
  • Water features and jets running
  • Warm water temperature

Slows it down

  • A pool cover
  • Calm, still air
  • Shade from trees or structures
  • Cooler water temperature
  • Running water features at night only

4 Evaporation or Leak? Try the Bucket Test

Not sure if you're looking at plain evaporation or an actual leak? Run the bucket test.

Fill a bucket about two-thirds full and set it on a pool step, so the water inside the bucket sits at the same level as the water in your pool. Mark both water lines with a strip of tape, one inside the bucket, one on the pool tile or the outside of the bucket. Let the pump run like normal. Wait 24 to 48 hours.

Compare the two lines. If your pool dropped about the same amount as the bucket, that's just evaporation. If your pool dropped noticeably more than the bucket, you likely have a leak. For a professional leak locate, American Leak Detection of Minneapolis can pinpoint the source.

Heads up: if your water level drops below the skimmer opening, your pump can suck air instead of water. That can burn out the motor fast. Keep an eye on the level and top off before it gets too low.

5 Top Off and Keep Your Chemistry Balanced

When you add fresh water to replace what evaporated, your chemistry shifts too. Chlorine, pH, and alkalinity all move a bit with a big top-off. Retest after every one.

Stop in for a water test — $7, or free with a $50 or more chemical purchase — and we'll get your numbers back where they belong before it turns into a bigger job.

Whether it's evaporation, a leak, or chemistry that's drifted from a big top-off, our team can help you sort it out. From testing to Balance Pak products to full-service checkups, we make pool ownership a joy!

Call Us Now — 763-542-9000

Your Happy Dolphin Pool & Spa Team