Pool evaporation calculator: how much water is my pool losing?

Every pool loses water to evaporation, more in hot, dry, windy weather and less when it is cool and humid. The loss shows up as a falling level, and the only number that matters for gallons is the surface area: a big, shallow pool loses as much as a big, deep one. Depth doesn’t enter into it.

How the evaporation calculator works

gallons per inch = surface (sq ft) × 0.623
water lost = surface × level drop
  (US: sq ft × inches × 0.623 = gallons; metric: m² × cm × 10 = litres)
heat carried off = gallons × 8.34 lb × 1,048 Btu (DOE, 80°F water)
cover saving = loss without a cover × the share it prevents

One inch of water over one square foot is 144 cubic inches, and a US gallon is 231 cubic inches, so each square foot loses 0.623 gallons per inch. In metric it is simpler: a centimetre over a square metre is 10 litres.

How much do pools typically lose?

• A hot, dry desert. The Southern Nevada Water Authority says the average in-ground pool in Las Vegas loses 48.6 gallons per square foot a year to evaporation. That is about 78 in a year, roughly 1.5 in a week on average — and much more in summer.
• Most places lose less. Cooler, more humid climates lose a fraction of that. There is no reliable single figure for your yard, which is why the rate here is an input and the bucket test is the way to measure it.
• With a cover. SNWA says covers reduce evaporation by 90%; EPA WaterSense says a cover can prevent up to 95%. DOE’s figure for the total make-up water a cover saves is lower, 30%–50%, because covers are off while people swim and splash-out and backwash still happen.

The bucket test: evaporation or a leak?

SNWA’s bucket test compares the pool with a bucket of pool water sitting in it, so both see the same sun, wind and temperature.

1. Turn off any auto-fill so the pool level isn’t topped up during the test.
2. Put a bucket on a pool step, weighted with a couple of rocks or bricks so it sits steady, with its rim above the water.
3. Fill the bucket with pool water to the same level as the pool, and mark both levels — inside the bucket and on the pool wall or skimmer.
4. Leave it for several hot days, running the pool as usual, then compare how far each level has dropped.

How much loss means a leak?

There isn’t a fixed number of inches. The bucket gives you the evaporation for that week; compare the pool against it. If the pool and the bucket drop by about the same amount, it is evaporation. If the pool drops faster than the bucket, the difference is water going somewhere else — a leak in the shell, liner, fittings or plumbing. Splash-out from swimmers and backwashing also lower the pool but not the bucket, so test on days with neither. A consistent gap of more than a fraction of an inch over a few days is worth a call to a leak detection specialist.

Gallons per inch

The result also gives the water in each inch (or cm) of level, the number for top-ups, for lowering the water before a winter cover, and for draining to dilute stabilizer or salt. The pool volume calculator shows it for any shape, and how many gallons per inch in a 24 ft round pool works one through.

Cutting the loss

• Cover it. The biggest lever by far. A solar cover also holds heat in overnight.
• Block the wind. Wind across the surface speeds evaporation; a fence or hedge on the windward side helps.
• Keep it cooler. Warmer water evaporates faster; DOE notes each extra degree costs 10%–30% more in heating energy as well.
• Check the top-up cost. The pool fill calculator prices water by the 1,000 gallons, and the pool running cost calculator puts it alongside the pump and chemicals.