Pool Water Loss Calculator

Short answer: gallons lost = surface area (sq ft) × inches dropped ÷ 12 × 7.48. A 16 × 32 ft pool that drops 1/4 inch a day loses about 80 gallons a day, or roughly 2,390 gallons a month. To tell evaporation from a leak, do a bucket test: if the pool drops more than a bucket of pool water sitting beside it, water is leaving another way.

How the water loss is worked out

The calculator turns a level drop into gallons over your pool’s water surface:

Gallons per day = surface area × (inches dropped ÷ days) ÷ 12 × 7.4805

Here 7.4805 is the number of US gallons in a cubic foot (NIST SP 811). Each inch over one square foot is 0.623 gallons, so the surface area does most of the work. That’s why the calculator asks for length and width rather than depth: a shallow pool and a deep pool with the same footprint lose the same gallons for the same drop. For a kidney or freeform pool, get the area from the pool surface area calculator and choose “I know the area”.

Worked example: a 16 × 32 ft pool (512 sq ft) dropped 1 3/4 inches over 7 days. That’s 0.25 inches a day, so 512 × 0.25 ÷ 12 × 7.4805 = about 80 gallons a day, 560 a week and 2,390 a month. At the EPA WaterSense national average of $16.83 per 1,000 gallons for water plus sewer (EPA), refilling that costs about $40 a month.

The table shows what each daily drop means in gallons per day, with the 30-day total underneath. The 16 × 32 and 20 × 40 ft pools are rectangles. The drops are examples, not targets: use the bucket test to decide whether your own drop is evaporation.

How to measure the drop

A good reading matters more than anything else here, since an eighth of an inch over a big pool is dozens of gallons.

1. Turn off the automatic fill valve if you have one, and don’t add water during the test.
2. Pick a fixed mark: the bottom of the tile line, a grout line, or the skimmer faceplate.
3. Measure from that mark to the water with a ruler, or stick a strip of tape at the waterline.
4. Measure again from the same mark two to seven days later, at about the same time of day.
5. Enter the drop and the number of days. A longer period averages out a windy afternoon or a heavy swim day.

Rain, a big pool party, backwashing the filter or draining for chemistry all change the level. If any happen during the test, start again.

How to do a bucket test

The bucket test compares your pool with a sample of the same water in the same spot. Both lose water to evaporation at about the same rate, so any extra drop in the pool is a sign of a leak.

1. Fill a 5-gallon bucket with pool water to about an inch from the top.
2. Set it on the first or second step so it sits in the pool water. Weigh it down with a brick if it floats.
3. Make the water inside the bucket level with the pool, then mark both: inside the bucket, and on the pool wall or skimmer.
4. Turn off the autofill and run the pump as you normally do. Don’t swim or add water.
5. After two or three days, measure how far each level dropped from its mark.
6. Enter the number of days and the pool drop above, then both drops under “Bucket test”.

If the drops match to within about 1/8 inch, the calculator calls it evaporation, because a ruler can’t reliably show less than that. If the pool dropped more, the difference is what’s leaving another way, and the calculator converts it to gallons a day.

Run it again with the pump off

When the pool loses more than the bucket, repeat the test with the pump off for the same number of days:

Result	What it suggests
Pool loses more only with the pump running	A leak on the pressure side: return lines, the filter, valves, or a heater
Pool loses more with the pump off too	A leak in the shell, liner, a fitting, the skimmer or the light niche
Pool and bucket drop the same both times	Evaporation, not a leak

The bucket test doesn’t find the leak, it only shows one is there. Check the easy places first: the skimmer throat, return fittings, the light, tears in a vinyl liner, and the pump and filter for drips or damp ground. If nothing shows, a leak detection company can pressure-test the lines and listen for the leak.

Evaporation or a leak?

Sign	Points to
Bucket and pool drop the same	Evaporation
Faster loss in hot, dry, windy weather	Evaporation
Pool drops more than the bucket	Leak
Level stops falling at the skimmer, a light or a return	Leak at or near that fitting
Wet or soft ground, or a drip by the equipment	Leak in the plumbing or equipment
Air bubbles from the returns, or the pump losing prime	Possible suction-side leak

How to cut evaporation

If the bucket test says evaporation, you can still lose less:

• Use a cover. A cover over the surface stops most evaporation while it’s on, and a solar cover also holds in heat.
• Turn off water features such as waterfalls, sheer descents and spillovers when nobody is enjoying them. Spraying water evaporates faster.
• Cut the wind. A fence, hedge or screen on the windy side slows evaporation.
• Don’t overheat. Warmer water evaporates faster, so a few degrees lower saves water as well as energy.

The pool fill cost calculator prices any top-up from your water bill, and the guide to how long it takes to fill a pool explains how to time it with a hose.

After topping up

Fresh water dilutes stabilizer and can shift pH and alkalinity, especially after a big top-up. Retest and use the pool chemistry calculator to see what needs adjusting. If you drained on purpose to lower stabilizer, the pool volume calculator gives the total gallons, so you can work out what share of the water you replaced. At the end of the season, the pool closing planner covers how far to lower the water for winter.