How pool evaporation is calculated
gallons = surface sq ft × inches dropped ÷ 12 × 7.48
A drop of one inch over one square foot is 1/12 of a cubic foot, and a cubic foot holds 7.48 gallons. A 400 sq ft pool that drops a quarter inch a day loses 62.3 gallons a day, about 1,870 gallons a month; an 800 sq ft pool dropping half an inch loses 249 gallons a day. Only the surface counts, so a deep pool and a shallow one of the same size lose the same water.
| Pool | 1/8 in a day | 1/4 in a day | 1/2 in a day | Month at 1/4 in |
|---|---|---|---|---|
| 12x24 (288 sq ft) | 22 | 45 | 90 | 1,346 |
| 15x30 (450 sq ft) | 35 | 70 | 140 | 2,104 |
| 16x32 (512 sq ft) | 40 | 80 | 160 | 2,394 |
| 18x36 (648 sq ft) | 50 | 101 | 202 | 3,030 |
| 20x40 (800 sq ft) | 62 | 125 | 249 | 3,740 |
| 24 ft round (452 sq ft) | 35 | 70 | 141 | 2,113 |
Evaporation or a leak?
Evaporation rises with sun, wind, dry air and warm water, so a pool can lose much more in a hot, windy week than in a mild one. When the drop looks too large, do the bucket test: fill a bucket with pool water, set it on a step, mark the water level inside and outside, and run the pump as usual. After 24 hours both should fall by the same amount; if the pool falls more, the difference is a leak. To refill, the pool fill time calculator gives the hours and cost; a lot of fresh water lowers salt and stabilizer, so check the salt calculator after a big top-up; heated pools also lose heat this way, see the pool heating cost calculator.
Formula: sq ft × inches ÷ 12 = cubic feet; 1 cubic foot = 7.48 gallons. The quarter-inch and 5 USD per 1,000 gallons defaults are example inputs, not averages: measure your drop and use the rate on your bill. Read October 23, 2026.
Frequently asked questions
How do you calculate pool water evaporation?
Multiply the surface area in square feet by the drop in inches, divide by 12 and multiply by 7.48 gallons per cubic foot. A 400 sq ft pool that drops a quarter inch a day loses 62 gallons; an 800 sq ft pool dropping half an inch loses 249 gallons.
How many gallons is a 40x20 pool?
At an average depth of 5.5 feet a 40x20 rectangle holds 40 × 20 × 5.5 × 7.48 = 32,900 gallons; at 4.5 feet, 26,900. Its 800 sq ft surface loses about 125 gallons for every quarter inch the level drops. The pool volume calculator works out the average depth for you.
How much water does a pool lose to evaporation in a month?
Gallons a day × 30. A 400 sq ft pool dropping a quarter inch a day loses about 1,870 gallons a month, which costs about 9 USD at 5 USD per 1,000 gallons. A 16x32 pool (512 sq ft) at the same rate loses about 2,390 gallons a month. Use the rate on your water bill.
Is it normal to have to add water to a pool every week?
Yes, in warm, dry or windy weather pools lose water to evaporation, and splash-out adds to it. If the level falls faster than you expect, do a bucket test: set a bucket of pool water on a step, mark both water levels, and compare after 24 hours. If the pool drops more than the bucket, look for a leak.
How can I reduce pool evaporation?
Cover the pool when it is not in use, which keeps the surface out of the sun and wind. A windbreak such as a fence or hedge helps, and cooler water evaporates less. Waterfalls, spillovers and fountains add surface and spray, so turn them off when no one is enjoying them.