Pool Evaporation Chart: Inches Lost to Gallons Table

Every inch of water your pool loses is a fixed number of gallons, and the surface area alone sets it. One square foot of surface, one inch deep, holds about 0.62 gallons (7.48 gallons per cubic foot divided by 12). Multiply that by your surface area and the inches you're down, which gives you the gallons to put back. That's the number you need before you choose between the hose and a truck.

Inches to gallons: the formula

Gallons lost = surface area in square feet x inches lost x 0.62.

Depth doesn't come into it, only the surface area at the water line.

• Rectangle: length x width.
• Round: diameter x diameter x 0.785.
• Oval: length x width x 0.9 for the usual straight-sided oval (0.785 for a true ellipse).
• Freeform: use the longest length, the average width and the oval formula.

For the pool's total volume rather than an inch of it, use the how many gallons is my pool calculator, where depth comes back into play.

Table: gallons per inch for common pool sizes

Figures are rounded.

A 20 x 40 pool a foot down has lost 6,000 gallons, a full load on a 6,000-gallon truck. An 18 ft round a foot down has lost 1,900, which a hose puts back in three to six hours.

Work out your own pool

Work out the surface area with the shape formula above, measure how many inches the water is below its normal mark, and multiply the two by 0.62. Divide the gallons by 5 to 10 gallons a minute for the hose time, and by 6,000 to 8,000 for the share of one tanker load.

How many inches one tanker load adds

Run the formula backwards and you get how far up the wall one load takes you.

On an 18 ft round pool, a 6,000-gallon load fills more than three feet of the wall, and an 8,000-gallon load is more than the whole pool holds at 4 feet of water. That's why a tanker for an above-ground top-off is rarely the right call unless the pool is nearly empty. Deliveries are priced per load, and a partial load often costs about the same as a full one, so the useful question is how close your gallons come to a load. The pool top-off page covers when the hose is the better choice.

Monthly loss in gallons and on your water bill

How fast a pool loses water depends on water and air temperature, humidity and wind, and on whether it's covered. The EPA's WaterSense program estimates that an uncovered 500-square-foot pool loses 12,000 to 31,000 gallons a year to evaporation, depending on climate, and more if heated. That's roughly 38 to 100 inches a year, or about 3 to 8 inches in an average month, with summer months higher. A solid cover cuts evaporation by more than 90 percent while it's on. The pool evaporation rate guide covers what to expect by climate and season.

Once you know the inches per month, the gallons come from the table. A 16 x 32 pool losing 6 inches in a month has lost about 1,900 gallons, and a 20 x 40 losing the same 6 inches has lost 3,000. On the bill, that's gallons ÷ 1,000 x your utility's rate per 1,000 gallons, plus sewer if it's billed on metered water. At typical utility rates, 1,900 gallons is a small line on the water bill, which is why routine evaporation is a hose or auto-fill job.

When the loss is more than evaporation

The inches lost can also point to a leak. The EPA's WaterSense guide treats a loss of more than 2 inches a week, or 3 inches in hot, dry areas, as a likely leak, and a pool that keeps dropping noticeably under a solid cover is losing water some other way. The 24-hour bucket test separates the two. The bucket evaporates at about the same rate as the pool and can't leak, so any extra drop in the pool is a leak. The why is my pool losing water guide walks through the decision tree, the pump-on versus pump-off comparison and the dye test.

Frequently asked questions

How many gallons is an inch of pool water?

Multiply the surface area in square feet by 0.62. A 16 x 32 ft pool holds about 320 gallons per inch, a 24 ft round about 280 and a 20 x 40 about 500.

How do I work out inches to gallons for a round pool?

Square the diameter, multiply by 0.785 for the surface area, then by 0.62 and by the inches lost. For a 24 ft round pool 3 inches down, 24 x 24 x 0.785 = 452 square feet, x 0.62 = 280 gallons per inch, x 3 = about 840 gallons.

Is a top-off of a few inches worth a truck?

On city water, usually not. A few inches on most pools is a few hundred to a couple of thousand gallons, or a few hours of hose, and a truck is priced per load even if you don't use all of it. A truck makes sense when the gallons approach a load, when a low-yield well can't spare the water, or when the tap is restricted.

Do I need to add chemicals after topping off?

Test after anything more than an inch or two, because fresh water dilutes sanitizer and stabilizer and shifts pH, alkalinity and calcium hardness. Run the pump for a few hours first, then adjust alkalinity before pH.

Know the inches? The quote form asks how many inches the water is down, along with your pool size and ZIP. We'll send back one delivered price, or tell you the hose is the better call. Get your delivered price.