Pool Volume: How to Work Out What You Actually Have

Every dose you will ever add to this pool is a multiplication with your volume in it. Get the volume wrong by 30% and every chemical you add is wrong by 30%, forever, and you will spend years blaming the products.

This is not a hypothetical. The classic forum story is an owner who added twelve 40 lb bags of salt to what they believed was an 8,403 gallon pool and still read 500 ppm. The reply from Inyo was not about salt. It was to go and recalculate the pool size.

Owners know this is the weak link, which is why there is a thread on Trouble Free Pool titled Calculate Volume of Pool by adding 1 bag of salt?. People are trying to reverse-engineer their own volume from a dose because no consumer tool does it for them.

Get your gallons with the volume calculator →

The formulas

Work in feet, and the answer comes out in US gallons. There are 7.48 gallons in a cubic foot.

Rectangle or square

length × width × average depth × 7.5

Circle or oval

length × width × average depth × 5.9

An oval and a circle use the same multiplier, because the area of an ellipse is π/4 × length × width, and for a circle length and width are both the diameter. If you see a guide quoting 6.7 for ovals, it is carrying an old rounding error.

Freeform and kidney shapes

Do not look for a magic constant. Break the pool into two or three rectangles and part-circles on a sketch, work each one out, and add them. You will be closer than any single-formula approximation.

The average depth trap

This is where most of the error comes in, and almost nowhere explains it properly.

If your floor slopes evenly from end to end, average depth is simply shallow plus deep divided by two. A 3 foot to 8 foot pool with a constant slope averages 5.5 feet.

Most pools are not built like that. The common shape is a flat shallow section, then a slope, then a flat deep section or a hopper. For those, the straight average overstates your volume, sometimes by a lot, because the deep section usually occupies less floor area than the shallow one.

The fix is to weight by area. Work out each section separately:

(shallow area × shallow depth) + (slope area × its own average depth) + (deep area × deep depth), all × 7.48

A worked example. A 16 × 32 pool with 18 feet of 3.5 foot shallow end, 8 feet of slope, and 6 feet of 7 foot deep end:

• Shallow: 16 × 18 × 3.5 = 1,008 cu ft
• Slope: 16 × 8 × 5.25 = 672 cu ft
• Deep: 16 × 6 × 7 = 672 cu ft
• Total 2,352 cu ft × 7.48 = 17,593 gallons

The naive version, using 16 × 32 × 5.25 × 7.48, gives 20,106 gallons. That is a 14% overstatement, which means every dose you calculate from it is 14% too high.

Check it against your fill time

A second, independent method is worth doing once, because it catches measurement mistakes.

Note your water meter reading, fill the pool or top it up a measured amount, and read the meter again. Most US residential meters read in cubic feet or in hundreds of gallons; multiply cubic feet by 7.48. This will not be perfect, but if it disagrees with your calculation by more than about 10%, one of them is wrong and it is usually the calculation.

Charts by pool size

Standard above-ground and prefabricated pools have known volumes, and looking yours up is faster than measuring. These are the pages in this hub:

• Round pool gallon chart: every common size
• Above-ground pool gallons by size
• 18 x 48 pool: gallons, salt and chemical doses
• 18 ft round pool gallons
• 15 x 30 pool gallons
• 14 ft x 48 in pool gallons
• What the average pool actually holds

A note on those last figures. “Average pool size” questions get answered with numbers between 10,000 and 20,000 gallons depending on who is answering, because the average depends entirely on whether above-ground pools are in the sample. Inground pools in the US average higher than the overall figure, and the installed base is roughly 5.4 million inground against 3.4 million above ground, so the mix matters.

The salt trace method

There is one more way to find your volume, and it is the only one that measures the water rather than the hole. Nobody sells it as a product, which is why we wrote it up.

The principle: add a precisely known weight of salt, let it dissolve and distribute fully, then measure the rise in salinity. Because 1 lb of salt in 1,000 gallons raises salinity by about 120 ppm, the rise tells you the volume:

volume in gallons = (pounds added × 120,000) ÷ ppm rise

Add 40 lbs, see a 400 ppm rise, and your pool holds 12,000 gallons. Add 40 lbs, see a 270 ppm rise, and it holds closer to 17,800.

It only works if you are careful. Test before and after with the same method on the same day, give it 24 to 48 hours with the pump running and the floor brushed, do not do it while it is raining, and do not do it if you are already near your target salinity. Used properly it is the most accurate number you will ever have for your own pool, and it costs one bag of salt.

The full procedure, with a worksheet and the error margins, is in the free True Pool Volume Workbook.

Pages in this hub

Beyond the charts above:

• How many gallons is my pool
• How to calculate pool volume, including irregular shapes
• Swimming pool volume explained
• The average depth method for sloped floors
• The salt trace method

Where to go next