Pool Volume Calculator

Three shapes, one average depth method

Pool volume is surface area times average depth. The shape only changes how you find the surface area. A rectangle is length times width. An oval is 3.1416 times half the length times half the width. A freeform pool, the kind with a curve on one side and a bench on the other, is length times an average width taken at several points across the water. Round above ground pools have their own page on this site, so this one handles the shapes that page does not.

Average depth is the step people skip. Most pools slope from a shallow end to a deep end, so the water is not one depth from wall to wall. Measure both ends at the water line and split the difference: (3.5 + 8) / 2 = 5.75 ft. Measure the water, not the top rail, because a pool that reads 48 inches on the wall may hold only 42 inches of water.

If the pool has a long flat shallow section, a plain average runs a little deep. Measure the depth in the middle of the pool as well and use that reading when it disagrees with the two end readings by more than a few inches.

Worked example: a 16 x 32 ft pool

Here is the full chain for a common in-ground rectangle, 32 ft long, 16 ft wide, running from 3.5 ft at the shallow end to 8 ft at the deep end. The surface is 32 x 16 = 512 sq ft, the average depth is 5.75 ft, and the volume is 512 x 5.75 = 2,944 cubic feet.

Convert with the standard factors: one cubic foot holds 7.480519 US gallons, and one gallon is 3.785412 liters, both from NIST Special Publication 811, appendix B. So 2,944 x 7.480519 = 22,022.6 gallons. In metric that is 22,022.6 x 3.785412 = 83,365 liters, or about 83.4 cubic meters. A gallon of water weighs 8.34 lb, so this pool holds about 183,700 lb of it.

Keep the decimals when you convert, and round only the last step. Rounding the cubic feet to 2,944 before the gallon conversion costs nothing here, but rounding 7.48 instead of 7.480519 loses about 0.3 gallons per 1,000, which is small. The real error comes from the tape measure. Measure the length and width to the nearest few inches, since 3 inches off a 16 ft width is 1.5% of the water, and 1.5% of 22,023 gallons is 330 gallons.

That 330 gallons matters at dosing time. It is more than a third of the water a shock dose treats at the 10,000 gallon mark, so a sloppy measurement shows up as cloudy water that will not clear or a chlorine level that will not hold.

Steps, ledges and freeform pools

Steps are the one place the average depth method misses water. A straight run of steps in the corner of a pool adds a wedge of water above the slope line. Take the width of the step block, the total run from the top step front to the bottom step front, and the total rise from the top step to the pool floor, then halve the result. An 8 ft wide block with 5 ft of run and 2.5 ft of rise adds 8 x 5 x 2.5 / 2 = 50 cubic feet, which is 50 x 7.480519 = 374 gallons.

Freeform pools reward patience with the tape. Take the longest run of water for the length, then pull three or four widths at even intervals and average them. A 20 ft pool measuring 11 ft, 12 ft and 13 ft across averages 12 ft, so the surface is 20 x 12 = 240 sq ft. At a 4.5 ft average depth the volume is 240 x 4.5 = 1,080 cubic feet, which is 1,080 x 7.480519 = 8,079 gallons.

Curved steps, a raised spa that spills into the pool, and a sun shelf all change the shape of the water body rather than the depth profile. Add the spa volume from its own dimensions, and treat a sun shelf as a shallow rectangle with its own depth, then add the pieces together. A single average depth over a pool with a big shelf will overstate the water by 5% to 10%.

From gallons to liters, fill time and dollars

The gallon figure is the pivot point for every other number. Liters and cubic meters come from it, along with the weight of the water, the time a hose needs, and the bill from the water utility. The table below uses the 16 by 32 ft example so you can check the calculator against the article.

Fill time surprises people. A garden hose delivers about 9 to 12 gallons per minute, so a 22,023 gallon pool needs 1,835.2 minutes of hose time, which is 30.6 hours. Fill overnight, then check the level in the morning, because a hose left running is a small flood.

The water bill follows the same gallon number. Divide by 1,000 to get thousand-gallon units, then multiply by the rate printed on your bill. In the example that is 22,022.6 / 1,000 = 22.02 units, and at $10 per 1,000 gallons the fill costs about $220. Some utilities bill sewer charges on water that goes into a pool and some waive them for a documented fill, so call before you start.

Chemical dosing: the formula behind the label

Every pool chemical dose starts from the same fact: 1 ppm is one part in a million by weight. A gallon of water weighs 8.34 lb, so 10,000 gallons is 10,000 x 8.34 = 83,400 lb of water, and 1 ppm of that is 0.0834 lb, which is 1.334 ounces. That gives the constant worth memorizing: 1.334 ounces of pure chlorine per 10,000 gallons raises the level by 1 ppm.

Product strength is the other half of the formula, because the bottle is not pure chlorine. Ounces of product equals gallons divided by 10,000, times the ppm rise you want, times 1.334, divided by the strength as a decimal. For the 22,023 gallon pool and a 2 ppm rise: 22,022.6 / 10,000 = 2.2023, then 2.2023 x 2 = 4.4046, then 4.4046 x 1.334 = 5.876 ounces of pure chlorine, and 5.876 / 0.10 = 58.8 ounces of a 10% product.

Two cautions belong here. Those are weight ounces, and liquid chlorine is sold by fluid ounces, so use the dosage chart on the label rather than converting the number yourself. And the target level is not a guess either: the CDC guidance on home pool water treatment recommends pH 7.0 to 7.8 with free chlorine of at least 1 ppm, or at least 2 ppm when cyanuric acid or a stabilized chlorine product is in the water. Test with a DPD kit, dose to the label, and never mix chemicals together.

Common pool sizes in gallons

These are the shapes and depths that show up most often in backyard pools, sized with the same average depth method. Use the table to sanity check your measurement before you trust the calculator, and remember that a pool with a deep well and a long shallow end will sit near the low end of its range.

The round pool page on this site gives 18,611 gallons for a 24 ft round pool at the same 3.5 to 8 ft profile, which is a useful cross check. A 24 ft circle covers 452 sq ft, a little less than the 512 sq ft of the 16 by 32 rectangle, and the gallon numbers follow the same ratio.

Small differences in area turn into big differences in cost. Moving from a 16 by 32 to a 20 by 40 adds 288 sq ft of surface, 56% more area, and the water grows from about 22,023 to 38,899 gallons. Chemical doses, filter run time, heating cost and the water bill all scale with that number.

How to use this calculator

Pick the shape first, then enter the length and width, or the average width for a freeform pool. Add the shallow and deep end water depths, measured at the water line with a pole or a marked broom handle. If your pool has a straight run of steps inside the water, fill in the step width, run and rise; leave those at zero to skip the step allowance. The last four boxes take your hose or pump flow, your water rate per 1,000 gallons, the chlorine rise you want and the strength printed on the product label.

The results cover gallons, liters, cubic meters, surface area, cubic feet, the weight of the water, the time to fill at your flow, the cost to fill at your rate, and the ounces of product needed for the chlorine rise. The line under the table repeats the shape and average depth the calculator used, so you can spot a typo in a depth before you order chemicals.

Measure twice, and measure to the water. A pool is not brim full, so the skimmer opening sets the real water line, and the water line is what you dose. Every number here is computed in your browser, and nothing you type is uploaded or stored.