To calculate pool gallons, multiply your pool's length, width, average depth, and a shape factor. Use 7.5 for rectangular pools and 5.9 for round or oval pools. For sloped pools, use average depth instead of the deepest point. Most residential pools land between 5,000 and 30,000 gallons, with 16 by 32 foot rectangulars holding about 19,200 gallons at a 5 foot average depth and 24 foot round above-ground pools holding about 13,600 at a 4 foot depth.
The exact figure matters because chemical dosing, heater sizing, filter selection, and even robotic pool cleaner choice all depend on it. The shape factor and the average depth are where most homeowners get the math wrong, and a guessed gallon count is one of the main reasons pools end up cloudy or chemically over-treated.
How Do You Calculate the Gallons of Water in a Pool
The shape factor in each formula combines pool geometry with a standard conversion. One cubic foot of water equals about 7.48 gallons. Rectangular pools use a flat 7.5 multiplier because their surface area is straightforward length times width. Round and oval pools use 5.9 because the formula folds the area of an ellipse or circle into the conversion.
Three working assumptions sit behind every formula. Measurements are in feet, the result is in US gallons, and any sloped pool uses average depth rather than the deepest point. In-ground pools that slope from a shallow end to a deep end are where the average-depth step matters most, since this is the slope homeowners tend to ignore when dosing chemicals.
Rectangular and Square Pools
The formula for a rectangular pool is length times width times average depth times 7.5. A 14 by 28 foot pool with a 4.5 foot average depth holds 14 times 28 times 4.5 times 7.5, which equals 13,230 gallons.
If the deep end is a separate hopper rather than a gradual slope, treat the two sections as separate rectangles, calculate each volume, and add them together. The full method for finding average depth on a sloped pool is in the next section.
Round Pools
Round pools use diameter times diameter times average depth times 5.9. A 24 foot round pool at 4 foot depth holds 24 times 24 times 4 times 5.9, which equals 13,594 gallons. The diameter is the distance straight across the widest point of the pool.
The 5.9 factor builds in the area of a circle and the cubic foot to gallon conversion. The radius version of the same formula is 3.14 times radius squared times depth times 7.5.
Oval, Kidney, and Free-Form Pools
Oval pools use length times width times average depth times 5.9, where width is the shortest distance across the pool. A 15 by 30 foot oval at a 4 foot depth holds 15 times 30 times 4 times 5.9, which equals 10,620 gallons.
Kidney pools change width along the length, so the standard approximation adds the two widest widths, multiplies by length, by 0.45, by average depth, and by 7.5.
Free-form pools with curves and lobes are easier to handle by surface area. Mentally divide the pool floor into a grid of one-foot squares, count the squares that fall mostly inside the pool, and treat that count as your surface area in square feet. A free-form pool with about 450 square feet of surface and a 5 foot average depth holds 450 times 5 times 7.5, which equals 16,875 gallons.
How Do You Find Your Pool's Average Depth
Average depth is the water depth, not the wall height. For a single-depth pool, that means measuring from the floor to the actual water surface, since most pools are filled below the coping.
For a sloped pool, add the shallow end depth and the deep end depth, then divide by 2. A pool with a 3 foot shallow end and a 7 foot deep end has a 5 foot average depth. If the pool has a flat shallow shelf, a slope, and a flat deep floor, split it into sections, calculate the volume of each using its actual depth, and add them together.
A pool wall can be 5 feet tall while the water sits at 4.5 feet. The 6 inch difference adds up to thousands of gallons in a larger pool. Length and width should also be measured from the inside walls, since outer dimensions include wall thickness and overstate the water-holding area.
Pool Size to Gallons Quick Reference
The table below covers the most common backyard pool sizes at a 5 foot average depth for in-ground pools and a 4 foot depth for above-ground pools.
|
Pool Shape and Size |
Average Depth |
Approximate Gallons |
|
12 ft round (above-ground) |
4 ft |
3,400 |
|
18 ft round (above-ground) |
4 ft |
7,650 |
|
24 ft round (above-ground) |
4 ft |
13,600 |
|
12 by 24 ft rectangular |
5 ft |
10,800 |
|
14 by 28 ft rectangular |
5 ft |
14,700 |
|
16 by 32 ft rectangular |
5 ft |
19,200 |
|
18 by 36 ft rectangular |
5 ft |
24,300 |
|
20 by 40 ft rectangular |
5 ft |
30,000 |
|
15 by 30 ft oval |
4 ft |
10,620 |
Read the table by pool shape and dimension, then adjust for your average depth. A 16 by 32 foot rectangular pool holds 15,400 gallons at 4 feet, 17,300 at 4.5 feet, 19,200 at 5 feet, 21,100 at 5.5 feet, and 23,000 at 6 feet. The same proportional shift applies to the other rectangular sizes.
What Pool Size Holds 10,000 or 20,000 Gallons
A pool that holds 10,000 gallons usually has a surface area between 270 and 360 square feet at typical residential depths. The most common 10,000 gallon pools are 12 by 24 foot rectangular at 4.5 foot average depth, 21 foot round at 4 foot depth, or 15 by 30 foot oval at 4 foot depth. All three land within a few hundred gallons of the 10,000 mark.
A 20,000 gallon pool is closer to 18 by 36 feet at 4 foot average depth, 16 by 32 feet at 5 to 5.5 foot depth, or 14 by 28 feet at 6 foot depth. The 20,000 gallon range is also the size band where pumps, heaters, and robotic pool cleaners often need to step up a tier in capacity.
Common Mistakes When Calculating Pool Volume
Five repeat errors account for almost every off-target gallon estimate.
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Using deep-end depth instead of average depth: Plugging the 8 foot deep end into the formula instead of the 5.5 foot average overstates volume by close to 50 percent.
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Measuring wall height instead of water level: Water typically sits 6 to 12 inches below the coping, so wall height adds gallons that are not actually there.
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Using outside dimensions instead of inside: Wall thickness adds 6 inches or more to length and width on a typical concrete pool, which can mean 1,000 extra gallons in a 16 by 32 foot pool.
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Using radius instead of diameter for round pools: The formula is diameter times diameter times depth times 5.9. Using the radius in the same formula produces a result four times too small.
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Ignoring steps, tanning ledges, and irregular shapes: A tanning ledge of 30 square feet at 12 inches of water removes about 220 gallons. Calculate the main basin first, then adjust for the feature volume.
Why Pool Volume Matters for Pool Maintenance
Pool volume drives almost every maintenance decision a homeowner makes. Chlorine, shock, algaecide, stabilizer, and pH adjusters are all dosed by gallon count, so an inaccurate volume number leaves chlorine ineffective in some zones and overshooting in others. Heater sizing follows the same logic. A 200,000 BTU heater might be right for 15,000 gallons but undersized for 25,000.
Filter and pump turnover rate also depend on volume. The standard target is to circulate the full pool volume every 8 hours, which means a 20,000 gallon pool needs a pump that moves at least 2,500 gallons per hour. A pump sized for an estimated 12,000 gallon pool that actually holds 20,000 will leave the water under-circulated and prone to algae build-up.
How to Choose a Robotic Pool Cleaner by Pool Volume
A robotic pool cleaner has to circulate, traverse, and filter every gallon in the pool, which is why pool volume changes the spec sheet you should be reading. Five spec dimensions track directly back to gallons.
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Runtime: 4 to 6 hours of floor-mode runtime can struggle to finish a single cycle in pools above 25,000 gallons. Bigger pools want 8 to 10 hours.
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Recommended max pool size: Listed in square feet, gallons, or both. Match your gallon count to the upper end of the range, not the average.
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Debris basket capacity: Larger pools generate more leaves, hair, and grit per cycle, so a 4L basket runs longer between empties than a 3L one.
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Filtration density: Finer filtration in microns picks up sand and pollen but clogs faster, which compounds the basket-emptying issue in larger pools.
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Cleaning mode coverage: Floor-only modes save time in smaller pools but leave waterline debris in larger ones, where surface area to volume ratios change.
For larger backyard pools approaching the upper end of typical residential volume, the iGarden Pool Cleaner K Pro 150 runs up to 15 hours in floor mode, lists a recommended maximum pool size of 8 by 15 m (about 47,551 gallons), and uses a 4L debris basket with 180 μm filtration. Pools under 15,000 gallons rarely need that much runtime, which is why the gallon count, more than the spec headline, decides whether the model is the right fit.
iGarden Pool Cleaner K Pro Series
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FAQs
How many gallons are in a 20 by 40 foot pool
A 20 by 40 foot rectangular pool holds 24,000 gallons at a 4 foot average depth, 30,000 gallons at 5 feet, and 36,000 gallons at 6 feet. This size sits at the upper end of typical residential pools, so check the maximum pool volume rating on any robotic cleaner or heater before buying.
Does pool volume change with the water level
Yes. Volume is calculated from the water level, not the wall height. Lowering the water by 6 inches in a 16 by 32 foot pool removes about 1,900 gallons, which is enough to throw off chlorine and stabilizer dosing.
How much does it cost to fill a 10,000 gallon pool
City water typically runs $4 to $7 per 1,000 gallons, which puts a 10,000 gallon fill at $40 to $70 plus any sewer or service fees. Trucked-in water from a bulk delivery service usually costs $200 to $400 for the same volume, depending on travel distance and local availability.
Is one cubic meter of pool water the same as 264 gallons
One cubic meter equals about 264.17 US gallons. Metric pool dimensions can be converted by multiplying length, width, and average depth in meters, then multiplying the result by 264.17 to get gallons.