How Many Gallons in an 18x48 Pool?
Water in the pool
7,615 gallons
Round, 18 ft across, filled to a 48 in wall. The 48 in the name is the wall height in inches, not a second width.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Pool shapeRound
- Length or diameter18 ft
- Width18 ft
- Shallow end depth4 ft
- Deep end depth4 ft
- Fill rate8 gal/min
- Turnover target8 hours
Water in the pool7,615 gallons
| Surface area | 254.5 sq ftpi r squared on the diameter |
|---|---|
| Average depth | 4 ftmidpoint of shallow and deep |
| Volume | 1,017.9 cu ft7.48 gallons in each |
| Water weight | 31.8 tons63,503 lb |
| Fill time | 15.9 hoursat 8 gallons a minute |
| Pump for turnover | 15.9 gal/minwhole volume once in 8 hours |
| Heater to raise 10 degrees | 635,000 BTU8.34 BTU per gallon per degree |
| Liquid chlorine for a 10 ppm shock | 0.61 gal12.5 percent, sold in gallon jugs, so buy 1 |
| Salt to reach 3,200 ppm | 204 lb6 bags at 40 lb, from zero |
The material list this produces
| Water, gallons | 7,615 |
|---|---|
| Liquid chlorine 12.5 percent, gal | 1 |
| Salt, 40 lb bags | 6 |
| Pump, gal/min | 16 |
Diameter compared
Same wall height as this page, at every diameter the round pool shelf comes in. Water goes up with the square of the diameter, so the jump from 24 to 27 feet costs more water than the jump from 8 to 15.
| Diameter | Water in the pool | Surface area | Fill time |
|---|---|---|---|
| 8 ft | 1,505 gallons | 50.3 sq ft | 3.1 hours |
| 12 ft | 3,385 gallons | 113.1 sq ft | 7.1 hours |
| 15 ft | 5,288 gallons | 176.7 sq ft | 11 hours |
| 18 ft | 7,615 gallons | 254.5 sq ft | 15.9 hours |
| 21 ft | 10,364 gallons | 346.4 sq ft | 21.6 hours |
| 24 ft | 13,537 gallons | 452.4 sq ft | 28.2 hours |
| 27 ft | 17,133 gallons | 572.6 sq ft | 35.7 hours |
| 30 ft | 21,151 gallons | 706.9 sq ft | 44.1 hours |
| 33 ft | 25,593 gallons | 855.3 sq ft | 53.3 hours |
What else matters here
The first thing to clear up is the name. An 18x48 pool is not eighteen feet by forty eight feet. It is eighteen feet across and forty eight inches tall, and that mixture of units in one product name is why the size gets searched so often. Read it as a diameter and a wall height and the 7,615 gallons stops being surprising.
This is one of the best selling round sizes made and the numbers show why. Nearly 255 square feet of surface, four feet of water, and it still fills in sixteen hours on an ordinary hose. It is the largest pool most people can put up in a weekend without machinery and without a permit for the water supply.
Sixteen hours means overnight, and overnight means nobody is watching it. Set an alarm. An overflowing above ground pool does not hurt the pool, it saturates the compacted pad the pool is standing on, and a soft pad under one side of a 31 ton load is how a wall goes out of round in its first week.
What this calculator assumes
- A cubic foot of water is 7.48 gallons and a gallon weighs 8.34 pounds. Those two numbers do all the work on this page.
- Average depth is taken as the midpoint of the shallow and deep ends. That is exact for a pool with one straight slope between them and slightly generous for a pool with a flat shallow shelf and a hopper, which is the safer way for it to be wrong when you are buying chemicals.
- A round pool uses the diameter and ignores the width. An oval is figured as an ellipse, which is length times width times pi over four, not as a rectangle.
- Turnover is the time the pump takes to move the whole volume once. Eight hours is the common residential target and some health codes for public water are stricter, so check yours before you size a commercial pump off this.
- Liquid chlorine trade percent is defined so that one gallon of it raises ten thousand gallons of water by that many parts per million. One gallon of 12.5 percent raises 10,000 gallons by 12.5 ppm, which is where the dose row comes from rather than a chart.
- Salt is figured from zero to 3,200 ppm, which is the middle of the range most chlorine generators want. A pool that already has salt in it needs a test first, because salt only leaves with the water.
Questions people ask
Does 18x48 mean 18 feet by 48 feet?
No. The first number is the diameter in feet and the second is the wall height in inches. An 18x48 is an 18 foot round pool with a 48 inch wall, which is four feet. An 18x52 is the same pool with a taller wall and about 600 more gallons in it. If a listing really meant an 18 by 48 foot rectangle it would be an olympic training pool, not a backyard one.
Is that the number printed on the box?
It is the same basis. A manufacturer quotes the pool filled to the top of the wall, which is what this page computes. In the yard you fill to the middle of the skimmer mouth, roughly six inches down, so the water actually in it lands about ten percent under the printed figure. Both numbers are useful: buy the cover and the liner off the printed one, and dose chemicals off the real one.
Where does the chlorine dose come from?
Liquid chlorine trade percent is defined so that one gallon of it raises ten thousand gallons of water by that many parts per million. One gallon of 12.5 percent raises 10,000 gallons by 12.5 ppm. The row on this page is the gallons for a 10 ppm shock at that strength, worked from the volume rather than read off a chart, which is why it moves when you change the size.
The pool volume calculator takes any dimensions you like and hands back the same list.