How Many Gallons in a 9x18 Pool?
Water in the pool
4,848 gallons
Rectangle, 18 ft by 9 ft, flat 4 ft bottom, which is how a plunge pool this size is normally built.
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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 shapeRectangle
- Length or diameter18 ft
- Width9 ft
- Shallow end depth4 ft
- Deep end depth4 ft
- Fill rate8 gal/min
- Turnover target8 hours
Water in the pool4,848 gallons
| Surface area | 162 sq ftlength by width |
|---|---|
| Average depth | 4 ftmidpoint of shallow and deep |
| Volume | 648 cu ft7.48 gallons in each |
| Water weight | 20.2 tons40,428 lb |
| Fill time | 10.1 hoursat 8 gallons a minute |
| Pump for turnover | 10.1 gal/minwhole volume once in 8 hours |
| Heater to raise 10 degrees | 404,000 BTU8.34 BTU per gallon per degree |
| Liquid chlorine for a 10 ppm shock | 0.39 gal12.5 percent, sold in gallon jugs, so buy 1 |
| Salt to reach 3,200 ppm | 130 lb4 bags at 40 lb, from zero |
The material list this produces
| Water, gallons | 4,848 |
|---|---|
| Liquid chlorine 12.5 percent, gal | 1 |
| Salt, 40 lb bags | 4 |
| Pump, gal/min | 11 |
Deep end compared
The shallow end stays where this page has it and the deep end moves. Average depth is the midpoint of the two, so every foot you add at the deep end adds half a foot of average depth and the water goes up with it.
| Deep end | Water in the pool | Average depth | Pump for turnover |
|---|---|---|---|
| 4 ft | 4,848 gallons | 4 ft | 10.1 gal/min |
| 5 ft | 5,454 gallons | 4.5 ft | 11.4 gal/min |
| 6 ft | 6,060 gallons | 5 ft | 12.6 gal/min |
| 7 ft | 6,666 gallons | 5.5 ft | 13.9 gal/min |
| 8 ft | 7,272 gallons | 6 ft | 15.1 gal/min |
What else matters here
4,848 gallons. A 9 by 18 is a plunge pool or a small spool, and it is built with a flat bottom rather than a slope, because there is not enough length to put a hopper in without losing the usable floor. That is why this page starts at a uniform four feet instead of the three to six a bigger rectangle gets.
The small volume is the reason people choose it and the reason it needs attention. 162 square feet of surface holding under five thousand gallons heats up fast, cools down fast, and swings on chemistry faster than any of it. A dose meant for ten thousand gallons is double strength in here.
Ten hours to fill and a turnover pump of ten gallons a minute. Both of those are small numbers, and they are the practical appeal: a plunge pool can be drained, cleaned and refilled inside a day, which nobody with a 24 foot round can say.
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
What is a plunge pool?
A small rectangle, usually somewhere between 8 by 16 and 10 by 20, built at one uniform depth for cooling off rather than swimming lengths. The flat bottom is the defining part. It keeps the whole floor usable and it is the reason this page assumes four feet everywhere instead of a shallow end and a deep end.
Can I heat a pool this small cheaply?
More cheaply than any other size, yes, because the energy scales with the volume. Ten degrees on 4,848 gallons is around 404,000 BTU, which is a small heater running a short time. The catch is the same small volume in reverse: the surface is large relative to the water, so it gives the heat back overnight unless it is covered.
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.