How Many Gallons in a 20x40 Pool?
Water in the pool
26,930 gallons
Rectangle, 40 ft by 20 ft, shallow end 3 ft, deep end 6 ft, so the average depth is 4.5 ft.
Filling this pool on a garden hose takes 2.3 days of continuous water. Most water utilities sell a bulk fill, and some will not meter a pool fill through the sewer charge if you call first, which is worth more than the hose.
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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 diameter40 ft
- Width20 ft
- Shallow end depth3 ft
- Deep end depth6 ft
- Fill rate8 gal/min
- Turnover target8 hours
Water in the pool26,930 gallons
Filling this pool on a garden hose takes 2.3 days of continuous water. Most water utilities sell a bulk fill, and some will not meter a pool fill through the sewer charge if you call first, which is worth more than the hose.
| Surface area | 800 sq ftlength by width |
|---|---|
| Average depth | 4.5 ftmidpoint of shallow and deep |
| Volume | 3,600 cu ft7.48 gallons in each |
| Water weight | 112.3 tons224,596 lb |
| Fill time | 56.1 hoursat 8 gallons a minute |
| Pump for turnover | 56.1 gal/minwhole volume once in 8 hours |
| Heater to raise 10 degrees | 2,246,000 BTU8.34 BTU per gallon per degree |
| Liquid chlorine for a 10 ppm shock | 2.15 gal12.5 percent, sold in gallon jugs, so buy 3 |
| Salt to reach 3,200 ppm | 720 lb18 bags at 40 lb, from zero |
The material list this produces
| Water, gallons | 26,930 |
|---|---|
| Liquid chlorine 12.5 percent, gal | 3 |
| Salt, 40 lb bags | 18 |
| Pump, gal/min | 57 |
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 | 20,946 gallons | 3.5 ft | 43.6 gal/min |
| 5 ft | 23,938 gallons | 4 ft | 49.9 gal/min |
| 6 ft | 26,930 gallons | 4.5 ft | 56.1 gal/min |
| 7 ft | 29,923 gallons | 5 ft | 62.3 gal/min |
| 8 ft | 32,915 gallons | 5.5 ft | 68.6 gal/min |
What else matters here
26,930 gallons on 800 square feet. A 20 by 40 is a large residential rectangle, long enough to swim proper lengths in, and the size at which a backyard pool starts to behave like a small commercial one in everything except the rules it has to follow.
Fifty six hours on a hose says the fill is a tanker job. Fifty six gallons a minute for an eight hour turnover says the equipment is inground equipment, sized off flow rather than off horsepower. And 720 pounds of salt from zero says the chlorine generator is a real purchase rather than an accessory.
800 square feet of surface is where a cover changes the running cost more than any other decision. Everything that leaves a pool leaves through the surface: the water by evaporation, the heat, and a large part of the chlorine burnt off by sunlight. A pool this size uncovered through a hot month loses enough water to be visible on the bill.
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 pump does a 20x40 need?
26,930 gallons in eight hours is 56 gallons a minute, roughly 3,360 gallons an hour, and at that flow the filter, the plumbing diameter and the number of returns all matter as much as the pump badge. This is the size where a variable speed pump earns its price, because it can run long and slow to hit the turnover instead of short and loud.
Is a 20x40 really bigger than a 33 ft round?
Slightly smaller, in fact, which surprises most people. This pool holds 26,930 gallons and a 33 foot round above ground with a 52 inch wall holds 27,704. The rectangle looks much larger because it is longer and it has a deep end, but a 33 foot circle is 855 square feet of surface against 800 here.
Why is the salt figure from zero?
Salt only leaves a pool with the water, through splash out, backwash and a leak. It does not evaporate. So the bag count here is what an empty pool needs to reach 3,200 ppm, and a pool that has run on salt before needs a test first or you will overshoot it. Adding salt is easy and taking it out means draining water.
The pool volume calculator takes any dimensions you like and hands back the same list.