The short answer is approximately 30,000 U.S. gallons is the capacity of a 20' x 40' pool.
Determine your pool volume by with these easy to use formulas:
Rectangular Pools: Length x Width x Average Depth x 7.5 = Gallons
Oval Pools: Length x Width x Average Depth x 5.9 = Gallons
Round Pools: Diameter x Diameter x Average Depth x 5.9 = Gallons
Gregg Dietz
<a href="http://www.dietzpools.com/">Dietz Pools &</a><a href="http://www.dietzpools.com/">Pool Supplies</a>
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Most 20 x 40 inground pools, assuming hopper depth of 8 feet, shallow depth of 42 inch, hold roughly 35,000 gallons. With Average depth of 5.5 feet, would hold about 34,760 gallons.