A 20-by-40-feet pool with an average depth of 5 feet holds 21,600 gallons of water. The same shape pool with an average depth of 6 feet holds 36,000 gallons.
A 20-by-40-feet pool with a 7-feet average depth holds 41,900 gallons, and a pool with an 8-feet average depth holds 47,900 gallons. To keep a pool filter working properly, it is important to keep the proper amount of water in the pool. The water level should reach the midpoint on the tile backsplash, or it should rise above the skimmer opening. Water often must be added to the pool periodically throughout the season.