The amount of gallons of water in a pool varies greatly depending on the size of the pool. The average in-ground backyard pool holds between 18,000 and 20,000 gallons of water.
An Olympic-sized swimming pool holds around 660,000 gallons of water, whereas a Junior Olympic-sized pool holds approximately 280,000 gallons. It takes over six years for the average American household to use the amount of water necessary to fill an Olympic pool.
A simple formula to use to calculate how many gallons of water a swimming pool holds is to multiply the length by the width by the average depth of the pool.