A study shows the average pool contains about 20 gallons of urine in it. According to a CBS News report, Canadian researchers did the study.
During the study a synthetic sweetener was used as a marker to detect urine. The research was done over three weeks in two different types of pools.
CBS News reports an Olympic-size swimming pool had an estimated urine volume of 75 liters, a 110,000-gallon pool had approximately a urine volume of 30 liters.
For more on the health concerns about swimming in a pool with urine click this link. It was also recommended in the CBS News report to take a shower before going into the pool.