Before diving in to the details, here are some numbers:

53: The number of city pools

900: The number of pool lifeguards

1.7 million: The number of users last summer

16 million: The total number of gallons in all the pools

$0: The entry fee

Yes, these urban oases are free, but before you plunge into that cool, chlorinated bliss, you must show an attendant at the entrance that you have a conventional bathing suit — shorts must have a lining — and a sturdy padlock to secure your belongings in a locker.

Pool hours are generally from 11 a.m. to 7 p.m., with an hour break at 3 p.m. for cleaning, and lunch time for lifeguards and other staff members.

The pools range in size from the 40-by-20-foot pool in Manhattan’s Tompkins Square Park to the mammoth Astoria Pool in Queens overlooking the East River. That pool is 330 feet by 165 feet and was designed to accommodate 3,000 swimmers. It was one of 11 Olympic-size pools the city opened in 1936, all under Robert Moses , the city’s original parks commissioner, with financing from the federal Works Progress Administration.

Now as then, the pools “help provide vacations for some people who don’t have the ability to leave the city in the summertime,” Liam Kavanagh, the first deputy parks commissioner, said.