What size gas pool heater do I need for a 15,000-gallon pool?

A 150,000 BTU heater is about right for a 15,000-gallon pool. That follows the general rule of 10 BTUs per gallon for decent heating in average climates, raising the water a couple of degrees per hour. If you live somewhere cold or want to swim in spring/fall, bump it to 200,000 BTUs. Anything over 250k wastes gas and shortens the heater’s life.