How do I determine the proper water heater size for a new whirlpool bath?

A general rule of thumb is that the water heater should be two-thirds the size of the water capacity of the bath. For example, a 60-gallon bath would require a 40-gallon water heater.  This rough formula does not take into consideration other demands that might be placed on the water heater.  We recommend consulting a plumber for your particular application.