It is measured in gallons per minute. 14*400= 5600.Multiply this answer with 500 5600*500 = 2,800,000.This is the equator size measured in BTUs per hour. Now divide this answer by 12,000, 2,800,000/12000. This is the correct value of evaporator size in tons. The evaporating coil sizing chart is very complicated to understand.