Goodman air conditioning unit tonnage can be determined by the unit's model number. This number is located on a plate attached to the back of the unit, near the power supply and coolant lines.
The model number includes the nominal cooling capacity in three digits beginning with zero. This three-digit number is the unit's capacity measured in British Thermal Units. For example, "018" indicates 18,000 BTUs. To calculate tonnage, divide the BTUs by 12,000, which is the weight of one refrigeration ton. For example, a unit with 18,000 BTUs has a tonnage of 1.5. The Goodman Manufacturing Company website provides product specifications, including model numbers, for all current air conditioning units and some older models.