Central air conditioning systems are rated by tons and British Thermal Units (BTUs). When replacing your old central air system, you will most likely want to replace it with the same sized unit. Most likely, however, the label on your central air system will not list this information. Luckily, with a simple math calculation, you can easily figure out the actual tonnage of your central air conditioning system without needing to hire a professional technician.
Find the label on your air condenser unit and look for the BTU rating and the tonnage rating. The air condenser in a split system air conditioning apparatus is the unit located outside your house. If you do not have an outside unit, the label will be located in the inside unit. If the label on your air conditioning system lists the tons, no more calculation is needed as you will have found the tonnage of your cooling system.
Divide the BTUs on the label by 12,000. You need to use this number as the divisor because there are 12,000 BTUs per ton. The result of this calculation will be the tons of your central air conditioning system. For example, assume that your system is labeled as having 60,000 BTUs. Dividing 60,000 by 12,000 will give you 5 tons. This is the most accurate method of determining the tons on your central air conditioning system when your label does not indicate the tons on it directly.
Find the model number on your label. If your system happens to not have the BTU or the tons information listed, locate the model number on the label. Contact a local Heating, Ventilating and Air Conditioning (HVAC) contractor and provide them with the model number. Most HVAC contractors can look up the tonnage of the system by using the model number.