Air Conditioning Compressor Tonnage

There is no exact formula for this.

However, it is possible to do some estimating. The LRA (locked rotor amps) will be about 6-8 times the RLA (rated load amps). Together with the voltage, this will tell you the HP of the motor, at least approximately. For HVAC applications, you get about 1 HP/TR (ton of refrigeration), again only approximately.

Such a calculaton is clearly imprecise. If possible, get the compressor model number from the nameplate, and the refrigerant; then consult a manufacturer's representative or a ratings catalog.