Measure the suction line temperature at the evaporator outlet or near the compressor inlet
Measure the suction pressure at the same point
Convert the suction pressure to the saturated evaporating temperature using the refrigerant pressure-temperature chart
Subtract the saturated evaporating temperature from the measured suction line temperature
Superheat = Suction line temperature – Saturated evaporating temperature
Example: 45°F suction line temperature – 35°F saturated evaporating temperature = 10°F superheat