• Aincrease in temperature of the cold fluid / decrease in temperature of the hot fluid
• Bactual exit temperature attained by the cold fluid / maximum exit temperature attainable by the cold fluid
• Cactual exit temperature attained by the hot fluid / minimum exit temperature attainable by the hot fluid
• actual heat transfer rate / maximum possible heat transfer rate from hot fluid to cold fluid

By definition, effectiveness ε=Qactual/Qmax, where Qmax is the heat duty that would be achieved by an infinitely long counter-current exchanger (limited by the fluid with the smaller heat capacity rate).