This Section Explains The Rep Max Calculator And Its Importance You can use the Rep Max Calculator to accurately find out how much weight you should lift if you know the number of repetitions and your one rep max. The Rep Max calculator can help you lift the right amount of weight which will ensure that your muscles get the right amount of contraction during the workout. The American College of Sports Medicine recommends that you do 8-12 reps if you plan to increase muscular strength and 10-15 reps if you plan to increase muscular endurance.