This calculator can be used to determine how much weight you should be lifting to perform the desired 12 - 15 repetitions. 12 - 15 repetitions has been determined as the range to facilitate muscle stimulation and help redefine your body. Finding which weight you should select to complete this range of repetitions can be done by filling in how much weight you used and how many repetitions you performed. This will give you a suggested weight to lift and take the guess work out.