Reps in Reserve (RIR) is a method used in weightlifting to measure the intensity of a lift by describing how many more repetitions you could perform before technical failure or an inability to perform the lift with good form. RIR is used to help lifters determine their intensity of effort and to personalize the prescription in each session to meet their demands. It is best to employ this method after you are comfortable with resistance training and have been familiarized with the scale. Leaving some "reps in the tank" by employing the RIR method will help you train close to failure, which is critical for muscular hypertrophy and strength development, without allowing you to go into failure each set. It is acceptable to train to failure every so often to reach a max strength goal, but training to failure often can cause changes in resting hormone concentrations and increases in strength may be compromised due to overtraining. RIR is also useful when you do not know your one-rep max for a given lift. To calculate your RIR weightlifting, all you have to do is subtract the number of reps you actually perform for a given lift from the number of reps you could have possibly performed with proper form. RIR is a skill that takes time to develop, and it can help teach you to gauge different intensity levels.