One repetition maximum (1-RM) is used by strength coaches to determine the intensity of a resistance training program.
It simply refers to the maximum amount of weight you can lift once, and only once, for a particular exercise.
When prescribing the amount of weight to use for a strength training program, coaches often use percentages of 1-RM.
For example, if your 1-RM for the bench press is 100kg (220lbs), bench pressing 70% of your 1-RM equates to a weight of 70kg (154lbs).
How do you determine your 1-RM?
Essentially, you choose a weight you estimate is slightly less than your 1-RM (i.e. that you will able to lift for 1-2 repetitions). You then add the smallest amount of weight possible and attempt another lift. You continue adding small increments of weight until you cannot perform a single repetition. The previous weight is classed as your 1-RM.
It goes without saying that you must have someone competent spotting each of your lifts. You also have to allow for complete recovery between attempts (5 minutes plus). So as you can imagine this isn't always practical and can be quite time consuming.
An alternative is to estimate your 1_RM by lifting a lighter weight for more repetitions. For example, your 10 repetition maximum (the amount of weight you can lift 10 times and no more) is typically about 75% of your 1-RM. In other words, if you can lift 60kg for 10 repetitions and no more, you should be able to lift about 80kg for a single repetition.
The One Rep Max Calculator below estimates your 1-RM based on a number of repetitions you lift. It assumes you lifted to failure i.e. you were unable to lift another repetition.