This calculator will estimate your one rep maximum based on weight and repetitions. It will also estimate 5, 10, and 15 rep maxes based on the one rep max that was computed for hypertrophy-specific training (HST).

Enter number of reps performed:
Enter weight used:
One rep max (formula one)
weight / ( 1.0278 – ( 0.0278 * reps ))
One rep max (formula two)
weight * ( reps * 0.033 + 1 )
5-rep max
10-rep max
15-rep max

What Is a One Rep Max?

One rep max is the amount of weight you can lift for a given exercise one time only. It is a great indicator of strength and, in some cases, fitness level. However performing a one rep max can be dangerous. It is

recommended that you use a safer method, such as finding out how much weight

you can lift for, say, 10 repetitions. This equation will convert that weight

into your one rep max.

Finding Your Other Maximum Weights

The calculator also estimates 5, 10, and 15 rep maxes. These should only serve as starting points. There are numerous factors that contribute to your ability to achieve a given load at a particular repetition range.

Those with local muscular endurance may be able to lift heavy loads for high repetitions, while others may find their 15-rep maxes are much lower than the calculator reports due to the fact that their muscles exhaust much earlier.

Keep a Journal

Always keep a detailed training journal and adjust to what you are actually able to lift during a workout (for this reason, I always consider my first new workout a learning session, where I hone in on the appropriate weight ranges based on how the workout progresses).

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Source: http://www.naturalphysiques.com/18/one-rep-max-calculator