One Rep Max Calculator

Gauging Your Strength: A Guide to the One-Rep Max (1RM) Calculator

In the world of strength training and weightlifting, the one-repetition maximum, or 1RM, is the gold standard for measuring an individual's maximum strength. It is defined as the maximum amount of weight that a person can lift for a single repetition of a given exercise with proper form. Knowing your 1RM is not about ego; it's a crucial piece of data for designing an effective strength training program. Many popular and proven programs, such as 5/3/1 or the Juggernaut Method, base their prescribed lifting weights on a percentage of your 1RM. This ensures that you are consistently training with the right intensity to stimulate muscle growth and strength gains (a principle known as progressive overload) while also managing fatigue and reducing the risk of injury.

However, performing a true one-rep max test—actually attempting to lift the absolute heaviest weight you possibly can—can be physically demanding, requires a spotter, and carries a higher risk of injury, especially for beginners. For this reason, it's often safer and more practical to *estimate* your 1RM. This calculator is designed to do exactly that. By taking a weight you can lift for multiple repetitions (ideally in the 2-10 rep range) and plugging it into a well-established predictive formula, you can get a reliable estimate of your 1RM without having to perform a risky maximal attempt. This tool is invaluable for lifters of all levels to set up their training programs, track their strength progress over time, and make smarter decisions in the gym.

How 1RM is Estimated: The Brzycki Formula

There are several different formulas used to estimate 1RM, but one of the most common and respected is the Brzycki formula, developed by Matt Brzycki. This calculator uses that formula:

1RM = Weight Lifted / (1.0278 - (0.0278 × Number of Reps))

Alternatively, it can be written as:

1RM = Weight Lifted × (36 / (37 - Number of Reps))

This formula is generally considered to be most accurate for repetition ranges between 2 and 10. Estimations based on a very high number of reps (e.g., more than 15) become less reliable.

Using Your 1RM for Percentage-Based Training

Once you have your estimated 1RM, you can use it to structure your workouts. Percentage-based training involves working with specific percentages of your 1RM for a set number of reps. This provides a clear structure for progression.

For example, if your 1RM for the bench press is 100 kg, a workout focused on hypertrophy might call for you to do 3 sets of 8-12 reps with 70-80 kg.