One-rep Max Calculator

One-Rep Max Calculator

The one-repetition maximum (one-rep max, or 1RM) is a fundamental concept in strength training. It refers to the maximum weight an individual can lift for a single repetition of some exercise while maintaining proper form. Athletes, coaches and other people who practice fitness use the 1RM to assess muscular strength and to design effective training programs.

Understanding your one-rep max can be especially useful for structuring workouts. Training programs sometimes prescribe loads as percentages of your one-rep max. For example, lifting at 60-80% of your 1RM for maximal muscle growth (Krzysztofik et al., 2019). It also provides a clear benchmark for tracking your progress over time.

It should be noted that (estimating) the 1RM is not without limitations. Accuracy varies depending on factors like fatigue, technique, and, to some extent, psychological factors (van Every et al., 2022). Additionally, focusing solely on maximal strength may overlook other important aspects of fitness such as endurance, mobility, and injury prevention. In practice, the one-rep max is best used as a guideline rather than an absolute measure. When applied thoughtfully, it does become a powerful tool for tailoring training intensity and monitoring improvements in strength.

Estimation methods

Usually, rather than testing a true maximum—which can be risky or impractical—many people estimate their one-rep max by lifting a lighter weight multiple times. There are many estimation methods to calculate the one-rep max, such as Adams', Lombardi's, Mayhew's, Brzycki's and Epley's formula (Simonsen et al., 2025). Research into the accuracy and reliability of these formulas shows that, generally, the difference between the formulas is small (Grgic et al., 2020; Wood, Maddalozze & Harter, 2002, Simonsen et al.). This Calkit calculator uses the popular Epley's formula for estimation, which has shown to be accurate, and fairly consistent across exercises (Epley, 1985; Reynold, Gordon & Robergs, 2006; Wood, Maddalozze & Harter).

Epley's formula:

Where W is the lifted weight in any unit of measurement, and r is the number of repetitions performed.

How to estimate

• Pick an exercise you would like to know your maximum weight for.
• Do a small warm-up with a low resistance.
• Have 2-3 minutes rest.
• Now, for the weight to do the estimation with, aim for a weight with which you can do approximately 5 reps. This is usually a weight around 60-80% of your estimated maximum. The formula gives you the best estimation around 5 reps, and declines in accuracy at more than 10 reps (Wood, Maddalozze & Harter, 2002; Reynold, Gordon & Robergs, 2006).
• Perform the exercise with that weight until failure and count your reps. Remember to keep proper form.
• Plug in the weight of the lifted object and your repetitions in the calculator to find out your one-rep max.

Example

Samuel is eager to know his one-rep max for his bench press. He goes to the bench press and performs a small warm-up with a light resistance. He rests for 3 minutes. He estimates that his maximum weight that he can bench is 180 lbs (~82kg). This means that he needs a weight of around 70% * 180 = 126 lbs (~57kg) to do the test with. He is able to perform 6 reps with that weight. Plugging in the values into Epley's formula gives:

1RM = 126 * (1 + 6/30) = 151.2 lbs (68.58 kg).