One Rep Max Calculator

A one-repetition maximum, or 1RM, is the heaviest load a person can lift for a single complete repetition of an exercise with correct technique. Testing a true 1RM directly is time-consuming and carries injury risk, so most training programs estimate it from a lighter set taken close to failure using a prediction formula. This calculator uses two of the most widely used equations, the Epley formula and the Brzycki formula, to convert a submaximal set (a given weight and rep count) into an estimated 1RM.

The formula

Epley: 1RM = weight x (1 + reps / 30). Brzycki: 1RM = weight / (1.0278 - 0.0278 x reps)

• weight = the load lifted on the working set, in kilograms or pounds (the output uses the same unit as the input)
• reps = the number of complete, controlled repetitions performed with that load before technical failure
• 1RM = the estimated single-repetition maximum for that lift, in the same unit as weight

Epley formula: Boyd Epley, published as a poundage chart used in US collegiate strength and conditioning programs, 1985. Brzycki formula: Matt Brzycki, "Strength Testing - Predicting a One-Rep Max from Reps-to-Fatigue", Journal of Physical Education, Recreation & Dance, 1993.

How the calculation works

Both formulas start from the same idea: the number of reps completed with a given weight is a rough proxy for how close that weight is to a true maximum. The Epley formula assumes each additional rep represents about a fixed fraction of the 1RM and scales the weight upward using a simple multiplier based on rep count divided by 30. The Brzycki formula takes a slightly different mathematical path, dividing the weight by a term that shrinks as rep count rises, and was derived from regression analysis of lifters performing sets to fatigue.

In practice the two formulas agree closely in the 1 to 6 rep range and start to diverge more noticeably as rep count increases, because they use different underlying curves to model fatigue. Neither formula "knows" anything about the specific lifter, their training history, or the exercise being performed; they are general statistical approximations, not physiological measurements.

To use the calculator, enter the weight lifted and the number of strict reps completed on that set, ideally a set taken to or very close to muscular failure with technique that did not break down partway through. A set stopped early because of caution rather than fatigue will underestimate the true 1RM.

How to read your result

The table below shows the approximate percentage of 1RM that a given rep count at failure typically represents, averaged across the Epley and Brzycki models. These percentages are commonly used in strength programming to select training loads for a target rep range without needing to test a true maximum.

Why accuracy drops at higher rep counts

Every 1RM prediction formula loses accuracy as the number of reps used in the estimate goes up, and this is well documented for both the Epley and Brzycki equations. There are several reasons for this. As a set gets longer, technical form is more likely to degrade before true muscular failure is reached, so the "reps to failure" number can reflect a breakdown in movement pattern rather than pure strength. Longer sets are also more influenced by local muscular endurance and metabolic fatigue, which are different physical qualities from the neuromuscular factors that determine a single maximal lift. Two lifters with an identical true 1RM can perform a very different number of reps at 70% of that 1RM depending on their fiber-type distribution and training background, which introduces error that the formulas cannot correct for.

For this reason, most strength coaches and exercise scientists recommend using sets of roughly 1 to 10 reps to estimate 1RM, and treating estimates built from sets of more than about 10 reps as increasingly approximate. Above roughly 10 reps, the gap between the Epley and Brzycki predictions widens and both tend to diverge further from directly tested maximums. An estimate from a 3-rep set is generally far more trustworthy than one from a 20-rep set.

These formulas were also derived primarily from barbell exercises such as the squat, bench press and deadlift performed by trained lifters. They transfer reasonably well to other free-weight and machine exercises but are less reliable for movements with a strong technical or balance component, such as single-leg or Olympic-style lifts, where fatigue changes the movement pattern rather than simply the force output.

Limitations

• Estimates lose accuracy as rep count rises; sets taken past about 10 reps produce a less reliable prediction than sets of 1 to 6 reps.
• The formulas were developed mainly from barbell lifts (squat, bench press, deadlift) in trained adults and transfer less well to technical, balance-dependent or single-limb movements.
• Results assume the set was taken to genuine muscular failure with unbroken technique; a set stopped early for caution, or one where form degraded before failure, will distort the estimate.
• The calculation does not account for training experience, muscle fiber type, warm-up fatigue, or the specific exercise variation, all of which affect how many reps a person can perform at a given percentage of their true maximum.
• This is not a substitute for a supervised, properly progressed true 1RM test where that is genuinely needed, and it is not a screening or diagnostic tool for any medical condition.
• Attempting near-maximal or maximal single lifts carries injury risk and is not appropriate for beginners, untrained individuals, or people with uncontrolled cardiovascular conditions, recent injury, or joint problems without qualified supervision.

Frequently asked questions

Which formula is more accurate, Epley or Brzycki?

Neither formula is consistently more accurate for every lifter or every rep range. They tend to agree closely for sets of about 1 to 6 reps and diverge more as reps increase, usually with Brzycki predicting a slightly lower 1RM than Epley at higher rep counts. Both are population-average estimates, so individual results can differ from either prediction.

How many reps should I use to get the most accurate estimate?

A set of roughly 3 to 6 reps taken close to failure with solid technique generally gives the most reliable estimate. Sets of more than about 10 reps introduce noticeably more error because fatigue and muscular endurance start to influence the result more than raw strength does.

Can I use this calculator for any exercise?

It works best for compound barbell and machine exercises like the squat, bench press, deadlift and overhead press. It is less reliable for exercises with a strong balance, coordination or technical-skill component, since fatigue can change the movement pattern before it changes the force a person can produce.

Is an estimated 1RM safe to train from?

Using an estimated 1RM to set training percentages is generally safer than repeatedly testing a true maximal single lift, because it avoids near-maximal loading. It is still general information rather than individual advice, and anyone new to a lift, returning from injury, or managing a health condition should get guidance from a qualified coach or clinician before working near estimated maximal loads.

Should beginners test a true 1RM instead of estimating it?

Most strength and conditioning guidance suggests beginners build technique and a base of training experience before attempting true maximal singles. An estimated 1RM from a moderate-rep set is usually a more practical and lower-risk way to set training loads while technique and confidence with the movement are still developing.

References

1. National Institute on Aging (NIH). Four Types of Exercise Can Improve Your Health and Physical Ability.
2. Centers for Disease Control and Prevention. Physical Activity Basics.
3. MedlinePlus (National Library of Medicine, NIH). Exercise and Physical Fitness.
4. American College of Sports Medicine. ACSM home.