Are 1RM Calculators Accurate? What Studies Show

A one-rep max (1RM) calculator turns a set you have done, such as 185 lb for 8 reps, into an estimate of the most you could lift for a single rep. We checked every study number in the study's abstract or full text and calculated every number in the tables below from the published formulas.

How do 1RM calculators work?

Most calculators use a formula that takes the weight and the number of reps you did to failure, meaning you could not have done another rep. The two you will see most often are:

• Epley: 1RM = weight × (1 + reps ÷ 30)
• Brzycki: 1RM = weight × 36 ÷ (37 − reps)

Epley's formula dates from 1985 and Brzycki's from 1993. Brzycki is also written as weight ÷ (1.0278 − 0.0278 × reps), and Epley's 1/30 is sometimes rounded to 0.033, as in a 2025 study that tested eleven of these equations (Simonsen et al.). The two forms of Brzycki differ by less than 0.1% up to 20 reps.

For example, a 5-rep max (5RM) of 225 lb gives an estimated 1RM of 253 lb with Brzycki and 263 lb with Epley. The studies below tested the formulas with sets taken to failure. A set you stop short of failure has fewer reps than you could do, so its estimate comes out lower.

Brzycki vs Epley: where they agree and where they split

Brzycki and Epley stay within 4% of each other up to 9 reps and match at 10. Above 10 reps, Brzycki gives more: 9% more at 15 reps and 27% more at 20. Both multiply the weight by a factor that grows with the reps (Table 1). At 1 rep, Brzycki returns the weight you lifted, while Epley adds 3.3%. Brzycki's divisor (37 − reps) reaches zero at 37 reps, so it cannot be used for very long sets.

Which 1RM formula is most accurate?

No single formula is the most accurate. The studies we checked found that different equations worked best for the people and exercises they tested:

• In a 2006 study of 70 adults aged 18 to 69 (Reynolds et al.), the researchers' own equations built from a 5-rep max predicted the bench press and leg press 1RM better than equations from 10 or 20 reps. The authors concluded that equations of this kind should use sets of no more than 10 reps.
• In a 2008 study of 103 young women (Mayhew et al.), published equations predicted the bench press 1RM more accurately when the set had fewer than 10 reps. Twelve weeks of training did not make them less accurate.
• In a 2025 study of 22 adults doing ten lower-body exercises (Simonsen et al.), from ankle and knee exercises to the leg press, with sets taken to failure at about 85% of 1RM, the average error was 5.8% with the Lombardi formula, 11.7% with Epley and 16.0% with Brzycki.
• In a 2006 study of 70 trained postmenopausal women aged about 57 (Kemmler et al.), an equation built for that group predicted the 1RM on four machines to within 1.5 to 3.1% on average. Its accuracy did not depend on the exercise or on the number of reps, from 3 to 20.

Two of the three studies that compared rep counts found closer estimates from sets of 10 reps or fewer (Reynolds 2006, Mayhew 2008). The third used an equation built for its own group and stayed accurate from 3 to 20 reps (Kemmler 2006).

Why can a 1RM estimate be off?

A formula assumes that everyone who lifts the same share of their max manages the same number of reps. The research shows they do not:

• The exercise. A 2024 meta-analysis of 269 studies (Nuzzo et al.) found that at 70% of 1RM people did about 14 reps on the bench press but about 19 on the leg press. In the 2006 study of 70 adults (Reynolds et al.), the researchers' own equations from a 5-rep max had a typical error of about 3 kg (7 lb) on the bench press and 16 kg (36 lb) on the leg press, which uses much heavier loads. One formula cannot fit every exercise.
• The number of reps. In two of the three studies that compared rep counts (Reynolds 2006, Mayhew 2008), the error was larger for sets of more than 10 reps.
• Effort. The formulas assume a set to failure. Reps left in reserve lower the estimate.
• The person. At 80% of 1RM, people in the 2024 meta-analysis differed by about 2.5 reps (one standard deviation), and at 60% by about 4.4 reps.
• The test itself. A 1RM you actually lift is not fixed either. In a 2020 systematic review of 32 studies of repeat 1RM tests (Grgic et al.), the median coefficient of variation, a measure of how much results differ between tests, was 4.2%, ranging from 0.5% to 12.1%.

A worked example

Here is what the formulas say for two fictional lifters who do the average reps reported for each load in the 2024 meta-analysis (Nuzzo et al.), rounded to whole reps. Their measured 1RM is 225 lb on the bench press and 400 lb on the leg press.

In this fictional example, built from the average reps in the 2024 meta-analysis, the bench press estimates from 4 and 9 reps land within 4% of the measured 225 lb, and Epley from 14 reps lands close too (103%). On the leg press, both formulas overshoot by about 16 to 17% at 9 reps, because people manage more reps there at the same share of their max. These are averages: real lifters differ, and the spread is larger at lighter loads.

How SetGrip estimates your 1RM

SetGrip estimates a 1RM with Brzycki for sets of 1 to 5 reps and Epley for sets of 6 to 10 reps, and shows no estimate above 10 reps. This applies to exercises you log with weight and reps. In the current version, 1.1.6:

• A single rep counts as exactly the weight you lifted.
• If an exercise has no sets of 10 reps or fewer, its statistics show total reps instead of an estimate.
• Warm-up sets and sets you did not complete are left out.
• Estimated 1RM is one of eight personal records the app detects and celebrates, and records are free.
• The Statistics tab shows your exercises with their latest estimate, a small chart and a trend arrow. The detailed charts for each exercise are part of SetGrip Premium.
• During a workout, an arrow next to a set logged with weight and reps compares it with the previous result shown for that set. It uses the same formulas, also above 10 reps, and shows only a direction, not a number.

The help article What set types, rest timers and records does SetGrip track? lists every record and set type. SetGrip is free to download on the App Store.

This article explains what studies found about 1RM estimates. It is not a training plan or medical advice.

Sources

1. Reynolds JM, Gordon TJ, Robergs RA. Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry. Journal of Strength and Conditioning Research. 2006;20(3):584–592.
2. Mayhew JL, Johnson BD, Lamonte MJ, Lauber D, Kemmler W. Accuracy of prediction equations for determining one repetition maximum bench press in women before and after resistance training. Journal of Strength and Conditioning Research. 2008;22(5):1570–1577.
3. Kemmler WK, Lauber D, Wassermann A, Mayhew JL. Predicting maximal strength in trained postmenopausal woman. Journal of Strength and Conditioning Research. 2006;20(4):838–842.
4. Simonsen MB, Jolas E, Pedersen SR, et al. Analyzing one-repetition-max predictions: load-velocity relationship vs. repetition to failure equation in ten lower extremity exercises. Sport Sciences for Health. 2025;21(1):343–353.
5. Nuzzo JL, Pinto MD, Nosaka K, Steele J. Maximal Number of Repetitions at Percentages of the One Repetition Maximum: A Meta-Regression and Moderator Analysis of Sex, Age, Training Status, and Exercise. Sports Medicine. 2024;54(2):303–321.
6. Grgic J, Lazinica B, Schoenfeld BJ, Pedisic Z. Test–Retest Reliability of the One-Repetition Maximum (1RM) Strength Assessment: a Systematic Review. Sports Medicine – Open. 2020;6(1):31.
7. Brzycki M. Strength Testing—Predicting a One-Rep Max from Reps-to-Fatigue. Journal of Physical Education, Recreation & Dance. 1993;64(1):88–90.
8. Epley B. Poundage chart. In: Boyd Epley Workout. Lincoln, NE: Body Enterprises; 1985:86.
9. MacDougall JD, Tuxen D, Sale DG, Moroz JR, Sutton JR. Arterial blood pressure response to heavy resistance exercise. Journal of Applied Physiology. 1985;58(3):785–790.
10. Riebe D, Franklin BA, Thompson PD, et al. Updating ACSM's Recommendations for Exercise Preparticipation Health Screening. Medicine & Science in Sports & Exercise. 2015;47(11):2473–2479.
11. Centers for Disease Control and Prevention. About Heart Attack Symptoms, Risk, and Recovery. 2024.

Sources checked on October 20, 2026.