One-rep max calculator

Do a set close to failure, add the reps you had left to the reps you did, and put the weight and that total into a prediction equation. With Epley’s, weight × (1 + reps / 30), 200 lb for 8 reps with 2 in reserve counts as 10 reps to failure and gives about 267 lb (121 kg). Estimates are closest from sets of about 10 reps or fewer, they drift as the reps climb, and the same set can mean a different max on another exercise or for another lifter. Read the number as a good guess, often within about 10 % of what you would lift, and test a true max only when you need the exact figure.

How the calculator works

The classic one-rep max formulas were all built from sets taken to failure. You lift a weight as many times as you can, and the count tells you how far below your max that weight sits. Most training sets stop short of failure, so the calculator also asks how many reps you had left (reps in reserve, RIR) and adds them: 8 reps with 2 in reserve counts as a 10-rep max. If you’re new to judging reps left, RIR and RPE in lifting explains the scale.

The main number is Epley’s estimate. It is one of the most common formulas, it gives the same answer as Brzycki’s at 10 reps, and it is the one Workout Wizard uses in the app. No formula has won every comparison, so the calculator also shows the range across all seven. The range tells you how much the choice of formula matters for your set. For a 200 lb (91 kg) set:

Reps to failure	Epley	Lowest to highest of the seven	Spread
5	233 lb	225–238 lb	6 %
10	267 lb	250–269 lb	7 %
15	300 lb	262–327 lb	22 %
20	333 lb	270–424 lb	46 %

Up to about 10 reps the seven stay within a few percent of each other. Past 12 they split: Brzycki’s and Lander’s climb steeply, while Lombardi’s and O’Connor’s flatten. A wide range is a sign that the set was too long to say much about your max.

The formulas

All seven are prediction equations. Each fits a curve to how many reps people manage at loads below their max and reads it backwards. In the table, w is the weight and r the reps to failure.

Formula	One-rep max	Original source
Epley	w × (1 + r / 30)	Epley’s poundage chart, 1985 (a coaching chart)
Brzycki	w × 36 / (37 − r)	Brzycki 1993, Journal of Physical Education, Recreation & Dance
Lander	100w / (101.3 − 2.67123r)	Lander, NSCA Journal, 1984–85
Lombardi	w × r^0.10	Lombardi 1989, Beginning Weight Training (textbook)
Mayhew	100w / (52.2 + 41.9e^(−0.055r))	Mayhew et al. 1992, college men and women on the bench press
O’Connor (often spelled O’Conner)	w × (1 + 0.025r)	O’Connor, Simmons and O’Shea 1989, Weight Training Today (textbook)
Wathen (often spelled Wathan)	100w / (48.8 + 53.8e^(−0.075r))	Wathen 1994, a load chart in Essentials of Strength Training and Conditioning

Brzycki’s is often written w / (1.0278 − 0.0278r), and Epley’s w × (1 + 0.0333r). They are the same equations. Most of these come from coaching charts and textbooks. A 2008 comparison of 14 such equations noted that most give no sign of the population they came from or how they were derived. Mayhew’s was fit to data from college students, and Wathen’s equation was later fit to the chart he published (Mayhew et al. 2008). We checked every equation above against that paper’s table, except Epley’s, which it didn’t include under that name.

How accurate is an estimated 1RM?

An estimate from a set of 10 reps or fewer is close on average, but it can miss by several percent for any one lifter, and by more on some lifts. The studies below show where the error comes from.

Different lifts, different errors

In the classic test of seven equations, 67 untrained college students (40 men, 27 women) practiced the bench press, squat and deadlift for four sessions before testing their 1RM and their reps to fatigue. Every equation’s predictions correlated strongly with the real max (r > 0.95), but a high correlation can hide a consistent error. On the bench press, the average error was significantly different from zero for five of the seven equations, and on the squat for six. On the deadlift, every equation underestimated the max (LeSuer et al. 1997).

More reps, more error

In 70 adults aged 18 to 69 (34 men, 36 women), a 1RM was predicted from a 5, 10 and 20-rep max on the bench press and the leg press. The 5RM predicted best, and accuracy fell at 10 and 20 reps. Even the best equation, from the 5RM, had a standard error of about 3 kg (7 lb) on the bench press and 16 kg (36 lb) on the leg press. The authors advised using no more than 10 reps (Reynolds et al. 2006).

In 103 college women who did sets to failure at 60–90 % of their 1RM, before and after 12 weeks of training, Brzycki’s and Lander’s equations overestimated by 23–27 % on average across the full rep range (up to about 30). With 10 reps or fewer, most of the 14 equations tested were within about 4 % on average. An equation identical to Epley’s was 0.5 % off on average, but individual errors had a standard deviation of about 10 % (Mayhew et al. 2008).

Same percentage, different reps

A meta-regression of 952 tests to failure in 7,289 people found more reps at a given percentage on the leg press than on the bench press: about 13 against 9 at 80 % of 1RM. Sex, age and training status made little clear difference. People varied widely, though: at 80 % of 1RM, the standard deviation between individuals was 2.5 reps (Nuzzo et al. 2024).

Our arithmetic from those numbers: Epley’s formula assumes 7.5 reps at 80 % for every lift. A typical leg press set at 80 % (13 reps) reads about 15 % high, and a typical bench press set (9 reps) about 3–4 % high. And 2.5 reps either way moves Epley’s estimate by about 7 %.

Adding reps in reserve

We found no study that tested a formula fed with reps plus RIR against a measured max. What is known is that lifters guess about one rep too few left, on average (Halperin et al. 2022). Around 10 reps to failure, each rep of misjudged reserve moves Epley’s estimate by about 2.5 %. If you underguess like the average lifter, the calculator’s estimate leans a little low.

Should you test a true 1RM?

If you train for muscle, usually not. This is our reading, which no study has tested: an estimate is enough to choose a starting weight, and the trend of your estimates shows whether a lift is going up. Test a true max when the exact number matters: a powerlifting meet, a program written in percentages of a tested max, or a lift where formulas tend to miss, such as the deadlift.

A tested 1RM is a repeatable measure. Across 32 studies with 1,595 people, test and retest agreed closely (median ICC 0.97, typical variation 4.2 %), with or without training experience (Grgic et al. 2020). The authors of the 2008 study found little evidence that a properly run 1RM test is more dangerous than other lifting, and noted that long sets to failure carry their own risk as form breaks down (Mayhew et al. 2008). If you test, the usual precautions apply:

• Warm up with a few lighter sets, then work up in singles
• Rest several minutes between heavy attempts (the 2008 study gave at least 5)
• Squat and bench press in a rack with safety pins set a little below your lowest point, or with a spotter
• Count only full-range reps with good form, and stop when form breaks down
• Aim to reach your max within 3–5 attempts after the warm-up, as that study usually did

Using percentages of your 1RM

The table under the calculator turns your estimate into weights from 100 % down to 50 %. For muscle growth, the percentage matters less than the effort. In a meta-analysis of 21 studies, light loads (60 % of 1RM or less) built about as much muscle as heavy ones when sets went to failure, though heavy loads built more 1RM strength (Schoenfeld et al. 2017).

What a percentage means in reps depends on the lift and on you. In the meta-regression above, the bench press averaged about 9 reps to failure at 80 % and 14 at 70 %, and the leg press about 13 and 19 (Nuzzo et al. 2024). Use a percentage for your first set, then let reps in reserve take over. Our reading: for sets of 8–12 reps with 1–3 in reserve, about two-thirds to three-quarters of your estimate is a reasonable first guess. If you get 14 reps with a weight meant for 10, go up next time.

An RIR target also adjusts itself as you get stronger. A fixed percentage of an old max gets easier each week, while “2 reps left” keeps the set about as hard (see progressive overload, week by week).

How Workout Wizard does it

Each exercise in Workout Wizard has a screen with its history. It shows each session’s estimated one-rep max, worked out from that session’s best set with the Epley formula, weight × (1 + reps / 30), and plots the sessions on a chart so you can see whether the lift is going up. Deload weeks are left out of the trend.

How far can you trust your estimated one-rep max?

Within about 10 % when it comes from a hard set of 10 reps or fewer, and less on long sets and on lifts like the deadlift and leg press. Use it to pick a starting weight and to follow a lift’s trend, let reps in reserve set the effort, and test a true max only when you need the exact number.

References

• Brzycki M. 1993. Strength testing: predicting a one-rep max from reps-to-fatigue. J Phys Educ Recreat Dance 64(1):88–90. doi:10.1080/07303084.1993.10606684. Not in PubMed.
• Epley B. 1985. Poundage chart. In: Boyd Epley Workout. Lincoln, NE: Body Enterprises. No DOI.
• Grgic J, Lazinica B, Schoenfeld BJ, Pedisic Z. 2020. Test–retest reliability of the one-repetition maximum (1RM) strength assessment: a systematic review. Sports Med Open 6:31. doi:10.1186/s40798-020-00260-z. PMID 32681399.
• Halperin I et al. 2022. Accuracy in predicting repetitions to task failure in resistance exercise: a scoping review and exploratory meta-analysis. Sports Med 52(2):377–390. doi:10.1007/s40279-021-01559-x. PMID 34542869.
• Lander J. 1984. Maximum based on reps. NSCA J 6(6):60–61. doi:10.1519/0744-0049(1984)006<0060:MBOR>2.3.CO;2. Not in PubMed.
• LeSuer DA, McCormick JH, Mayhew JL, Wasserstein RL, Arnold MD. 1997. The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. J Strength Cond Res 11(4):211–213. doi:10.1519/00124278-199711000-00001. Not in PubMed.
• Lombardi VP. 1989. Beginning Weight Training: The Safe and Effective Way. Dubuque, IA: W.C. Brown. No DOI.
• Mayhew JL, Ball TE, Arnold MD, Bowen JC. 1992. Relative muscular endurance performance as a predictor of bench press strength in college men and women. J Appl Sport Sci Res 6(4):200–206. doi:10.1519/00124278-199211000-00002. Not in PubMed.
• Mayhew JL, Johnson BD, LaMonte MJ, Lauber D, Kemmler W. 2008. Accuracy of prediction equations for determining one repetition maximum bench press in women before and after resistance training. J Strength Cond Res 22(5):1570–1577. doi:10.1519/JSC.0b013e31817b02ad. PMID 18714230.
• Nuzzo JL et al. 2024. 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 Med 54(2):303–321. doi:10.1007/s40279-023-01937-7. PMID 37792272.
• O’Connor B, Simmons J, O’Shea P. 1989. Weight Training Today. St Paul, MN: West Publishing. No DOI.
• Reynolds JM, Gordon TJ, Robergs RA. 2006. Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry. J Strength Cond Res 20(3):584–592. doi:10.1519/R-15304.1. PMID 16937972.
• Schoenfeld BJ, Grgic J, Ogborn D, Krieger JW. 2017. Strength and hypertrophy adaptations between low- vs. high-load resistance training: a systematic review and meta-analysis. J Strength Cond Res 31(12):3508–3523. doi:10.1519/JSC.0000000000002200. PMID 28834797.
• Wathen D. 1994. Load assignment. In: Baechle TR (ed). Essentials of Strength Training and Conditioning. Champaign, IL: Human Kinetics, 435–439. No DOI.