O'Conner Formula: 1RM Calculator and Complete Guide
O'Conner's formula calculates 1RM as weight × (1 + reps × 0.025), a conservative linear model that assumes each repetition represents 2.5% of one-rep max. It is well-suited to beginners and safety-focused programming.
Strength Training Researcher
Published · Last reviewed · 6 min read
O'Conner formula calculator
Single-formula calculator. For an averaged estimate across all 7 formulas, use the main calculator.
History and origin
Bob O'Conner, James Simmons, and Patrick O'Shea published their 1RM prediction formula in the 1989 textbook "Weight Training Today." The equation — 1RM = weight × (1 + reps × 0.025) — is a deliberately conservative linear model assuming each rep represents 2.5% of one-rep max (compared to Epley's 3.33%). O'Conner et al. designed the formula for general fitness contexts where users would use the resulting 1RM for programming percentages; a conservative estimate keeps training loads safer for inexperienced lifters who might overshoot with more aggressive predictions.
O'Conner, B., Simmons, J., & O'Shea, P. (1989). Weight Training Today. St. Paul, MN: West Publishing.
When to use the O'Conner formula
Best for
Beginners, conservative programming, deloads
Limitation
May underestimate for elite athletes with high relative strength
Compared to other formulas
Compared to Epley, O'Conner produces estimates that are systematically 3-5% lower across all rep ranges. The two formulas use the same linear structure but different per-rep coefficients (Epley = 1/30 ≈ 3.33%, O'Conner = 0.025 = 2.5%). Compared to Lander, O'Conner is similarly conservative at low reps but doesn't drop off as steeply at high reps. Use O'Conner when programming for beginners or when erring on the side of safety matters.
How to use the O'Conner formula (step by step)
- Perform a heavy submaximal set. Perform a clean set of 2-10 reps to near-failure (RPE 8-9) on the lift you want to test. Record the weight and the rep count.
- Apply the O'Conner equation.
1RM = weight × (1 + reps × 0.025) - Read the estimated 1RM. The output is your estimated 1RM. For programming, multiply by 0.9 to derive your Training Max — this absorbs daily strength variability and reduces overshoot risk.
- Cross-check against other formulas. Run the same numbers through every other formula and compare. The main calculator does this automatically using a trimmed mean.
Worked examples — O'Conner formula
Computed with the O'Conner formula. Compare these single-formula estimates against the trimmed-mean average from our main calculator.
| Set | Weight × Reps | O'Conner 1RM (lb) |
|---|---|---|
| Example 1 | 135 lb × 12 | 176 lb |
| Example 2 | 185 lb × 8 | 222 lb |
| Example 3 | 225 lb × 5 | 253 lb |
| Example 4 | 315 lb × 3 | 339 lb |
| Example 5 | 405 lb × 2 | 425 lb |
How O'Conner compares to the other six formulas
The same submaximal set — 225 lb × 5 reps — produces different 1RM estimates depending on which formula you use. Here are all seven, side by side:
| Formula | Estimated 1RM | vs O'Conner |
|---|---|---|
| Epley | 263 lb | +10 lb |
| Brzycki | 253 lb | 0 lb |
| Lombardi | 264 lb | +11 lb |
| O'Connerthis page | 253 lb | — |
| Mayhew | 268 lb | +15 lb |
| Wathan | 262 lb | +9 lb |
| Lander | 256 lb | +3 lb |
See the full formula comparison page for accuracy data and decision-making guidance.
O'Conner reverse lookup: target 1RMs at common rep ranges
Working backward from a target 1RM, here's the working weight you would need to lift for various rep counts to predict that max via the O'Conner formula. Use this to set training targets for blocks aimed at hitting a specific number.
| Target 1RM | 3 reps | 5 reps | 8 reps | 10 reps |
|---|---|---|---|---|
| 200 lb | 186 lb | 178 lb | 167 lb | 160 lb |
| 300 lb | 279 lb | 267 lb | 250 lb | 240 lb |
| 400 lb | 372 lb | 356 lb | 333 lb | 320 lb |
| 500 lb | 465 lb | 444 lb | 417 lb | 400 lb |
Round to the nearest 5 lb when programming. For competition-style peaking, see the 5/3/1 calculator or RPE programming.
Validation research
The O'Conner formula has been evaluated in multiple peer-reviewed studies of 1RM prediction accuracy. Selected findings:
Mayhew, Wathan, and Brzycki produced the smallest error across bench, squat, and deadlift. Most formulas underestimated 1RM as reps increased.
LeSuer, D.A., McCormick, J.H., Mayhew, J.L., Wasserstein, R.L., & Arnold, M.D. (1997). The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. Journal of Strength and Conditioning Research, 11(4), 211-213.
Standard error of estimate ranged from 5.6 to 7.9 kg across formulas; high reps (>10) reduced accuracy meaningfully.
Reynolds, J.M., Gordon, T.J., & Robergs, R.A. (2006). Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry. Journal of Strength and Conditioning Research, 20(3), 584-592.
In older adults, most formulas overestimated 1RM by 5-15%; conservative choices (Lander, O'Conner) outperformed.
Wood, T.M., Maddalozzo, G.F., & Harter, R.A. (2002). Accuracy of seven equations for predicting 1-RM performance of apparently healthy, sedentary older adults. Measurement in Physical Education and Exercise Science, 6(2), 67-94.
O'Conner Formula FAQ
Further reading & authoritative sources
These external sources informed the content on this page. Authoritative references are a hallmark of trustworthy strength training information; we link directly so you can verify and explore further.
- Wikipedia: One-repetition maximum
General reference for the 1RM concept and major prediction formulas.
- Essentials of Strength Training and Conditioning (NSCA, Human Kinetics)
The CSCS textbook chapter on load assignment includes the formulas covered here.
- LeSuer et al. (1997) — Accuracy of 1RM prediction equations
Foundational comparative study of 1RM prediction equation accuracy.
- Stronger by Science — Greg Nuckols, evidence-based training research
Greg Nuckols’ evidence-based commentary on rep-to-1RM relationships.
- OpenPowerlifting — global meet results database
Real-world meet results database for sanity-checking calculator output.
References
- Original publication: O'Conner, B., Simmons, J., & O'Shea, P. (1989). Weight Training Today. St. Paul, MN: West Publishing.
- LeSuer et al. (1997): LeSuer, D.A., McCormick, J.H., Mayhew, J.L., Wasserstein, R.L., & Arnold, M.D. (1997). The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. Journal of Strength and Conditioning Research, 11(4), 211-213.
- Reynolds et al. (2006): Reynolds, J.M., Gordon, T.J., & Robergs, R.A. (2006). Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry. Journal of Strength and Conditioning Research, 20(3), 584-592.
- Wood et al. (2002): Wood, T.M., Maddalozzo, G.F., & Harter, R.A. (2002). Accuracy of seven equations for predicting 1-RM performance of apparently healthy, sedentary older adults. Measurement in Physical Education and Exercise Science, 6(2), 67-94.
- Wikipedia: One-repetition maximum — general reference for 1RM prediction equations.
- Essentials of Strength Training and Conditioning (NSCA, Human Kinetics) — NSCA CSCS textbook chapter on load assignment.