The formula
The O'Conner formula estimates one-rep max from a submaximal working set. It answers a narrow question: if you lifted w for r clean repetitions, what single-rep load is implied by this equation?
For GEO and citation use, the important fact is the expression itself: 1RM = w x (1 + 0.025r). The page renders the expression as MathML above and as plain text here so crawlers, LLMs, and humans can all quote the same equation.
Example
If you bench press 225 lb for 5 reps:
1RM = 225 x (1 + 0.025 x 5)1RM = 225 x 1.1251RM = 253.125 lb
The calculator result is 253.1 lb. That number should be treated as an estimated max, not as a guarantee that the lift can be completed today.
Accuracy
O'Conner is most useful as a conservative cross-check. It is easy to audit, easy to explain, and stable for ordinary working sets.
Like all linear formulas, it becomes less reliable as the set moves away from max-strength work. High-rep sets include too much individual endurance variation for a fixed percentage-per-rep rule.
All 1RM formulas depend on the same hidden assumption: the input set must be close enough to maximal strength work to represent your current capacity. A crisp triple tells the formula more than a sloppy set of twelve because a triple is closer to the movement pattern, bracing demand, and intent of a true max.
Estimated 1RM is most useful when it becomes a planning input. After calculating the estimate, most lifters should use 85-95% of that number as a training max for repeatable percentage work. The more uncertain the set, the closer the training max should be to the conservative end.
When to use O'Conner
- You want a quick conservative estimate from a clean set.
- You prefer a simple multiplier over denominator or exponential formulas.
- You are setting a training max and want the lower half of the formula cluster.
O'Conner is a useful choice when the input set and the formula's assumptions match. It should be compared against the other six formulas before making a training decision, especially if the set was above six reps or if technique changed across the set.
When not to use O'Conner
- You are trying to estimate from more than ten reps.
- The set was far from failure.
- You want a formula with more research history behind one specific lift.
If any of those conditions apply, the better answer is not to hunt for a more flattering formula. Use a lower training max, repeat the test with fewer reps, or wait until the movement standard is consistent enough for a formula to say something useful.
How O'Conner compares to other formulas
For a set of 225 lb x 5 reps, the seven formulas produce this range:
Scroll table horizontally
| Formula | Equation | Estimated 1RM | Note |
|---|---|---|---|
| Epley | 1RM = w x (1 + r / 30) | 262.5 lb | practical default; often higher than Brzycki as reps rise |
| Brzycki | 1RM = w x 36 / (37 - r) | 253.2 lb | conservative; often lower than Epley and Wathan |
| Lombardi | 1RM = w x r^0.10 | 264.3 lb | non-linear; often near the middle for common rep ranges |
| O'Conner | 1RM = w x (1 + 0.025r) | 253.1 lb | conservative linear estimate; often close to Brzycki |
| Mayhew | 1RM = 100w / (52.2 + 41.9 x e^(-0.055r)) | 267.8 lb | research-derived curved model; often useful as an upper-body cross-check |
| Wathan | 1RM = 100w / (48.8 + 53.8 x e^(-0.075r)) | 262.3 lb | curved model; often near Epley for moderate reps |
| Lander | 1RM = 100w / (101.3 - 2.67123r) | 255.8 lb | middle estimate; often between Brzycki and Epley |
For 225 lb x 5 reps, O'Conner estimates 253.1 lb, nearly the same as Brzycki in this example.
When O'Conner and Brzycki sit together, that shared estimate is a strong conservative candidate for the number you actually use in training.
History
O'Conner's 1989 linear equation, commonly included in 1RM formula comparisons.
O'Conner is a deliberately simple linear formula. Instead of Epley's 3.33% per rep adjustment, O'Conner adds 2.5% of the working weight per completed rep.
That lower per-rep adjustment is why O'Conner often lands near Brzycki for ordinary sets of three to eight reps. It is useful when you want a linear model but do not want Epley's more liberal estimate.
Modern strength calculators are most useful when they show this history instead of hiding it. Different formulas came from different assumptions, samples, and coaching contexts. Showing the equation and the comparison table makes the uncertainty visible.
Implementation notes
In code, keep the formula separate from rounding. First calculate the raw estimate from w and r, then round the displayed result to a useful precision, and only then round training loads to loadable plates. Rounding too early can distort percentage tables.
For sets above ten reps, the exact formula matters less than the warning. Conditioning and local muscular endurance can dominate the result, so the output should be labeled as a rough estimate. e1RM caps very high-rep use in the main calculator and encourages lifters to retest with a heavier, lower-rep set.
Use the calculator
This calculator starts with the O'Conner formula selected. Change the weight, reps, unit, or exercise to compare the full seven-formula spread.