How does the OConner formula differ from other 1RM equations?

The OConner formula uses a constant 2.5% increment per repetition, resulting in a purely linear relationship. This contrasts with the Epley (3.33% per rep), Brzycki (curvilinear decay), and Lombardi (exponential) models, each of which applies different coefficients and produces divergent estimates, especially above 6–8 repetitions.

Why do estimates become less accurate above 10 repetitions?

Higher-repetition sets increasingly reflect muscular endurance and metabolic fatigue rather than pure force production. The OConner formula's linear structure cannot capture the nonlinear fatigue accumulation seen in longer sets, leading to systematic overestimation of 1RM when using lighter loads for 12+ reps.

Does exercise selection affect the accuracy of this calculator?

Yes. Prediction formulas were typically validated on multi-joint barbell movements like the squat, bench press, and deadlift. Single-joint exercises, machine variants, and movements with different stability demands may produce estimates that deviate from true 1RM values due to differences in skill, coordination, and muscle recruitment patterns.

Can this tool be used to set training percentages?

The calculator provides a starting reference. Many strength programs prescribe training loads as percentages of 1RM; using an estimated 1RM from this tool allows percentage-based programming without performing a true maximum attempt. However, adjustments based on bar speed, perceived exertion, and session-to-session performance remain necessary.

How much variation exists between prediction formulas?

For the same input—75 kg lifted for 5 reps—the OConner, Epley, Brzycki, and Lombardi formulas can produce 1RM estimates that differ by 5–10 kg. No single equation is universally superior; each reflects the specific population and exercise conditions in which it was developed. Testing multiple formulas and comparing results to actual performance offers the most insight.

Strength Training

One-Rep Max Calculator (OConner Formula)

Estimate your one-rep max using the OConner linear regression formula for sub-maximal lifts.

Last updated: 22 April 2026

What this tool does

This calculator estimates your one-rep max (1RM) using the O'Connor formula, a linear regression equation developed for predicting maximal strength from sub-maximal lifts. It takes the weight lifted and the number of repetitions performed to failure as inputs, applying a fixed 2.5% multiplier per rep to generate an estimated 1RM in kilograms or pounds. The formula is most reliable for lifts in the 1–10 rep range and serves as a population-average estimate rather than a direct measurement.

Inputs

Result

—

Formula Used

1RM = w \times (1 + 0.025 r)

w

Weight in kg

r

Reps performed

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How One-Rep Max Calculator (OConner Formula) works

This calculator estimates your one-repetition maximum (1RM) using the OConner formula, one of several regression equations developed to predict maximal strength without performing a true maximum lift. By entering the weight lifted and the number of repetitions completed to momentary muscular failure, the tool applies a linear coefficient of 0.025 per repetition to project the heaviest single lift the same muscle group could perform under identical conditions.

The formula

The OConner equation is expressed as:
1RM = weight × (1 + 0.025 × reps)
where weight is the load lifted in kilograms, reps is the number of complete repetitions performed, and 1RM is the estimated one-rep maximum in kilograms. The constant 0.025 reflects a 2.5% increase in projected maximal load for each additional repetition completed.

Where this method is most accurate

The OConner formula was derived from observation of trained individuals performing compound exercises in the 1–10 repetition range. Estimates tend to be most reliable when repetitions fall between 2 and 8, the set is taken to concentric failure, and exercise technique remains consistent across the set. Prediction error increases substantially above 10 repetitions, with lighter loads producing systematically inflated 1RM projections. The formula does not account for training age, fiber-type distribution, exercise tempo, rest interval before the set, or inter-individual variation in fatigue resistance.

What this tool does not do

This calculator provides a numerical estimate only. It does not prescribe training loads, recommend program design, diagnose strength deficits, or predict injury risk. The tool cannot account for differences in recovery status, muscle-group specificity, equipment variation (barbell versus dumbbell versus machine), or whether the set ended at technical failure versus absolute failure. Results are projection models, not measured outcomes.

Disclaimer

This tool is for educational and informational purposes only. It is not a substitute for professional coaching, medical evaluation, or individualized program design. All strength-training activities carry inherent risk; consult qualified professionals before beginning or modifying resistance-training protocols.

Questions

How does the OConner formula differ from other 1RM equations?: The OConner formula uses a constant 2.5% increment per repetition, resulting in a purely linear relationship. This contrasts with the Epley (3.33% per rep), Brzycki (curvilinear decay), and Lombardi (exponential) models, each of which applies different coefficients and produces divergent estimates, especially above 6–8 repetitions.
Why do estimates become less accurate above 10 repetitions?: Higher-repetition sets increasingly reflect muscular endurance and metabolic fatigue rather than pure force production. The OConner formula's linear structure cannot capture the nonlinear fatigue accumulation seen in longer sets, leading to systematic overestimation of 1RM when using lighter loads for 12+ reps.
Does exercise selection affect the accuracy of this calculator?: Yes. Prediction formulas were typically validated on multi-joint barbell movements like the squat, bench press, and deadlift. Single-joint exercises, machine variants, and movements with different stability demands may produce estimates that deviate from true 1RM values due to differences in skill, coordination, and muscle recruitment patterns.
Can this tool be used to set training percentages?: The calculator provides a starting reference. Many strength programs prescribe training loads as percentages of 1RM; using an estimated 1RM from this tool allows percentage-based programming without performing a true maximum attempt. However, adjustments based on bar speed, perceived exertion, and session-to-session performance remain necessary.
How much variation exists between prediction formulas?: For the same input—75 kg lifted for 5 reps—the OConner, Epley, Brzycki, and Lombardi formulas can produce 1RM estimates that differ by 5–10 kg. No single equation is universally superior; each reflects the specific population and exercise conditions in which it was developed. Testing multiple formulas and comparing results to actual performance offers the most insight.

Sources & Methodology

The OConner formula calculates estimated one-rep max as 1RM = weight × (1 + 0.025 × reps). Originally presented in strength-training literature for sub-maximal load prediction, the equation applies a fixed 2.5% multiplier per repetition performed to failure.

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