{
  "slug": "one-rep-max-oconner",
  "title": "One-Rep Max Calculator (OConner Formula)",
  "category": "Strength Training",
  "category_slug": "strength-training",
  "tier": 2,
  "summary": "Estimate your one-rep max using the OConner linear regression formula for sub-maximal lifts.",
  "description": "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.",
  "page_url": "https://fitmetriclab.com/en/tools/strength-training/one-rep-max-oconner/",
  "api_url": "https://fitmetriclab.com/api/tools/one-rep-max-oconner.json",
  "formula": {
    "id": "one_rep_max_oconner",
    "expression": "\\text{1RM} = w \\times (1 + 0.025\\,r)",
    "variables": [
      {
        "symbol": "w",
        "description": "Weight in kg"
      },
      {
        "symbol": "r",
        "description": "Reps performed"
      }
    ]
  },
  "inputs": [
    {
      "id": "weight_kg",
      "label": "Weight",
      "type": "number",
      "unit": "kg",
      "default": 75,
      "min": 20,
      "max": 300,
      "step": 0.1
    },
    {
      "id": "reps",
      "label": "Reps",
      "type": "number",
      "unit": "reps",
      "default": 5,
      "min": 1,
      "max": 20,
      "step": 1
    }
  ],
  "worked_example": {
    "inputs": {
      "weight_kg": 75,
      "reps": 5
    },
    "expected_output": "84.4 kg"
  },
  "result_summary_template": "Lifting {weight_kg} for {reps} repetitions, the OConner formula estimates a one-rep max of {result}.",
  "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.",
  "sources": [
    {
      "label": "LeSuer DA, McCormick JH, Mayhew JL, Wasserstein RL, Arnold MD. The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. J Strength Cond Res. 1997;11(4):211-213.",
      "url": "https://journals.lww.com/nsca-jscr/Abstract/1997/11000/The_Accuracy_of_Prediction_Equations_for.1.aspx"
    },
    {
      "label": "Reynolds JM, Gordon TJ, Robergs RA. Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry. J Strength Cond Res. 2006;20(3):584-592.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/16937972/"
    },
    {
      "label": "Baechle TR, Earle RW, editors. Essentials of Strength Training and Conditioning. 3rd ed. Human Kinetics; 2008.",
      "url": "https://www.nsca.com/education/books/essentials-of-strength-training-and-conditioning-4th-edition/"
    }
  ],
  "faqs": [
    {
      "question": "How does the OConner formula differ from other 1RM equations?",
      "answer": "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."
    },
    {
      "question": "Why do estimates become less accurate above 10 repetitions?",
      "answer": "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."
    },
    {
      "question": "Does exercise selection affect the accuracy of this calculator?",
      "answer": "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."
    },
    {
      "question": "Can this tool be used to set training percentages?",
      "answer": "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."
    },
    {
      "question": "How much variation exists between prediction formulas?",
      "answer": "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."
    }
  ],
  "related_tools": [
    {
      "slug": "one-rep-max-brzycki",
      "title": "One-Rep Max Calculator (Brzycki Formula)",
      "api_url": "https://fitmetriclab.com/api/tools/one-rep-max-brzycki.json",
      "page_url": "https://fitmetriclab.com/en/tools/strength-training/one-rep-max-brzycki/"
    },
    {
      "slug": "one-rep-max-comparison",
      "title": "Multi-Formula 1RM Comparison Calculator",
      "api_url": "https://fitmetriclab.com/api/tools/one-rep-max-comparison.json",
      "page_url": "https://fitmetriclab.com/en/tools/strength-training/one-rep-max-comparison/"
    },
    {
      "slug": "one-rep-max-epley",
      "title": "One-Rep Max Calculator (Epley Formula)",
      "api_url": "https://fitmetriclab.com/api/tools/one-rep-max-epley.json",
      "page_url": "https://fitmetriclab.com/en/tools/strength-training/one-rep-max-epley/"
    }
  ],
  "last_updated": "2026-04-22",
  "price": {
    "value": 0,
    "currency": "USD"
  },
  "runs_in_browser": true,
  "disclaimer": "Educational estimate produced from a published formula. Not medical, nutritional, or training advice. Calculations are deterministic — apply the formula to the inputs to reproduce the result."
}