{
  "slug": "rep-max-calculator",
  "title": "Rep Max Calculator (Any Rep Max)",
  "category": "Strength Training",
  "category_slug": "strength-training",
  "tier": 2,
  "summary": "Estimate your X-rep max from your known 1RM using the reverse Epley formula.",
  "description": "This calculator estimates the maximum weight a person can lift for a specified number of repetitions, given their known one-rep max (1RM), using the reverse Epley formula. It takes a 1RM in kilograms and a target rep count as inputs, then outputs the estimated load for that rep range and its percentage of the 1RM. The reverse Epley equation—derived from Boyd Epley's 1985 model—assumes a linear relationship between load and repetitions, and is most commonly applied to compound barbell lifts in the 1–10 rep range.",
  "page_url": "https://fitmetriclab.com/en/tools/strength-training/rep-max-calculator/",
  "api_url": "https://fitmetriclab.com/api/tools/rep-max-calculator.json",
  "formula": {
    "id": "rep_max_calculator",
    "expression": "w_r = \\frac{\\text{1RM}}{1 + r/30}",
    "variables": [
      {
        "symbol": "\\text{1RM}",
        "description": "One-rep max in kg"
      },
      {
        "symbol": "r",
        "description": "Target reps"
      }
    ]
  },
  "inputs": [
    {
      "id": "one_rm_kg",
      "label": "1-Rep Max",
      "type": "number",
      "unit": "kg",
      "default": 100,
      "min": 1,
      "max": 500,
      "step": 0.5
    },
    {
      "id": "target_reps",
      "label": "Target Reps",
      "type": "number",
      "unit": "reps",
      "default": 5,
      "min": 1,
      "max": 30,
      "step": 1
    }
  ],
  "worked_example": {
    "inputs": {
      "one_rm_kg": 100,
      "target_reps": 5
    },
    "expected_output": "85.7 kg"
  },
  "result_summary_template": "For a one-repetition maximum of {one_rm_kg} and a target of {target_reps} repetitions, the reverse Epley formula estimates the maximum load at {result}.",
  "methodology": "Applies the reverse Epley formula: X-rep max = 1RM / (1 + reps / 30). The Epley equation was introduced by Boyd Epley in 1985 for estimating 1RM from submaximal loads; the reverse form solves for the load at a given rep count from a known 1RM.",
  "sources": [
    {
      "label": "Epley B. Poundage Chart. Boyd Epley Workout. Lincoln, NE: University of Nebraska Press; 1985.",
      "url": "https://www.nsca.com/education/tools-and-resources/1rm-calculator/"
    },
    {
      "label": "LeSuer DA, McCormick JH, Mayhew JL, et al. 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://pubmed.ncbi.nlm.nih.gov/27775469/"
    },
    {
      "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/"
    }
  ],
  "faqs": [
    {
      "question": "What is the reverse Epley formula?",
      "answer": "The reverse Epley formula calculates the maximum load for a given number of repetitions from a known 1RM. It divides the 1RM by (1 + reps / 30), where 30 is a constant derived from empirical strength data collected in the 1980s."
    },
    {
      "question": "Why does the formula use the constant 30?",
      "answer": "The constant 30 originates from the original Epley equation (1RM = weight × (1 + reps / 30)), which was calibrated against observed strength performance in trained lifters. This divisor reflects the average rate at which load capacity declines as reps increase."
    },
    {
      "question": "Is this formula accurate for high-rep sets?",
      "answer": "The reverse Epley formula is most reliable for 1–10 repetitions. Beyond 12 reps, factors such as local muscular endurance, lactate tolerance, and cardiovascular conditioning become more influential, and the predictions may diverge from actual performance."
    },
    {
      "question": "Can I use this calculator for all exercises?",
      "answer": "The formula was developed primarily for multi-joint barbell movements like squats, bench presses, and deadlifts. It may be less accurate for isolation exercises, machine lifts, or movements with different fatigue profiles."
    },
    {
      "question": "How do I find my 1RM to use this calculator?",
      "answer": "A 1RM can be tested directly under supervision, or estimated from a submaximal lift using a 1RM prediction formula. Both methods carry inherent error; direct testing is generally more accurate but requires proper warm-up and spotting."
    }
  ],
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  ],
  "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."
}