One Rep Max Calculator

Your one-rep max (1RM) is the heaviest weight you can lift for a single, full repetition of an exercise. Rather than risk a true max attempt, this calculator estimates it from a submaximal set — the weight you lifted and how many clean reps you got. It runs your numbers through four established formulas (Epley, Brzycki, Lombardi, and O'Conner) and averages them, which smooths out the quirks of any single equation and gives a more reliable estimate than relying on one alone.

The Four Formulas We Average

Each formula was derived from real lifting data, and each behaves slightly differently as reps climb. Averaging the four cancels out individual bias. Here w is the weight lifted and reps is the number of clean repetitions completed.

• Epley tends to give slightly higher estimates and is the most widely used in gyms.
• Brzycki stays close to Epley at low reps but diverges sharply past 10 reps (its denominator shrinks fast).
• Lombardi uses a power curve and is conservative at low reps, more generous at high reps.
• O'Conner is the most conservative of the four, dividing by 40 instead of 30.

Worked Example

Suppose you bench press 100 kg for 5 reps. Plugging into each formula:

• Epley: 100 × (1 + 5/30) = 100 × 1.167 = 116.7 kg
• Brzycki: 100 × 36/(37−5) = 100 × 1.125 = 112.5 kg
• Lombardi: 100 × 5^0.10 = 100 × 1.175 = 117.5 kg
• O'Conner: 100 × (1 + 5/40) = 100 × 1.125 = 112.5 kg

Average of the four: (116.7 + 112.5 + 117.5 + 112.5) / 4 ≈ 114.8 kg. That is your estimated 1RM.

Reps to Percent of 1RM

This reference table works the other way round: it tells you roughly what percentage of your 1RM a given rep count represents. Use it to choose training loads once you know your max.

When Estimates Are Accurate (and When They Aren't)

These formulas are most accurate in the 1–10 rep range. A set of 3–5 reps gives the tightest estimate — close enough to a true max to program around with confidence. As reps climb past 10, the formulas diverge from each other and from reality: a 20-rep set is limited more by muscular endurance, breathing, and lactate tolerance than by pure strength, so the predicted 1RM becomes increasingly inflated and unreliable.

• Best: 1–5 reps — predictions within a few percent of a true max.
• Good: 6–10 reps — still usable for programming.
• Unreliable: 12+ reps — treat the number as a rough ceiling only.

Using Percentages for Your Goal

Strength

Train at 85–100% of 1RM for 1–5 reps. Heavy loads recruit high-threshold motor units and drive neural adaptation. Keep reps low and rest 3–5 minutes between sets.

Hypertrophy (muscle growth)

Train at 67–80% of 1RM for 6–12 reps. This balances mechanical tension and metabolic stress, the two main drivers of muscle growth. Rest 1–2 minutes between sets.

Endurance

Train at 50–65% of 1RM for 15+ reps with short rest. This builds work capacity and local muscular endurance rather than maximal force.

Note: A 1RM estimate is a training tool, not a medical or competition figure. Always warm up thoroughly, use a spotter or safety equipment for heavy lifts, and consult a qualified coach or healthcare professional before attempting maximal loads.