One Rep Max Calculator
Predict your one-rep max (1RM) — the heaviest weight you could lift for a single perfect rep — from a recent set you actually performed. Used by powerlifters, strength coaches, and bodybuilders to prescribe percentage-based training (e.g., "work up to 5×5 at 80% of 1RM") without the risk and recovery cost of repeatedly testing true maxes.
About this calculator
All 1RM prediction formulas use the empirical relationship between the maximum load you can lift for a given number of reps and your true single-rep maximum. The four most cited formulas: Epley: 1RM = weight × (1 + reps/30). Brzycki: 1RM = weight × (36 / (37 − reps)). McGlothin: 1RM = weight × (1.0278^reps). Lombardi: 1RM = weight × (reps^0.10). For sets of 1–5 reps they agree within 2–3%; for 6–10 reps they diverge by 5–10%; for 11+ reps they all break down because muscular endurance, lactate buffering, and mental tolerance increasingly determine the rep count rather than pure strength. This calculator multiplies the predicted 1RM by a small training-experience factor (0.90–1.00) to correct for the fact that beginners under-express their true neural-strength potential in any single set (their 5-rep set is closer to their max than an experienced lifter's), while advanced lifters can grind out reps closer to their true ceiling. The output is most reliable when the test set is 3–6 reps, performed to genuine concentric failure or with 0–1 reps in reserve, with good form, on a compound movement you train regularly (squat, bench, deadlift, overhead press). Edge cases: form breakdown (back rounding, partial reps, range-of-motion shortening) inflates the test-set rep count and overpredicts 1RM. Sets above 10 reps have a 10–20% error band; testing at 1–3 reps gives the tightest prediction but carries the most injury risk. Fatigue, time of day, sleep, and nutrition all affect single-session strength by ±5–10%, so any single 1RM estimate is best treated as a planning input with ±5 kg uncertainty, not a precise number.
How to use
Example 1 — Bench press, intermediate lifter. You benched 80 kg for 5 reps in good form with about 1 rep in reserve, using the Epley formula, intermediate experience (multiplier 0.95). Epley: 1RM = 80 × (1 + 5/30) = 80 × 1.1667 ≈ 93.3 kg. Apply experience factor: 93.3 × 0.95 ≈ 88.6 kg. ✓ Estimated 1RM ≈ 89 kg. You can now build a training cycle: 5×5 at 80% = 71 kg, 3×3 at 90% = 80 kg, peak attempt at 100% = 89 kg. Verify by retesting your top set every 4–6 weeks; if a 5-rep set at 80 kg starts feeling like 3 reps in reserve, your true 1RM has moved up and the prediction is now low. Example 2 — Deadlift, beginner using Brzycki. You deadlifted 100 kg for 8 reps, beginner experience (multiplier 0.90), Brzycki formula. Brzycki: 1RM = 100 × 36 / (37 − 8) = 100 × 36 / 29 ≈ 124.1 kg. Apply experience factor: 124.1 × 0.90 ≈ 111.7 kg. ✓ Estimated 1RM ≈ 112 kg. Beginners often see the prediction undershoot their actual max within a few months because rapid neural adaptation closes the gap between submaximal and maximal performance — recheck the estimate every 4 weeks during the first 3–6 months of training.
Frequently asked questions
Which 1RM formula is the most accurate?
For sets of 1–5 reps, all four formulas (Epley, Brzycki, McGlothin, Lombardi) agree within about 2–3%, and any of them is fine. For sets of 6–10 reps, Brzycki tends to be slightly more conservative (lower predictions) and Epley slightly more aggressive (higher predictions); the truth usually sits between them. At 11+ reps all four become unreliable because the underlying assumption (that muscular endurance follows a smooth predictable relationship with maximal strength) breaks down. Studies validating prediction equations against tested 1RM (e.g., LeSuer et al., 1997) consistently find that the choice of formula matters less than the rep range — testing with 3–5 reps in any formula beats testing with 10 reps in the "best" formula. Some coaches average the four formulas for a single number, which works fine but is no more accurate than picking one and being consistent.
How often should I retest my actual 1RM?
Most strength programs avoid testing true single-rep maxes more than twice a year for non-competitive lifters and never more than once per training cycle (8–16 weeks) even for competitive athletes. The reason: a true max attempt carries higher injury risk than a near-max attempt, requires several days of recovery, and tells you less than a well-designed AMRAP (as-many-reps-as-possible) or 3RM test. For ongoing programming, predict from a top set of 3–5 reps every 4–6 weeks. Competitive powerlifters test true 1RMs during meet-day attempts (every 3–6 months), and use predicted maxes from training in between. Olympic weightlifters test true maxes more often (some programs include weekly singles) because the technical complexity of the lifts rewards frequent practice at competition load. For everyone else, more frequent prediction beats more frequent true testing.
Why does my 1RM estimate seem too high or too low for my actual strength?
Predicted 1RMs systematically overestimate true 1RMs when test-set form breaks down (partial reps, sloppy bar path, momentum), when reps are pumped out far past true mechanical failure, or when the test set is higher than 5–6 reps and the formula extrapolates beyond its reliable range. They underestimate when you stopped well before failure (3+ reps in reserve), when you tested on a poor day (under-recovered, dehydrated, late evening after a long workday), or when you are still very early in training and lack the neural drive to express true strength even in a maximal attempt. Calibration over time: track your predicted 1RM against your tested 1RM at every meet or peak day, note the systematic bias for your training style and form, and adjust the experience multiplier accordingly. For honest lifters with strict form, the standard formulas land within 5 kg of tested 1RMs on the major lifts.
What are the most common mistakes people make with 1RM estimates?
Testing with too many reps — a 10-rep set has 10%+ prediction error compared to a 3-rep set, so the resulting 5×5 percentages will be slightly off. Allowing form to break down to get more reps, which inflates the prediction and produces working weights that are actually too heavy. Treating the predicted number as exact — your true 1RM has natural ±5% day-to-day variation, so chasing a predicted PR by 1 kg is a chase after noise. Using predicted 1RMs across different exercises — your barbell bench 1RM does not predict your dumbbell-bench 1RM or your incline-bench 1RM; each variation needs its own test. Programming peak attempts at 100% of predicted 1RM on day one of a new cycle — give a buffer (95–97%) the first time, especially if it has been more than a few weeks since the test. And ignoring training experience entirely — the same 5-rep set means a different thing for a beginner who is mostly learning the movement and an advanced lifter near their genetic ceiling.
When should I not use this calculator?
Skip it for higher-rep accessory work (curls, rear delt flies, lateral raises) — the 1RM concept is meaningful for compound barbell lifts but not for isolation exercises that nobody trains in the 1–3 rep range. Avoid it during deloads, illness, or significant under-recovery; the prediction will be artificially low and you will program weights that are too easy. Do not use it on a lift you have not trained in the last four weeks — your 1RM on a movement decays without practice and the prediction will overshoot. Skip it if you cannot lift with consistent technique to failure or near-failure — without a clean test set, the prediction is meaningless. And do not use it for novice lifters who are still acquiring technique on the major lifts (typically the first 3–6 months); for them, "linear progression" programming (add 2.5–5 kg per session as form allows) outperforms any percentage-based scheme. Finally, for competitive powerlifters and weightlifters preparing for meets, predicted 1RMs are a useful planning input but tested singles in the final 4–8 weeks of a peak are what actually counts.