One Rep Max Calculator
Enter the weight you lifted and how many clean reps you managed, and see what the four classic prediction equations — Epley, Brzycki, Lombardi, and O’Conner — estimate your one-rep max to be. The result is shown as a range, because the formulas genuinely disagree, plus conventional training percentages for programming.
Estimate your one-rep max
Example: 100 kg × 5 reps → Epley 116.7 kg, Brzycki 112.5 kg.
Four formulas, one honest range
The four equations share a shape — scale the weight up by a factor that grows with rep count — but were fitted to different data: Epley (1985) uses w × (1 + reps/30), Brzycki (1993) uses w × 36 ÷ (37 − reps), Lombardi (1989) uses w × reps0.10, and O’Conner et al. (1989) uses w × (1 + reps/40). For 100 kg lifted 5 times the engine computes: Epley 116.7 kg, Brzycki 112.5 kg, Lombardi 117.5 kg, O’Conner 112.5 kg — a spread of 5 kg. That disagreement is real information: your true max most likely sits somewhere inside the band, and no single formula deserves the extra decimal places it prints.
An estimate, not a measurement
These are prediction equations, not a bar you actually lifted. Validation research finds them most accurate for low-rep sets — roughly 3–5 reps — with error growing as reps increase, which is why the calculator accepts at most 15 reps and flags results from more than 10 reps as noticeably less reliable. Fatigue resistance also differs by person and exercise: high-rep sets on squats and deadlifts overestimate for some lifters and underestimate for others. Use the range to plan training, then let real bar speed and real sets calibrate it.
Training percentages (a convention, not a law)
Programs prescribe working weights as percentages of 1RM. Applied to the Epley estimate from the example above: 90% ≈ 105 kg, 85% ≈ 99.2 kg, 80% ≈ 93.3 kg, 75% ≈ 87.5 kg. The percentage-to-rep correspondence (for example, "about 3 reps at 90%") is a coaching convention with wide individual variation — treat the table as a starting point and adjust to performance.
If you ever test a true single
A genuine max attempt is the riskiest thing most lifters do in a normal training year. Never test a true one-rep max alone: use a competent spotter or safety pins set at the right height, build up with progressively heavier singles, and stop the session on any technique breakdown. For programming purposes, a heavy set of three to five with an estimate from this page gives you almost all of the information at a fraction of the risk. The equations were characterized in adults; young and novice lifters should avoid maximal testing altogether.
Frequently asked questions
Why do the four formulas disagree?
Each was fitted to different lifters, exercises, and rep ranges: Epley (1985) came from a University of Nebraska training manual, Brzycki (1993) from a coaching article, and Lombardi and O’Conner (both 1989) from other practitioner data. At 5 reps they typically differ by a few percent; the spread grows with rep count. Showing all four as a range is more honest than picking one.
How accurate are the estimates?
Validation studies find prediction is most accurate from low-rep sets (about 3–5 reps) and gets worse as reps rise — which is why this calculator accepts at most 15 reps and flags anything above 10 as noticeably less reliable. Even at best, treat the result as a range with a few percent of uncertainty, not a number your next lift is guaranteed to match.
Should I actually test my true 1RM?
For most people, most of the time, no — the estimate is enough for programming. A true max attempt is a maximal skill and strain test: if you do it, do it well-rested, with warm-up singles, solid technique, safety pins or a spotter, and never alone. The formulas exist precisely so you can train off a heavy set of 3–5 instead.
What are the training percentages for?
Programs commonly prescribe loads as a percentage of 1RM — for example around 90% for heavy strength doubles and triples, 75–85% for most strength and hypertrophy work. The table applies those conventional percentages to the Epley estimate. They are programming conventions, not physiology: adjust to how the weight actually moves.
Does this apply to teenagers?
The equations were characterized in adults, and maximal testing is generally discouraged for young or novice lifters. Teens can train effectively with submaximal sets and should treat these estimates — and especially true max attempts — with extra caution and qualified supervision.
Estimates, not measurements: all figures on this page are population-level predictions with real error bands. Train sensibly, never max out alone, and consult a qualified professional before major changes to training intensity. Values are processed locally in your browser and never transmitted. See the methodology page.