How it works
Epley: 1RM = weight × (1 + reps ÷ 30); Brzycki: 1RM = weight × 36 ÷ (37 − reps)
Both formulas extrapolate a single-rep max from a sub-maximal set. Epley (1985) scales the lifted weight up by the reps performed: 1RM = weight × (1 + reps ÷ 30). Lifting 100 kg for 5 reps gives 100 × (1 + 5/30) = 100 × 1.1667 ≈ 116.7 kg. Brzycki (1993) uses a different shape: 1RM = weight × 36 ÷ (37 − reps), which for the same set gives 100 × 36 ÷ 32 = 112.5 kg. The headline number is the average of the two. Both are most accurate in the 1–12 rep range and drift apart as reps climb, because predicting a max from a 20-rep set is inherently shaky — so we cap the input at 12 reps. From the estimated max we also derive standard training loads: roughly 85–95% of 1RM for strength, 70–80% for hypertrophy. The weight unit you enter does not affect the maths — the formulas are linear in weight — so kg and lb give the same proportional result.
Sources
- Epley formula Epley B. Poundage Chart. Boyd Epley Workout. Lincoln, NE: Body Enterprises, 1985 — 1RM = w·(1 + reps/30).
- Brzycki formula Brzycki M. Strength testing — predicting a one-rep max from reps to fatigue. JOPERD. 1993;64(1):88–90 — 1RM = w·36/(37 − reps).
- Accuracy range Rep-max prediction equations are most accurate at low reps (≈1–10) and diverge at higher reps; estimates from very high-rep sets are unreliable.
FAQ
How do I calculate my one-rep max?
Take a set you can do with good form, then apply a rep-max formula. This calculator uses Epley, 1RM = weight × (1 + reps/30), and cross-checks it with Brzycki, 1RM = weight × 36/(37 − reps). For example, 100 kg for 5 reps estimates roughly 113–117 kg. You never have to attempt a true max to get the number.
Which is better, Epley or Brzycki?
Neither is universally best — they agree closely at low reps and diverge as reps rise. Epley tends to read slightly higher, Brzycki slightly lower, beyond about five reps. Using the average of the two, as this calculator does, is a reasonable hedge. For the most reliable estimate, base it on a heavy set of 5 reps or fewer.
How many reps should I use for the estimate?
Fewer is more accurate. A set of 3–6 reps gives the best estimate; the formulas hold to about 12 reps and become unreliable beyond that, which is why this tool caps reps at 12. If you want a precise 1RM, use the heaviest set you can manage with clean technique.
Why should runners care about one-rep max?
Strength training improves running economy, power and injury resistance, and most evidence-based programs prescribe loads as a percentage of 1RM. Estimating your max lets you train at the right intensity — heavy enough to build strength, controlled enough to stay safe — without the risk of testing a true max.
What percentage of 1RM should I train at?
It depends on the goal. Roughly 85–95% of 1RM (low reps) develops maximal strength, 70–85% builds muscle, and lighter loads with more reps build endurance. The load table in this calculator does the arithmetic for you from your estimated max.
Is it safe to actually test my one-rep max?
Testing a true max is demanding and riskier than a sub-maximal set, especially without a spotter, warm-up and solid technique. For most runners and recreational lifters, estimating from a moderate set is safer and accurate enough. If you do test, build up gradually and prioritise form over the number.
Estimated one-rep max figures are approximations from prediction formulas, not measured maxes, and they grow less accurate as reps increase. This is general fitness information, not coaching or medical advice. Lift within your ability, warm up, use good technique and a spotter where appropriate, and consult a qualified coach or clinician before heavy strength work if you are new to it or returning from injury.