How it works
Epley: 1RM = weight × (1 + reps ÷ 30); Brzycki: 1RM = weight × 36 ÷ (37 − reps)
Both equations project what one all-out rep would be from a set you stopped short of failure. Epley (1985) adds a thirtieth of the weight per rep — lift 100 kg for 5 and you get 100 × (1 + 5/30) ≈ 116.7 kg. Brzycki (1993) reaches the same territory along a different curve, 36 ÷ (37 − reps), which puts that identical set at 112.5 kg. The headline result averages the two, since neither is consistently right for every lifter. Accuracy is best at low reps and decays as they climb; past a dozen the formulas stop meaning much, which is why the reps field is capped at 12. From the estimated max, the table derives the standard prescriptions — around 85–95% of 1RM for maximal strength, 70–85% for muscle growth. Both formulas scale linearly with weight, so the kg/lb toggle changes only the display, never the proportions.
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
When should I use the one rep max calculator?
At the start of a strength block, or whenever a program asks for something like five reps at 80% and you have no tested max to anchor it. Take a recent honest set — say 100 kg for 5 clean reps — and the estimate lands around 113–117 kg depending on the formula. Re-estimate every few weeks as your working sets get heavier.
What inputs matter most?
Rep honesty. The formulas assume the set was taken near its true limit with solid form, so counting grinding, half-depth reps inflates the estimate. Rep count also drives accuracy more than the load does: a heavy triple or a set of five predicts far better than a light set of twelve.
How should I read the result?
As a working ceiling for programming, not a challenge to attempt. Use it to set training loads — the percentage table does that arithmetic for you — and to track your strength trend across a season. When the Epley and Brzycki figures sit close together, your set was well inside the range where the formulas behave.
Does it matter whether I enter kilograms or pounds?
No — both formulas multiply the weight by a rep-based factor, so the unit passes straight through. Enter 225 lb or its metric twin and the estimated max comes back in the same unit, the same percentage above your set. Just keep the load table in whichever unit your gym’s plates use.
Why do runners need a 1RM at all?
Because the strength work that improves running economy and injury resistance is the heavy kind, and heavy is defined relative to max. Squats or deadlifts at 85–95% of 1RM for a few reps build force without bulk; 70–85% suits muscle-building phases. Without an estimate, most runners drift toward weights too light to change anything.
Is it safer to estimate than to test a real max?
For most runners, yes. A true max attempt concentrates injury risk into one maximal effort and really wants a spotter, a long warm-up, and practiced technique. A sub-maximal set of 3–6 reps carries ordinary training risk and, run through these formulas, gets you within a few percent of the same answer.
The 1RM shown here is a statistical projection from your set, not a measured lift, and it degrades as reps rise. Heavy resistance training loads joints, tendons, and the cardiovascular system: progress gradually, prioritize technique, and use a spotter on barbell lifts. If you are new to lifting, managing a heart condition or an injury, or returning after a long break, clear the program with a clinician or qualified coach first — this page is general fitness information only.