If you've ever pulled a "one-rep max" estimate from a spreadsheet or calculator app, there's a good chance one of two formulas was running under the hood: Epley (1985) or
Brzycki (1993). They look similar on paper but give different answers under fatigue. Which matters when you're programming percentages.
## The formulas
Epley:
1RM = weight × (1 + reps / 30)
Brzycki:
1RM = weight × 36 / (37 − reps)
## Worked example
Bench 100 kg for 5 reps:
-
Epley:
100 × (1 + 5/30) = 116.7 kg -
Brzycki:
100 × 36 / 32 = 112.5 kg
Same set, 4 kg gap in the estimate. That gap matters when you're prescribing "5x5 at 85%" — the two formulas produce different working loads.
## When to use each
Research comparison (LeSuer et al. 1997, Wood et al. 2002):
| Rep range | Better formula | Why |
|-----------|---------------|-----|
| 1–3 reps | Epley or either | Both fit tightly at low reps |
| 4–8 reps | Similar accuracy | Convergence zone |
| 9–10 reps | Brzycki | Epley starts overpredicting |
| 12+ reps | Neither | Both break down |
## Practical rule
Average the two. For programming safety:
function estimate1RM(weight, reps) {
const epley = weight * (1 + reps / 30);
const brzycki = weight * 36 / (37 - reps);
return {
epley: epley.toFixed(1),
brzycki: brzycki.toFixed(1),
avg: ((epley + brzycki) / 2).toFixed(1),
conservative: Math.min(epley, brzycki).toFixed(1),
};
}
estimate1RM(100, 5);
// { epley: '116.7', brzycki: '112.5', avg: '114.6', conservative: '112.5' }
Use conservative for heavy percentages you're about to attempt, avg for programming references.
## Why this matters
Neither formula was validated above 10 reps. The relationship between rep count and load isn't linear at the extremes — at 15+ reps, endurance and glycogen dominate rather than neural
drive, and no formula accounts for that.
Full breakdown with rep-load percentage table and lift-specific accuracy data at ffmicheck.com.
Built as part of FFMI Check — a free fitness calculator hub citing peer-reviewed formulas (Kouri 1995, Mifflin-St Jeor, Epley, Brzycki) instead of proprietary
math.
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