One Rep Max Calculator
Estimate your one-rep max (1RM) from any weight and rep count between 1 and 12 using the Epley and Brzycki formulas — get both estimates plus a combined training number.
How It's Calculated
Formula
\text{Epley: } 1RM = weight \times \left(1 + \frac{reps}{30}\right)
\text{Brzycki: } 1RM = weight \times \frac{36}{37 - reps}A one-rep max (1RM) is the heaviest weight you could theoretically lift for a single complete repetition of an exercise. Most lifters never test it directly — maxing out is fatiguing and carries more injury risk than working with lighter, higher-rep sets — so training programs instead estimate 1RM from a set you've actually performed (a weight and a rep count) and use that estimate to set percentage-based training loads (e.g. "work up to 80% of your estimated 1RM for 5 sets of 3"). This calculator applies two independently developed, widely cited estimation formulas — Epley (1985) and Brzycki (1993) — to the same weight and rep count, and reports both individually plus a combined estimate. Estimates get progressively less reliable as rep count rises, because fatigue and technique breakdown at high reps don't scale linearly with the load-to-failure relationship either formula assumes; this calculator restricts input to a conservative 1–12 rep range for that reason and does not extrapolate estimates beyond it.
Worked Examples
100 kg for 5 reps
- Epley: 1RM = 100 × (1 + 5/30) = 100 × 1.16667 = 116.67 kg
- Brzycki: 1RM = 100 × 36 / (37−5) = 100 × 36/32 = 112.5 kg
- Combined estimate (average of both): (116.67 + 112.5) / 2 = 114.58 kg
100 kg for 1 rep
- Epley: 1RM = 100 × (1 + 1/30) = 103.33 kg
- Brzycki: 1RM = 100 × 36/36 = 100.00 kg — an exact identity, because the Brzycki formula's denominator (37 − reps) equals 36 at reps=1, which cancels the 36 in the numerator
- Note the two formulas do not have to agree: Epley's model still applies a small ~3.3% adjustment even at 1 rep, since it isn't built around the same reps=1-collapses-to-identity structure Brzycki has
Frequently Asked Questions
Why do the Epley and Brzycki formulas give different answers for the same set?
They're two independently developed mathematical models (Epley, 1985; Brzycki, 1993) that both approximate the same underlying relationship — how load-to-failure declines as rep count rises — using different curve shapes. Neither is universally more accurate across every rep range; they simply weight the reps-to-fatigue relationship slightly differently. This calculator reports both, plus their average as a single reference number, so you can see the spread rather than treating either formula as the single correct answer.
Why is the rep count capped at 12?
Both formulas were validated primarily on low-to-moderate rep sets (roughly 1–10 reps), and general strength-training literature considers 1RM estimates from sets beyond about 10–12 reps unreliable — fatigue and technique breakdown at high rep counts don't scale the way either formula's fatigue curve assumes. Rather than silently return a number from an unsupported input, this calculator hard-rejects rep counts outside 1–12.
Why does Brzycki give exactly the input weight back at 1 rep, but Epley doesn't?
It's a property of the two formulas' structures, not a bug. Brzycki's formula is weight × 36 / (37 − reps); at reps = 1 the denominator is exactly 36, which cancels the numerator's 36 and returns the input weight unchanged — a true mathematical identity. Epley's formula is weight × (1 + reps / 30); at reps = 1 that's weight × 1.0333, a deliberate ~3.3% upward adjustment even for a single rep. Neither behavior is wrong — they're just different models — but it's a genuinely interesting distinction worth knowing if you ever wonder why a 1-rep set doesn't return identical numbers from both formulas.
Is the number this calculator gives me safe to attempt?
No — this tool estimates a number for training-percentage programming purposes only. It is not a safety or medical clearance and does not account for your technique, warm-up, equipment, injury history, or whether a qualified spotter is present. Treat the estimate as a planning reference, not as a weight to attempt without proper form and supervision.
What if I enter 0 reps or a negative weight?
The calculator rejects both. Repetitions must be a whole number from 1 to 12, and weight must be a number greater than zero — there's no meaningful one-rep-max estimate for zero reps or a non-positive weight, so these are treated as invalid input rather than silently producing a number.