1RM (One-Rep Max) is the maximum weight you can lift for a single repetition. Because attempting an actual one-rep max carries a high risk of injury, it's common practice to estimate it with a formula based on a set of several reps at a lighter weight.
Here are the most common formulas. w is the weight lifted, and r is the number of reps.
- Epley —
1RM = w × (1 + r/30). The most widely used formula; works well at moderate rep counts. - Brzycki —
1RM = w × 36 / (37 − r). Slightly more conservative than Epley at low rep counts. - Lombardi —
1RM = w × r^0.10. An exponential formula that flattens out at higher rep counts. - O'Conner —
1RM = w × (1 + r/40). Estimates lower than Epley.
Because each formula behaves slightly differently depending on rep count, GymCalc shows the average of several formulas as the representative value. Remember it's most accurate at 1–10 reps, and the margin of error grows once you go beyond 10.