One-rep max (1RM) calculators often disagree. A set that one calculator turns into a 300 lb max might come out at 285 lb on another. The reason isn't a bug: each calculator uses a different formula, and the formulas are only approximations of how many reps a person can do at a given share of their maximum.
Common 1RM Formulas
Brzycki formula:
1RM = Weight ÷ (1.0278 − 0.0278 × Reps)
Example: 200 lb × 5 reps = 1RM of 225 lb
Epley formula:
1RM = Weight × (1 + Reps ÷ 30)
Example: 200 lb × 5 reps = 1RM of 233 lb
Lombardi formula:
1RM = Weight × Reps^0.10
Example: 200 lb × 5 reps = 1RM of 235 lb
Same lift, a 10 lb spread between formulas.
Why the Formulas Disagree
Each formula draws a different curve through the same idea: the heavier the weight, the fewer reps you can do.
- Brzycki (1993) assumes reps fall in a straight line as the load rises, which fits well up to about 10 reps
- Epley (1985), from strength coaching at the University of Nebraska, adds about 3.3% for every rep
- Lombardi (1989) uses a power curve, which gives higher estimates from low-rep sets
The spread is smallest in the 3–6 rep range and widens at higher reps. For a 200 lb set, the three formulas give 212–223 lb at 3 reps and 252–267 lb at 10 reps.
| Reps at 200 lb | Brzycki | Epley | Lombardi |
|---|---|---|---|
| 3 | 212 | 220 | 223 |
| 5 | 225 | 233 | 235 |
| 10 | 267 | 267 | 252 |
What Else Changes the Answer
The formulas assume a set taken close to failure with good form. Several things move your real number away from the estimate:
- Rep range: estimates are most accurate from sets of about 2–6 reps and get less reliable past 10
- The lift: most people can do more reps at a given percentage on lower-body lifts, such as the squat or leg press, than on the bench press, so the same formula can underestimate one and overestimate the other
- Training background: endurance-trained lifters tend to do more reps at a given load than powerlifters, who are trained to express maximum strength in one rep
- Fatigue and technique: a set done after other heavy work, or with a shorter range of motion, gives a misleading estimate
Best Practice
- Estimate from a hard set of 3–5 reps rather than a high-rep set
- Use the same formula and the same lift each time, so changes reflect real progress rather than a switch in method
- Average a few formulas if you want a single number; the one-rep max calculator shows seven side by side
- Use the estimate to set training loads (for example 80% for sets of about 8), not as a target to test every few weeks
- If you do test a true max, warm up thoroughly and use a spotter or safety bars
An estimated max is a planning tool. Within 5% or so, it tells you everything you need to choose your training weights.