You lifted 80 kg for 5 reps and one calculator says your one-rep max is 93 kg. Another says 90 kg. Neither number is a guarantee, and knowing why keeps your training both effective and safe.
What a one-rep max actually is
Your one-repetition maximum, or 1RM, is the heaviest weight you can lift for a single repetition of an exercise. It is a snapshot of your maximal strength during one contraction. Lifters use it for three main things: gauging their maximum strength, deciding winners in powerlifting and weightlifting competitions, and setting training loads as a percentage of that maximum.
The most reliable way to find it is a direct test: you lift progressively heavier weights until you reach the heaviest one you can complete once. The downside is that this takes time and carries a real risk of injury without correct technique and a spotter. That is why most people estimate it instead, from a lighter weight they can lift several times.
The two formulas most calculators use
Most one-rep max estimates come from one of two formulas. In both, w is the weight you lifted and r is the number of reps you completed with it.
- Epley formula: 1RM = w × (1 + r ÷ 30). In plain words, every rep you complete adds one thirtieth of the weight to your estimated max.
- Brzycki formula: 1RM = w × 36 ÷ (37 − r). Here, the more reps you do, the smaller the divisor gets, so the estimate climbs further above the weight you actually lifted.
A worked example with real numbers
Say you complete 5 reps at 80 kg.
- Epley: 80 × (1 + 5 ÷ 30) = 80 × 1.1667, which is about 93.3 kg
- Brzycki: 80 × 36 ÷ (37 − 5) = 80 × 36 ÷ 32 = 90 kg
Same lift, two estimates that sit 3.3 kg apart. Now take a set of 10 reps at 60 kg. Epley gives 60 × (1 + 10 ÷ 30) = 80 kg, and Brzycki gives 60 × 36 ÷ 27 = 80 kg. At exactly 10 reps the two formulas agree, but below 10 reps Epley comes out slightly higher.
Why your real max can be 10% away from the estimate
Estimation formulas are faster and safer than a max test, but they remain estimates. The gap between the calculated number and your true 1RM can reach 10 percent or more. On a 90 kg estimate, that means your real max could sit anywhere from roughly 81 kg to 99 kg.
The error is not evenly spread either. Estimates tend to work best for experienced lifters. Beginners often reach a real maximum lower than the formula predicts, because their nervous system is not yet used to handling very heavy loads. If you are new to lifting, treat the number as a ceiling rather than a target.
How to use your estimate without getting hurt
- Pick one formula and stick with it, so your progress is measured on the same scale from week to week.
- When a program asks for a percentage of your max, work from the more conservative figure. If a plan calls for 70 percent, that is 63 kg from a 90 kg estimate, but about 65.3 kg from 93.3 kg.
- Do not read a formula result as permission to load that weight right away, especially as a beginner.
- If you do test a true single, use correct technique and a spotter, since the direct test is where the injury risk sits.
The bottom line
A one-rep max formula turns one ordinary set into a useful strength number in seconds. Just read it as a range, not a verdict: two formulas can disagree by a few kilos, and your real max can land 10 percent away from either one.
