How the estimate is worked out
The calculator averages the two most widely used formulas, Epley (1985) and Brzycki (1993). Both turn the weight and reps of a hard set into the weight you could probably lift once. They agree closely up to about 10 reps and drift apart after that, which is why a set of 10 reps or fewer gives the best estimate.
The training weights come from the percentage chart published by the National Strength and Conditioning Association (NSCA): most people can do about 8 reps at 80% of their max and about 10 at 75%. People vary, so treat the rep counts as a guide.
Why estimate instead of testing?
A true one-rep max test is a skill of its own. It needs a good warm-up, spotters or safety bars, and it is tiring to recover from. For most people building muscle, an estimate from a normal working set tells you everything you need: whether you are getting stronger, and what weights to use.
- Use one exercise at a time. A bench press estimate tells you nothing about your squat.
- Compare like with like. Re-check with the same exercise, the same form and a similar number of reps, and the trend over weeks shows whether you are progressing.
- Stop a set when your form breaks down. Reps done with poor form make the estimate meaningless and raise the risk of injury.
Next steps
Strength goes up when the training adds a little more over time. Read progressive overload explained and how many sets per week you need to build muscle, or apply for coaching to get a program written around your numbers and adjusted every week.
General guidance for healthy adults. If you have an injury or a medical condition, check with your doctor or physiotherapist before lifting heavy.