I want to predict 1RM using a formula but I am not sure which is the best to use for the male athletes ages rages from 18 to 30 years old. For example, if I can bench press 80kg 4 times what could my 1RM be?
There are many different formulae for calculating one's 1RM.
I'll list some of the easiest-to-use ones. They'll all use two single variables;
w = the weight used for testing, measured in kilograms (kg)
r = the number of repetitions managed by the athlete at
Let's say I can do 4 reps at 80kg, my expected one-rep max would be
I feel comfortable with rounding to nearest integer here, because of the overall inaccuracy of any such formula.
Let's use the same example, from your comment. With 4 repetitions at 80kg, my expected one-rep max would be
There's a list of these at the One-repetition maximum article on Wikipedia.
Which is best?
Quite frankly, there is no way of telling, simply because of the diversity of people. One formula might be best for me, but for you, it might be a different one. But I think if you try them all, they should all land pretty close to each other.
In these dealings, there will always be inaccuracies. You can't rely on any such formula to give you anything too precise, so the whole "which is more precise" question seems unanswerable.
From: One-repetition maximum at Wikipedia, in the 'Further Reading' section is this link, which should help you understand the issues involved.
Campanholi Neto, José; Cedin, Luísa; Dato, Carla Cristina; Rodrigues Bertucci, Danilo; de Andrade Perez, Sérgio Eduardo; Baldissera, Vilmar (Jun 2015). "A Single Session of Testing for One Repetition Maximum (1RM) with Eight Exercises is Trustworthy" (PDF). Journal of Exercise Physiology - JEPonline.
Once the idea of 1RM is understood, we can take multiple repetition tasks and attempt to extrapolate further information. As with any statistical inference method, the more data we have, the more accurate the prediction.
So, if for example you can benchpress 70kg six times, we have much better information to work with.
Another method is to consider an average of several of the formula listed on Wikipedia.