If you lift a weight for five reps to failure, how you calculate what your one-rep maximum would be from that? That’s the purpose of this article.
If you just want to run a calculation rather than read this, try my one-rep max calculation page.
The first thing I should say as that all one-rep maximum calculators are prone to error. They are just estimates based on formulae and you’ll often find your actual one-rep maximum to be different from the one calculated by the formula. They are nevertheless a good measure of progress. Most weight trainers don’t shoot for one-rep maximums at every workout but if, for example, you increase the weight you can use for five reps, then the one-rep maximum calculated from that will increase, thus showing you’ve progressed.
The other thing to note about one-rep maximum formulas is that they become more inaccurate the higher the number of reps you calculate them from. A one-rep maximum calculated from five reps is likely to be much more accurate than a one-rep maximum calculated from 15 reps and any calculation made for over 20 reps is likely to be fairly useless.
Then again, some of them even produce inaccurate values if you enter your values for one rep! Such is the nature of formulae.
There are two main ways you can calculate one rep maximums: via a formula or by using a table of coefficients, one for each rep count that when multiplied by the coefficient gives the one rep maximum. A table of coefficients will generally be more accurate because they can be altered specifically for the number of reps and you can personalise the coefficients to match what you actually lift. A formula is, however, easier to apply because you just plug in the weights and the reps and it gives you your one rep maximum.
Remember that to use these things to best effect, you need to lift almost to failure. So if you want to plug in five reps and find your one-rep maximum from that, you need to use a weight you can only just lift for five reps and a sixth rep would not be possible.
As I said, I have developed a calculator based on coefficients you can test out here.
For the various formulae used take a look at this Wikipedia page.