How accurate are one repetition maximum calculators?

Calculators and tables necessarily contain a margin of uncertainty because they cannot account for our individual differences in genetics and training.

A greater proportion of Type I and Type IIa fibres gives our muscles lower fatigability, and hence results in our overestimating our one-repetition maximum. This is particularly true when our sub-maximal effort is in the higher repetition ranges. The same applies when we have done a lot of strength-endurance training, maximising ‘time under tension’. Conversely, of course, a greater proportion of Type IIb fibres, and/or a dominance of power training will tend to result in our underestimating our one-repetition maximum—again particularly if our sub-maximal effort is in the higher repetition ranges.

The accuracy of the formulae employed by these calculators varies, but all of them produce high average error, both absolute and relative. Thus, it should be understood that they are only suitable for broad estimation. For an accurate assessment of our one-repetition maximum, we should perform a maximal strength test.

I hope that helps.

In my experience they become less accurate the higher rep count you put in. So if you put in your 3RM in to the calculator, it will provide a fairly accurate idea of what your 1RM is. If you put in a 15RM, then it'll be way off.

The thing about rep-maxes is they can vary slightly day to day. Especially the low-rep maxes. It could be that on that day you got your 3RM you were at your peak, but on the day you attempt your 1RM you are slightly off your game. This will also throw off a calculator because you're inputting data for your peak day, so it's going to calculate as if you are always on your peak day.

A true 1RM attempt should be prepared for. It's by definition the maximum amount of weight you are physically capable of holding. It's going to be at a level of difficulty, uncomfort, and strain that you're not used to. So you need to put your body and mind in a place that can perform.

It sounds like you were close. You could have probably hit 46kg if you had prepared for it. If your sleep was spot on, if your diet was on point, if you were properly rested and recovered, if you really grinded through it. Maybe use a bit more caffeine or sugar than normal. So you can deload for a week and try again.