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It is quite obvious that every online calculator is off because of individual differences, but math tends to be spot on everything and when it doesn't it's because we lack the needed data... With enough data a computer can simulate the entire universe at 100% accuracy... But we don't have enough data for that.

strengthlevel.com tells me that my one rep max pull up is with 46 kilograms of added weight or 101 pounds.

I tried it, and well..kinda failed the rep...failed the rep on 8 attempts that same day and I always got to almost chin to bar...almost chin to bar but not quite. Which counts as failed rep. Specially since I'm used to do repetitions where I touch my sternum bone to the bar.

Seems the calculator was just slightly wrong, but not by much. Does anyone know by how much exactly can one rep max calculators be off?

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    I think you're putting too much stock in these calculators. They're not based on huge principles or anything, they're generally based on making some observations (with these types of calculators often with a fairly small sample set) and working backwards from there. – Cubic Jun 29 at 10:20
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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.

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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.

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