# Do 1RM calculation formulae start to become inaccurate as the rep counts go way up?

Since I did a set of 20x50kg barbell bench press tonight, according to my workout tracking app, my 1RM for BBBP is therefore 150kg. However, I simply know this to be false as I’ve never been able to bench more than 90kg. My app says that it uses the Brzycki 1RM estimation formula.

A trusted mate at the gym has told me that 1RM extrapolation formulae are pretty well studied and refined, and seem to hold accurate pretty consistently across various different individuals and contexts.

Obviously I generally don’t do such high rep counts, and I know that most of the yard stick guides to rep ranges tend, after “strength” (3-5) & “hypertrophy,” (8-12) to max out at “15+” for “endurance.”

Are these formulae known to become less reliable and applicable when one gets into especially high rep ranges like the 20 I did today (admittedly with a very very brief break that I needed in order to realign the weight plates as I’d forgotten to fasten a clip onto the ends)?

• Does 1-rep-max not have a topic tag? Jan 6 at 23:56
• Not the question, but I’d highly recommend benching without collars. Jan 7 at 0:59
• Lol that formula says I can bench 507 based on my 225 AMRAP. And it’s negative based on my 135 AMRAP. Jan 7 at 1:03
• Of course some formula can never accurately predict 1RM from other rep ranges but one that I've used a few times and is accurate enough to be slightly useful is - strengthlevel.com/one-rep-max-calculator Jan 7 at 17:29
• @ThomasMarkov yes I’d certainly agree it is advisable though in this case it was screwing my up (rapid jerky movements, low weights). Jan 7 at 22:27

Yes, the formulas are certainly less accurately beyond 10 reps. This is because the formulas are estimates. If you plotted them on a curve, you'll notice that by 20 reps they are giving very different answers.

The most relevant research here is "The Accuracy of Prediction Equations for Estimating 1-RM Performance in the Bench Press, Squat, and Deadlift", which concludes:

The data suggest that repetitions to fatigue (10 or fewer reps) can accurately predict 1-RM lifts. In general, the formulas most accurately predicted the 1-RM bench press, followed by the squat and then the deadlift. A follow-up study to determine a specific formula for the deadlift would contribute to the accuracy of predictions for that lift.

One-rep max formulae are all unreliable past about 10 reps. Brzycki specifically advises against using his formula with that many reps.

It appears the relationship [between anaerobic endurance and strength] is not quite linear beyond about 10 reps. Therefore, this formula is only valid for predicting a 1-RM when the number of reps-to-fatigue is less than 10.

Matthew Brzycki, Strength testing-- predicting a one-rep max from reps-to-fatigue, Journal of Physical Education, Recreation & Dance, January 1993, page 90

Note, this is a limitation of the phenomenon being estimated, not the specific calculation. Predictions diverge between methods, especially beyond about 10 reps, because output is highly variable given that input.

1RM calculation formulae start to become inaccurate when the reps performed are greater than or equal to 2

All 1RM calculation formulas assume strength and endurance increase exactly in step with each other. They have no way to account for the fact that some people have muscles that are better at endurance efforts, while others have muscles that are better at brief, maximal strength efforts. As the reps get higher, these errors get more and more significant, and another source of error appears, in that the energy used to perform the lift shifts from coming entirely from the totally anaerobic ATP-CP energy system, to the anaerobic glycolytic energy system and even a little from the aerobic energy system. These rate at which these systems can provide energy is highly dependent on your breathing, and so a set where you take a few breaths between each rep becomes a very different effort to one where you strictly only breathe out and in again between each rep.

As an example of this known inaccuracy, there's a traditional bodybuilding workout called "20 rep squats", in which you take your predicted 10RM weight and squat it for 20 reps, breathing as much as you need between reps. The fact that you can squeeze 20 reps out of a supposed 10 repetition maximum weight shows how much leeway there is in these estimates when things like breathing between reps and individual differences aren't accounted for.

All up, if you're trying to estimate 1RM, you're likely to get highly inaccurate results if you're basing that on anything other than a maximal effort set with perhaps 3-5 reps maximum. However, these formulas are still quite useful the other way around, in that if you know your 1RM, they can be used to select a ballpark-accurate weight for a 10 rep set, which can then be adjusted on the basis of whether the first set felt too easy or too hard.