It's a comparison unit, not a prediction. That distinction is the whole page.
An estimated one-rep max projects the single you'd have managed from a set you actually did. Lift 100 kg for 5 and a formula turns that into a number you can compare against 90 kg for 8, which raw weight on its own can't do. It saves you from testing a true max, which costs a session and the fatigue that follows it.
Every estimator works the same way: it assumes a relationship between reps and load, and extrapolates. The assumptions differ, so the answers differ. Epley is linear in reps. Brzycki is a ratio that steepens as reps rise. Lombardi is a power curve. None of them is measuring anything. They're all fitting a curve to a population and applying it to you.
The useful way to hold it is this: an e1RM is a unit of comparison against your own past e1RMs, computed the same way. It is not a number to walk up to a bar and attempt.
Read the trend, not the value. Whether your e1RM says 140 or 148 matters far less than whether the same formula on the same lift said 132 six weeks ago. The absolute figure carries the formula's error. The direction mostly doesn't, because the error is roughly constant for you at a given rep range.
Feed it good sets. Estimates come from sets taken reasonably close to failure. A 100 kg set of 5 stopped at 5 reps in reserve produces a projection that's simply wrong, and the formula has no way to know. This is the point where e1RM and RPE stop being separate topics.
Keep the rep range steady when you compare. An e1RM from a set of 3 and an e1RM from a set of 12 are being produced by very different parts of the curve, and switching between them will manufacture progress or regression that didn't happen.
The most expensive mistake is comparing an e1RM across apps or across formulas and concluding something about your training. A tracker showing 108 kg and ours showing 111 kg for the same set are not disagreeing about you. They're using different arithmetic. Comparison is only meaningful within one formula.
The second is trusting a projection from a high-rep set. Epley is linear, so it has no upper limit and will happily convert a set of 20 into a confident-looking number. It's noise. The rep-to-load relationship stops behaving linearly somewhere around 10 to 12 reps for most people, and past that any estimator is guessing about a region it was never fitted for.
The third is treating the number as a lift you own. An estimated max is what the maths says was available on that day, from that set, under that formula's assumptions. Plenty of lifters have an e1RM they've never touched, and a few have a tested max above it.
Take 100 kg and run the same set through Epley, which is what we use, and Brzycki, which is what a lot of other calculators use.
At 5 reps, Epley gives 100 x (1 + 5/30) = 116.7 kg. Brzycki gives 100 x 36/(37-5) = 112.5 kg. Epley reads 4.2 kg higher, about 3.7%.
At 10 reps they agree exactly. Epley gives 100 x (1 + 10/30) = 133.3 kg. Brzycki gives 100 x 36/27 = 133.3 kg. Same number.
At 15 reps the order flips. Epley gives 150.0 kg and Brzycki gives 100 x 36/22 = 163.6 kg, so now Brzycki is the higher of the two, by 13.6 kg. The common claim that Epley runs high at high reps is the wrong way round when you actually compare it against Brzycki.
One more, and it's the honest limitation of the formula we picked. At 1 rep, Epley gives 100 x (1 + 1/30) = 103.3 kg. You lifted 100 and it says 103. Epley does not return your actual single, which is a real flaw and the reason it should never be used to settle an argument about a tested max.
Fire Your Coach uses Epley: weight x (1 + reps / 30). It's a single function in trajectory.js and every e1RM anywhere in the product comes from it, so the number on your strength trend, the number on your session receipt and the number in your intake summary are all the same measurement. The app names the formula on screen rather than hiding it.
Epley was chosen because it's the most common formula across the trackers our users import from, which means an imported history lands on the same scale as the training you do here. That's a compatibility argument rather than an accuracy argument, and it's worth being clear about which one it is.
It runs on completed working sets only. Warm-ups, protocol rows and time or distance entries are excluded, so a 12-minute row can't manufacture a strength number. The strength trend takes your best e1RM per lift per week across your five most-trained compound lifts, deliberately preferring compounds because lateral raises produce tiny values that drag an average around without telling you anything.
Beat your own best inside a rolling 13-week window and your session receipt prints an Estimated 1RM PR, showing the mark it beat and the date it was set. That record is written at completion and never recalculated afterwards, because the window moves and a record that silently expired would be worse than no record.
The papers behind the empirical claims on this page, with the population each one studied. Where we could confirm only what a study set out to measure and not the size of what it found, the note says so rather than borrowing the paper's authority for a number nobody read.
Jerry L Mayhew et al. (2007). Using absolute and relative muscle endurance to estimate maximal strength in young athletes. International Journal of Sports Physiology and Performance 2(3): 305-314.
All 7-to-10-RM prediction equations correlated with actual 1RM at r > .98, higher than the absolute-load equation at r = .95. Despite those correlations, only 3 of 11 equations produced predicted values that were not significantly different from actual 1RM.
High school male athletes. Bench press only.D A LeSuer et al. (1997). The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. Journal of Strength and Conditioning Research 11(4): 211-213.
Correlations above 0.95 across the equations tested, and all tested equations significantly underestimated the deadlift.
67 untrained college students. This limit matters and the page carries it.Epley: weight multiplied by (1 + reps / 30). It's named on screen and it's the only estimator in the product, so every e1RM you see comes from the same arithmetic. We picked it because it's the most common formula across the apps people import from, which keeps an imported history on the same scale as new training.
Roughly up to 10, and treat anything past 12 as a curiosity. The relationship between reps and load stops behaving predictably beyond that range, and Epley in particular is linear, so it will keep producing larger numbers from longer sets without any limit. A set of 20 will give you a figure. It will not give you information.
Not necessarily, and we record them separately for that reason. A rep PR means more reps at a weight you've handled before. An Estimated 1RM PR means the projected ceiling beat your previous best, which can also happen from a heavier set at fewer reps. One session can set both, one, or neither.
Yes, and that is usually the honest read on progress. Epley multiplies the weight by one plus reps over thirty, so 100 kg for 5 estimates 116.7 and 100 kg for 6 estimates 120. Adding a rep at a fixed load is real progress, and a number that only moved when the weight moved would miss most of it. It also means a set taken far past your usual rep range inflates the estimate, which is why we treat e1RM as a trend line rather than a load to program from.
Fire Your Coach reads the intensity your plan asks for, shows it on the set, and adjusts it when your readiness says today is not the day. Nothing to configure.