What a one-rep max actually is
Your one-rep max (1RM) is the heaviest weight you can lift for a single complete repetition with correct technique. It's the reference point strength programs are built around: instead of writing "lift heavy today," a program can say "work at 80% of your 1RM for 5 sets of 5," and that number means the same relative effort whether you bench 60 kg or 200 kg. The problem is that finding your 1RM by actually testing it — loading a bar until it stops moving — is slow, fatiguing, and carries real injury risk if your technique or spotting isn't dialed in. That's why almost every lifter uses a submaximal set (something you can do for a handful of reps, not just one) and a prediction formula instead.
Why an estimate beats testing for most lifters
A true 1RM attempt asks your nervous system, joints, and connective tissue to handle a load you've never handled, on a day when you can't yet know if you're fully recovered. Get it wrong and you either miss the lift (wasted warm-up, wasted session) or grind out a rep with breakdown in form that puts a shoulder or lower back at risk. An estimate, by contrast, comes from a set you were always going to do anyway — a top set of 5, or a final AMRAP set — so there's no extra fatigue cost and no extra risk. You can recalculate it every week from ordinary training data instead of testing it once and hoping it's still accurate three months later as you get stronger.
The four formulas this calculator uses
All four formulas take the weight you lifted and the reps you got, and project what one more rep of intensity would have cost you. They disagree slightly because each was derived from a different lifter dataset and fits a different curve to it.
Epley and O'Conner are linear — they add a fixed percentage of the lifted weight per rep, so they scale smoothly but tend to overestimate as rep counts climb. Brzycki and Lombardi are curved, which makes them more conservative at higher reps but slightly less forgiving of very light, very-high-rep sets. Notice that at exactly 1 rep, Brzycki and Lombardi return the weight you lifted unchanged, while Epley and O'Conner still add a small bump — a known quirk of the linear formulas, not a bug.
Which formula is most accurate for your rep range
Brzycki is generally considered the strongest match to a directly tested 1RM in the 1-to-10 rep range, which is also where it was originally validated. Epley tracks it closely up to about 6 reps, then starts running progressively higher. Lombardi sits close to the pack under 8 reps but its curve flattens out at higher reps, and O'Conner — the most conservative of the four, adding only 2.5% per rep — is the safer read once you're working past 10 reps, where the other three's assumptions start to strain. This is exactly why the calculator shows all four instead of picking a winner: the "true" answer usually sits somewhere in that spread, and the average is a more honest single number than trusting any one formula in isolation.
Worked example 1 — bench press
You bench 80 kg for 8 reps to fatigue.
Worked example 2 — back squat
You squat 140 kg for 5 reps to fatigue.
Notice the spread is proportionally tighter than in the 8-rep bench example above (about 4% of the estimate versus 5%) — at lower reps the four formulas agree more closely, and that agreement is itself a signal that the estimate is more reliable.
Reading the percentage table: training zones
Once you have a 1RM estimate, the more useful number day to day is a percentage of it, matched to what you're training for. Strength work generally sits at 85% or above for 1-to-6 reps, hypertrophy (muscle growth) work sits roughly 65-85% for 6-to-12 reps, and muscular-endurance work drops below about 65% for 12+ reps, per the NSCA's published training load chart. These bands are guides, not hard cutoffs — current research on the "repetition continuum" shows real overlap between zones, and hypertrophy in particular can happen across a wide rep range as long as sets are taken close to failure. Use the table above to pick a load, then adjust by feel: if the prescribed rep range feels much easier or harder than it should, your estimated 1RM has probably shifted.
A safety note
This tool does strength math — it is not medical, coaching, or physical-therapy advice. It cannot account for injury history, joint health, or technique quality. If you choose to test a true 1RM rather than estimate one, do it with a qualified spotter or safety bars, a full warm-up ramp, and only on lifts where your technique is already solid at heavy weight. When in doubt, train from the estimate and skip the max-out.
Frequently asked questions
How many reps should I use to get the most accurate 1RM estimate?
Somewhere between 2 and 8 reps. Every prediction formula assumes a roughly linear or hyperbolic drop in strength as reps go up, and that assumption holds well under about 10 reps. Push it to 15 or 20 and the estimate drifts further from what you could actually lift once, because fatigue and technique breakdown stop scaling predictably.
Why do the four formulas give different numbers for the same set?
Each one was built from a different dataset and a different curve. Epley and O'Conner are straight lines that add a fixed percentage per rep, so they run a little high. Brzycki and Lombardi curve, so they run more conservative as reps climb. At 2-3 reps the four are usually within 1-2% of each other; by 10-12 reps they can spread 5-8%.
Is an estimated 1RM as accurate as actually testing my max?
No, it's an estimate, not a measurement. A true 1RM test captures your actual neural readiness, technique under a true maximal load, and psychology on that specific day, none of which a formula can see. What the formula gives you instead is a repeatable, low-risk number you can recalculate every week from ordinary training sets, which for programming purposes is usually more useful than one risky true-max attempt a few times a year.
Which of the four formulas should I trust most?
For sets of 6 reps or fewer, Brzycki and Epley are the most widely cited and tend to track true 1RM closely. For sets past 10 reps, O'Conner's flatter curve is usually the safer read since Epley and Lombardi both start overestimating. This calculator shows all four plus their average specifically so you aren't locked into one formula's blind spot.
How often should I retest or recalculate my one-rep max?
Recalculate it from your top working set every 2-4 weeks, or any time a set feels noticeably easier or harder than your current percentages predict. Because the formula only needs a normal working set, not a maximal attempt, there's no fatigue cost to updating it often.
Is it safe to attempt a true 1-rep max in the gym?
It carries more injury risk than submaximal training, especially on the squat and bench press, and it should only be attempted with a spotter or safety bars, a proper warm-up ramp, and solid technique already in place at heavy weights. Beginners and anyone lifting alone are better served by an estimate from a hard set of 3-8 reps.