How the 1RM Calculator Works

Your one-rep max is the heaviest weight you could lift for a single repetition with full range of motion and no assistance. Testing one is demanding and carries real risk, so the usual approach is to work backwards from a set you have already done: if you know what you managed for five reps, the relationship between reps and load lets you estimate the single.

The Two Formulas

Two published models are calculated and averaged. Each was fitted to real lifting data, and they behave differently as the reps climb — averaging them cancels some of the bias either carries alone. These are the same two formulas every lift-specific calculator on this site uses, so a set entered here and in the Bench Press or Back Squat calculator gives the same answer.

The pairing is deliberate: Brzycki is strongest at low reps, Wathan holds up better at moderate ones, and together they cover the full 1–10 range the calculator accepts. The headline number is their average, rounded to the nearest 5 lb or 2.5 kg — roughly the smallest jump most gyms can actually load. The calculator also shows each formula's own answer, because the gap between them is useful information: when they disagree by a lot, your estimate is soft.

Why a Single Rep Skips the Maths

This is worth stating plainly. Enter one rep and Brzycki returns exactly the weight you lifted, but Wathan returns 1.3% more. Averaged, they would tell you your max is about 0.7% above a weight you have demonstrably already lifted for a single.

That is nonsense, so the calculator doesn't do it. Enter 1 rep and your weight is used directly. By definition, the most you can lift for one rep is your one-rep max — and if you have a tested single, that is always the most accurate thing you can put in here.

How Accurate Is It?

Accuracy depends almost entirely on how many reps you did. At low reps you are extrapolating a short distance and the models agree closely; at high reps you are extrapolating a long way and the answer starts to depend on something the formulas cannot see — how well you personally tolerate fatigue.

• 1 rep — not an estimate at all. Exact.
• 2–3 reps — strong. The two formulas land within a few percent of each other.
• 4–5 reps — good, and comfortably inside the range these models were built for.
• 6–8 reps — moderate. Two lifters with identical sets can have genuinely different maxes by this point.
• 9–10 reps — weak. Rep endurance is doing much of the work in the answer.

Entries are capped at 10 reps for that reason. There are formulas that claim to work at 15 or 20, but the number they produce says more about your work capacity than your maximum strength.

The set also has to be honest. These models assume a set taken to the point where another rep was not available. If you stopped two reps short, the estimate will be low; if the last rep involved a bounce, a heave or a spotter's fingertips, it will be high.

Does the Exercise Change the Answer?

No — and the calculator says so rather than implying otherwise. Neither formula is lift-specific; enter 100 kg for 5 reps and you get the same number whether you selected bench press or leg press. The exercise dropdown selects a reliability note, because how well a rep-max estimate describes a lift varies a great deal:

• Bench press — the best case. Both formulas were validated primarily against bench data, so this is the lift they actually know.
• Back squat — also strong. Sets end because you run out of force, which is exactly what the formulas assume.
• Deadlift — the weakest of the barbell lifts here. Every rep starts from a dead stop and fatigue accumulates faster, so high-rep sets tend to overestimate.
• Front squat and overhead press — sets often end for positional reasons (the rack position collapses, the torso starts leaning back) rather than because the working muscles quit. The formulas cannot tell the difference.
• Leg press and machines — the models were never validated on these at all. Sled weight, angle and friction vary between machines, so the number is only meaningful against your own previous results on the same machine.

What This Calculator Deliberately Doesn't Do

It gives you a number and stops there. It doesn't ask your bodyweight, age or sex, and it doesn't tell you whether the result is good. That is on purpose: a general-purpose estimate applied across every exercise cannot support percentile claims or strength tiers.

If you want to know where a lift stands, the lift-specific calculators on this site do that properly — scored against percentile data for that exact movement, at your bodyweight and age.

Limitations

• It is an estimate, not a measurement. The only way to know your one-rep max is to lift it.
• Rep endurance varies between people. Two lifters with the same true max will manage different rep counts at the same load, and these formulas assume one shared relationship for everybody.
• The formulas were built on trained lifters doing free-weight compound lifts. Novices, isolation work and machines all sit outside what they were fitted to.
• Fatigue on the day matters. A set done fresh at the start of a session and the same set done at the end are not the same evidence.
• Technique drift is invisible to the maths. Range of motion, bounce, and assistance all change what the number means, and none of them are inputs.

Disclaimer:
This calculator provides an estimate based on established rep-to-1RM formulas and the values you enter. Real performance depends on technique, range of motion, training history, recovery, fatigue and individual variation. Estimating is safer than testing, which is the point of the tool — but if you do attempt a maximal lift, warm up thoroughly, use appropriate safety measures, and don’t attempt heavy singles beyond your experience. This tool is for general informational purposes only and should not be considered medical, fitness, or training advice. Consult a healthcare provider before beginning a new training program, especially if you have pre-existing injuries or conditions.