Skip to content
ToolBoxGeniehome

One-Rep Max Calculator

Calculators · Added

Six published formulas estimate a one-rep max from a submaximal set, and they disagree — which is the most useful thing about them. Where they spread widely the estimate is weak, and showing one number would hide that.

Clean reps to near failure

Unit

How to use the one-rep max calculator

  1. 1Enter the weight and how many clean repetitions you completed with it.
  2. 2Read the median estimate and, more importantly, the range around it.
  3. 3Check the loadable figure, which is rounded to plates that exist.
  4. 4Use the percentage table to set working weights for a programme.

Examples

A heavy set of five

Input
100 kg × 5
Result
About 115 kg, with the six formulas spanning 112.5 to 117.5

A spread of 4.3% — tight, because five repetitions is well inside where the formulas were fitted.

A light set of fifteen

Input
60 kg × 15
Result
The formulas spread by more than 15%, and the page says the estimate is weak

About the one-rep max calculator

Why estimate rather than test

A true one-rep max is the most technically demanding and highest-risk repetition available: maximum load, maximum fatigue, and the greatest chance of form breaking down at the worst moment. Testing one properly needs a warm-up protocol, spotters and a reason.

Estimating from a set of three to five gives you a number good enough to set working weights from, at a fraction of the risk and without the days of recovery a true max attempt costs. That is the trade, and it is why almost every training programme is written in percentages of an estimated maximum rather than a tested one.

Reading the percentage table honestly

The repetition figures beside each percentage are the conventional ones from strength coaching literature, and they are a guide rather than a measurement. They vary by lift, by training age, by how rested you are and by how close to failure you are willing to work.

The practical use is not to hit the listed repetitions exactly but to notice when you are a long way off. Consistently getting far more repetitions than the table suggests means the estimated maximum is low; consistently getting fewer means it is high, or that the day is not going well.

Frequently asked questions

Which formula should I use?
None of them alone. They were fitted to different populations on different lifts, so the median of the six is a more robust starting point than any single one, and the spread tells you how much to trust it. Wathan tends to sit closest to tested maxima in published comparisons; O'Conner is the most conservative; Brzycki and Lander climb steeply at high repetitions.
Why does it refuse more than 20 repetitions?
Because past roughly ten the formulas stop measuring strength and start measuring endurance, and none of them was fitted that far out. Brzycki makes the problem visible: its denominator reaches zero at 37 repetitions and the estimate becomes infinite. That is the model announcing it has left its range, and 20 is a generous stopping point well before it.
How accurate is an estimate like this?
Good enough to programme from, not good enough to load a bar at. From a heavy set of three to five the estimates usually agree within a few per cent; from a set of twelve they can differ by 15% or more. The honest reading is the range rather than the midpoint, which is why the range is shown as prominently as the number.
Is it different between lifts?
Yes. Most people get more repetitions at a given percentage on a squat or a deadlift than on a bench press or an overhead press, so a formula fitted on one lift over-predicts on another. Training age matters too: an experienced lifter typically gets fewer repetitions at 90% than a novice, because they can actually express their maximum.

Related tools