The formula
How to calculate one-rep max calculator
A one-rep max (1RM) is the heaviest weight that could theoretically be lifted for a single rep. Rather than testing that directly — which carries real injury risk at high loads — it is normally estimated from a lighter set taken close to failure, using one of several published formulas that model how strength falls off as reps increase.
All six formulas here take the same two inputs — the weight lifted and the number of reps completed with it — and extrapolate to a single-rep equivalent. They agree closely in the 1-10 rep range, which is where they were originally derived and validated, and diverge more the further out they are pushed. The rep-max table (2RM through 15RM) is built by re-running the same formula backwards from the estimated 1RM using the Epley relationship, so it stays internally consistent with whichever formula is selected, rather than mixing formulas within one table.
What to enter:
- Weight lifted (kg)
- Repetitions performed
- Formula
No submit button: type and the answer moves. Your inputs end up in the link, so the page can be shared already filled in.
The order the fields are filled in makes no difference to the result — the calculator recomputes the whole formula from whatever is currently in every field, not step by step. That means it is safe to adjust one number, watch the result change, and adjust it back, without worrying about resetting anything first.
Why one-rep max calculator matters
A calculation like this usually gets used at a decision point rather than out of curiosity — comparing two real options, checking a number a lender or adviser has quoted, or working out whether a plan that sounded fine in conversation still holds up once it is written down with actual figures. The maths itself is rarely complicated; what is hard is remembering which figures to use and in what order, which is exactly what a dedicated calculator is for.
It is also useful as a sense check before signing anything. A quote, an offer letter or a spreadsheet from someone else can contain an error, an optimistic assumption, or simply a different convention for rounding — running the same inputs through an independent calculator is a quick way to confirm a number before relying on it.
The reason a page like this exists at all, rather than leaving the calculation to a spreadsheet or a textbook appendix, is that the formula behind one-rep max calculator is fiddly enough to get wrong by hand but not complicated enough to need specialist software. That middle ground — real enough maths to matter, simple enough to check instantly — is exactly what a dedicated calculator is for, and it is why the same figure recalculated here should match a careful manual calculation almost exactly.
A calculator like this one is often bookmarked and returned to repeatedly over months rather than used once, particularly for anything tied to an ongoing plan such as a mortgage, a savings goal or an investment being tracked. Because the figures live in the web address rather than only in memory, coming back to the same page with updated numbers is quicker than starting from a blank spreadsheet each time.
Worked example
A concrete run-through, using the values already in the fields:
- Weight lifted: 100 kg
- Repetitions performed: 5
- Formula: epley
That gives:
- Estimated 1RM: 0 kg
- 2RM: 0 kg
- 3RM: 0 kg
- 5RM: 0 kg
- 8RM: 0 kg
- 10RM: 0 kg
- 12RM: 0 kg
- 15RM: 0 kg
- 85% of 1RM: 0 kg
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
Treat the estimated 1RM as a planning number for setting training percentages, not as a literal prediction of what could be lifted today — actual performance also depends on fatigue, technique breakdown at heavier loads, and daily readiness. The 85%-of-1RM figure is a commonly programmed heavy-set intensity; the rep-max table gives rough target weights for building out a full training block across different rep ranges.
Where this goes wrong. All 1RM formulas are most accurate for sets of roughly 1-10 reps; pushed out to 15+ reps, the estimate becomes noticeably less reliable because the relationship between reps and fatigue stops being linear. The six formulas can also disagree by a few percent from each other at the same weight and reps — none of them is definitively "the correct" one, they are simply different statistical fits to different original datasets, so switching formulas is a legitimate way to sanity-check a figure rather than a sign something is broken.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
Epley and Brzycki are the two most widely used and tend to agree closely in the 1-10 rep range; there is no universal "most accurate" formula across every lifter and every exercise. If in doubt, Epley is a reasonable default, and comparing it against one or two of the others gives a useful range rather than a single point estimate.
Every formula assumes a specific curve for how much weight has to drop as reps increase to keep failing at the same effort level. That curve is well-supported by data at low-to-moderate reps but becomes shakier at high reps, where technique, pacing and local muscular endurance start to matter as much as raw strength — so the same set of formulas naturally produces a wider spread of estimates the higher the rep count goes.
The headline figure is estimated 1RM. With 100 kg weight lifted, that comes to 0 kg. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.