One rep max calculator

What a one rep max is and how it is calculated

Your one rep max, or 1RM, is the heaviest weight you can lift once with full range of motion and no help from anyone else. It is the standard way lifters and coaches compare strength across people and across time, because it strips training down to one number on the bar.

Testing it directly means working up to a genuine single, which takes a proper warm up, fresh legs and usually a spotter. Most training days do not call for that, so the practical route is to estimate it from a set you already did: a few reps at a weight that was hard but not a grind to failure.

How the estimate works

Every formula on this page starts from the same idea: as the number of reps in a set goes up, the weight you can move for it goes down in a fairly predictable curve. Ten reps is roughly 75 percent of what you could lift once; three reps is closer to 90 percent. The formulas are simply different mathematical shapes fitted to that curve.

Worked example

Say you squatted 100 kg for 5 reps and picked Epley, the calculator's default. Epley's formula is weight multiplied by one plus reps divided by 30: 100 × (1 + 5 ÷ 30) = 100 × 1.167 = 116.7 kg. That is your estimated 1RM, before rounding to a sensible plate jump.

Now compare a higher rep set on the same lift. If that 100 kg had instead moved for 10 reps, Epley gives 100 × (1 + 10 ÷ 30) = 133.3 kg. Epley adds a fixed weight ÷ 30 for every rep, so each extra rep adds the same amount here, 3.3 kg: 116.7 kg at 5 reps, 133.3 kg at 10. The reason a 10 or 12 rep input set is still treated as a rough ceiling rather than a precise figure is not the shape of the curve, since Epley is a straight line, but the set itself, where fatigue and technique breakdown start to limit how many reps you get before pure strength does.

Which formula to use, and why Epley is the default

Five formulas are built into the calculator: Epley, Brzycki, Lombardi, Wathan and O'Conner. All five were tested against real, tested one rep maxes in the bench press, squat and deadlift by LeSuer and colleagues in 1997, which is why they are worth using instead of one you found on a forum.

Epley

Weight × (1 + reps ÷ 30). The most widely used formula in gyms and apps, and a reasonable middle ground across rep ranges. It is the default here because it is the number most lifters will have seen elsewhere, not because it is the most accurate. LeSuer et al. found Wathan the closest to tested maxes, with most equations including Epley underestimating the squat and deadlift.

Brzycki

Weight × 36 ÷ (37 minus reps). More conservative than Epley up to about 10 reps, after which its denominator collapses and it overtakes every other formula on this page, which is why it should never be used on high rep AMRAP sets.

Lombardi

Weight × reps to the power of 0.10. An older formula with a flatter curve than the others. It reads slightly higher than Epley and Brzycki in the 2 to 5 rep range, then crosses below Epley at around 6 reps and below Brzycki at around 8, giving the most conservative estimate of the five by 11 or 12 reps. Worth checking against the others rather than trusting alone.

Wathan

A curve fitted with an exponential term rather than a straight ratio. It agrees closely with Epley across the 3 to 8 rep range this calculator is aimed at, and separates out more at very high or very low reps.

O'Conner

Weight × (1 + 0.025 × reps). The most conservative of the five from about 6 to 10 reps. A sensible pick if you would rather under promise, for example when setting a training max for a new block.

None of the five is right for every lifter. Run your numbers through two or three and look at the spread rather than trusting a single figure to the kilogram.

Why the five formulas disagree

Run the same 100 kg set through all five formulas and the spread is small at low reps and widens as reps climb. At 3 reps, Brzycki gives 105.9 kg, O'Conner 107.5 kg, Wathan 109.0 kg, Epley 110 kg and Lombardi 111.6 kg, a spread of under 6 kg across all five. At 6 reps the gap is still around 5 kg in absolute terms, from O'Conner's 115 kg to Wathan's 120.3 kg, but that is only 4 to 5 percent of the estimate. At 10 reps it opens up properly: O'Conner sits at 125 kg while Wathan reaches 134.7 kg, a gap of almost 10 kg, close to 8 percent of the number. None of the five is wrong. They are different mathematical shapes fitted to the same underlying load-rep relationship, and they diverge more the further the input set sits from a true single, which is exactly where the underlying relationship itself gets harder to pin down.

That is the practical reason to run two or three formulas instead of one. A tight cluster of results tells you the estimate is well supported by the input set. A wide spread is itself useful information, a sign that the set sat far enough from a true max that any single figure from it deserves a pinch of salt rather than being loaded straight into next week's programme.

1RM percentages at a glance

Programming by percentage is only as good as the max you calculate it from, but once you have a number this is the range you will actually train in. The table assumes a 100 kg estimated 1RM; the calculator above works it out from your own numbers.

Testing your 1RM directly, and when to bother

An estimate is a working number, not a substitute for the real thing when it actually matters, for example before a powerlifting meet where you need to know your true max to pick opening attempts.

A simple testing protocol

Warm up with the bar, then climb in jumps of roughly 50, 70 and 80 percent of your estimated max for a few reps each, resting 2 to 3 minutes between sets. From about 90 percent, move in small jumps of 2.5 to 5 percent per attempt, resting 3 to 5 minutes, until a lift fails or form breaks down. Stop there; the last good single is your 1RM.

When to skip it

Testing a true max is fatiguing and carries more injury risk than submaximal training, particularly in the squat and deadlift. Most lifters are better served estimating from a hard set of 3 to 6 every few weeks and saving an actual max test for a specific date, like a competition or the end of a training block.

Equipment and warm up matter

The same person can post a different 1RM on the same lift depending on footwear, whether a belt was used, and how thorough the warm up was. Keep these consistent between the sessions you use to estimate or test a max, otherwise a change in the number might just be a change in set-up rather than a change in strength.

How rep range affects accuracy

Every prediction formula gets less reliable the further you push the reps, because fatigue starts to change your technique and bar speed in ways the maths cannot see.

Estimates from 1 to 6 reps are the most trustworthy: the set is short enough that form and effort stay close to a true single. Between 7 and 10 reps the formulas start to pull apart, with the spread approaching 8 percent of the estimate by the top of that range, so treat the number as a solid working figure rather than an exact one. Past 10 reps the five formulas spread further still, sometimes by well over 8 percent, and the result becomes a rough guide rather than a number you should load precise percentages from.

If you only have a high rep set to work from, treat the estimate as a ceiling rather than a target, and confirm it with a lower rep set the next time you train that lift.

Reps in reserve and RPE

The formulas assume the input set stopped somewhere close to failure without actually reaching it, roughly what lifters describe as RPE 8 or 9, or 1 to 2 reps in reserve. A set left further from failure than that, say RPE 6 or 7 with 3 or 4 reps still in the tank, gives the formula less information than it thinks it has: it reads the set as if it were closer to your ceiling than it actually was, so the resulting estimate comes out lower than your true max. A set pushed all the way to failure has a different problem. The last rep or two of a true failure set is rarely clean, bar speed slows and form drifts, and the rep that technically counted may not represent the same movement the earlier reps in the set did.

The most useful input set is one where you stopped because the next rep would have been a genuine grind, not because you physically could not have attempted it and not because you were still moving fast with plenty left. If you are unsure how close to failure a set actually was, err on the side of logging it as harder than it felt. An estimate built from a set you slightly undersell is a small, safe error to carry into your training max. One built from a set pushed past failure is not, since it folds a breakdown in technique into a number you are about to base a training block on.

How to use the number

The estimate is only useful once you turn it into training weights, which is what the percentage table above is for.

Building a training max

Most programmes do not load 100 percent of your true 1RM for regular work. Take your estimated max and knock 5 to 10 percent off it to set a training max, then base your working percentages on that instead. It leaves room to add reps or weight over a block rather than grinding from week one. The wider the spread you saw between formulas when you ran your numbers, the more of that 5 to 10 percent buffer you should take, since a wide spread is itself a sign the estimate carries more uncertainty than usual.

Picking a percentage for your goal

Strength work lives at 80 to 95 percent for low reps. Hypertrophy work sits comfortably at 65 to 80 percent for moderate reps. Technique and warm up work stays under 60 percent. Match the percentage to what the session is actually for, not to a number that just feels satisfying to lift.

Common mistakes

Testing to true failure for the input set. The formulas assume you had a rep or two left. A set taken to failure under-predicts, because form and bar speed collapsed before the final rep.

Using a rep range the formula was not built for. These equations are most accurate under about 10 reps, which is why this calculator caps input at 12. A 20-rep AMRAP is better logged as conditioning work than fed into a 1RM estimate.

Chasing the estimate as a real number. It is a planning tool. Treat a difference of a kilogram or two between formulas as noise, not a discrepancy to solve.

Never retesting. Strength changes every few weeks with consistent training. An estimate from three months ago is no longer your training max.

Ignoring technique on the input set. A grindy, uneven single with a bounce or a half rep is not the rep the formula assumes. Garbage in, garbage out.

Comparing your number to someone else's without context. Body weight, limb length and training age all change what a given 1RM means. A number that looks unremarkable next to a training partner can still represent excellent progress for you.

How coaches use a one rep max calculator

For a coach running more than a handful of clients, a 1RM calculator is how programming percentages turn into real plates on real bars without a spreadsheet for every lifter.

A workable routine: log a hard set of 3 to 6 on the main lifts every 4 to 6 weeks, recalculate the estimate, and let the programme's percentages update from there instead of resetting everything by hand.

Scraler does that inside the workout builder: set a lift's percentages once, and each client's assigned weight recalculates from their own logged numbers instead of you doing the maths for every person on your roster. See workout programming.