The quick way: the Epley formula
Your one-rep max, or 1RM, is the heaviest weight you could lift for a single repetition of an exercise. You rarely need to actually attempt one, because you can estimate it from a set you can already do for several reps. The most common method is the Epley formula:
1RM = weight x (1 + reps / 30)
You take the weight you lifted, multiply your rep count by the weight, divide by 30, and add that to the original weight. The idea is simple: each rep you can perform beyond the first implies a little more strength in reserve, and the formula scales the lifted weight up to estimate the single you could manage. It is gym maths, low risk, and quick enough to do on the back of a training log.
A worked example
Suppose you squat 80 kg for 5 clean reps and want to know your estimated max. Plug the numbers into Epley: reps divided by 30 is 5 divided by 30, which is about 0.167. Add 1 to get 1.167, then multiply by the weight: 80 multiplied by 1.167 is about 93 kg. So a 5-rep set at 80 kg points to an estimated 1RM of roughly 93 kg.
Notice what the formula is doing. Five reps adds about 17 percent on top of the lifted weight. If you had only managed 3 reps at 80 kg, the uplift would be smaller, around 10 percent, giving about 88 kg, because fewer reps imply less in reserve. More reps push the estimate higher, which is exactly why very high-rep sets become unreliable: the formula keeps adding, but real bodies do not extend in such a tidy straight line. You can run your own lifts through our one-rep max calculator to get the figure without the arithmetic.
The Brzycki alternative
Epley is not the only formula. The other popular option is Brzycki, which approaches the same problem from a slightly different angle:
1RM = weight x 36 / (37 - reps)
Run the same 80 kg for 5 reps through Brzycki: 37 minus 5 is 32, and 36 divided by 32 is 1.125, so 80 multiplied by 1.125 is 90 kg. That is close to the 93 kg Epley gave, and at low rep counts the two formulas almost always land within a couple of kilos of each other. The gap widens as reps rise, with Brzycki tending to read a little lower on high-rep sets. There is no single correct answer here. Both are well established estimates, and many people simply average the two or pick one and stay consistent so they can compare results over time.
Turning your 1RM into training weights
The reason a 1RM is so useful is that it lets you set every other working weight as a percentage of it. Different rep ranges and goals correspond to different percentages, and the table below gives the rough relationship most lifters use as a starting point.
- About 100 percent of 1RM: 1 rep, pure max strength.
- About 90 percent: around 3 to 4 reps, heavy strength work.
- About 80 percent: around 7 to 8 reps, strength and muscle.
- About 75 percent: around 10 reps, classic muscle building.
- About 67 percent: around 12 to 15 reps, muscle and endurance.
- About 60 percent or less: 15-plus reps, technique and endurance.
Using the worked example, an estimated 1RM of 93 kg means a 75 percent training weight is about 70 kg, which you would expect to handle for roughly 10 reps. An 80 percent day would be about 74 kg for sets of 7 to 8. This is how structured programmes prescribe load without you maxing out every session: they reference a percentage of your 1RM and let the formula do the scaling.
The percentages above are guidelines rather than fixed laws, because the exact number of reps a given percentage allows varies from person to person and from one exercise to another. Big compound lifts like the deadlift often let you grind out a couple of extra reps at a given percentage compared with a more technical lift such as the bench press, and well trained lifters tend to hold higher percentages for more reps than beginners. Treat the table as a sensible starting framework, then adjust the percentages slightly based on what you actually manage in the gym over a few sessions.
Why low-rep sets give the best estimate
All of these formulas are extrapolations, and the further they reach, the looser they get. A set of 2 to 5 reps is only a short hop from a true single, so the estimate sits on solid ground. A set of 15 reps is a long way from a max, and at that distance individual differences in pacing, breathing, fatigue tolerance and technique start to swamp the maths. Two people who both grind out 15 reps at the same weight might have very different true maxes, and the formula cannot tell them apart.
For the most trustworthy estimate, test with a weight you can lift for around 3 to 5 reps with good form and a rep or two still in the tank, then apply the formula. Re-test every few weeks rather than daily, since the number only moves meaningfully over training blocks. Used that way, an estimated 1RM is a genuinely handy planning tool: it turns a single sub-maximal set into a reference point you can program your whole week around, all without the risk and fuss of attempting a true maximal lift.