How the calculator works
A one-rep max is the heaviest weight you can lift once with good form. Testing it directly is hard on your body and only tells you about that one day, so most lifters estimate it instead, from a set of several reps taken close to failure. The formulas here all do the same job: they take the weight and the reps and work out the single that set implies.
The headline number uses Boyd Epley’s formula, weight × (1 + reps ÷ 30). For 100 kg lifted 5 times, that is 100 × (1 + 5 ÷ 30), or about 116.7 kg. The other six formulas sit beside it so you can see how much the answer depends on the method, and the range under the result shows the spread from lowest to highest.
Which formula is most accurate?
There is no single winner. Each formula was fitted to a different group of lifters and exercises, and each one is a little better or worse depending on the lift, your training and how many reps you did. For sets of about 2 to 6 reps they mostly land within a few percent of each other, and Epley and Brzycki give exactly the same answer at 10 reps.
They diverge as reps climb. Lombardi and O’Conner come out lower for long sets, Brzycki and Landers come out higher, and Mayhew runs high on very short sets. That is less a flaw in any one formula than a sign that long sets measure something else: at 12 or 15 reps, how long you can keep going matters nearly as much as how strong you are. For the most dependable estimate, use a set of about 3 to 5 reps that ended close to failure.
Why Epley gives more than the weight for a single
Put 100 kg for 1 rep into Epley and you get about 103.3 kg, not 100. The formula adds a thirtieth of the weight for every rep, including the first, so a single reads about 3% above the weight on the bar. Some calculators special-case one rep to return the weight itself. This one doesn’t, because Arclift’s Est. 1RM uses Epley exactly as written, and the two should show the same number for the same set.
Estimating from sets that weren’t to failure
Every formula here assumes the set went to failure, or very close to it. If you stopped with reps in the tank, the estimate comes out too low. A rough fix is to add the reps you had left: 5 reps with about two more possible is close to a 7-rep max, and Epley on 7 reps gives a higher, fairer estimate. The RPE calculator does this properly with a published RPE chart, and for 100 kg × 5 at RPE 8 it lands within a kilo of the adjusted Epley number.
Using your max
The percentage table turns your estimate into working weights. Many programs write their heavy sets as a percentage of a training max rather than your true max, often 90% of it, so that the planned weights stay doable on an ordinary day. To load any of those weights, the plate calculator shows the plates for each side of the bar.
How Arclift tracks your estimated max
Arclift works out this same Epley number from every set you log, so you never need to test a true max to see your strength move. With Arclift Pro, the progress charts for each exercise plot your estimated max over time beside the heaviest set and set volume, and the progression coach uses it to tell a real stall from a slow week: if neither your top weight nor your estimated max has risen in three sessions, it backs you off and rebuilds. Logging the sets that feed all of it is free.
