What is 1RM (one-repetition maximum)?

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1RM is the heaviest weight you can lift once with good form, but not twice; it is the yardstick for measuring strength and setting training weight.

What is 1RM (one-repetition maximum)?
The thing people mean by "What's your 1RM?" at the gym
Workout plans often say things like "3 sets of 10 at 70% of 1RM". This post explains 1RM and the words around it (rep, set, RM, %) one by one, then how 1RM is measured and how it is calculated.
In one line
The heaviest weight you can lift just once
1RM is the heaviest weight you can lift once with good form, but not twice. If someone lifts 100 kg once in an exercise and cannot do a second rep, their 1RM for that exercise is 100 kg. It is the greatest strength that person shows in that exercise, written as a weight.
First, what is a rep?
Lifting a weight up and lowering it once counts as 1
In exercise, a 'repetition' is one movement of lifting a weight up and lowering it. It is also called a rep. If you do 10 push-ups without stopping, you did 10 reps.
Then what is a set?
A group of reps done in a row without stopping
A set is a group of reps done in a row without resting. "3 sets of 10" means lift 10 times and rest, lift 10 times and rest, then lift 10 times once more. People rest a few minutes between sets.
What is RM?
The weight you can lift only a certain number of times
RM is short for 'repetition maximum'. The number in front tells how many times at most you can lift that weight. 5RM is a weight you can lift 5 times and no more; 10RM is a weight you can lift 10 times and no more. So 1RM is a weight you can lift just once.
Measuring it directly
Add weight little by little to find the limit
After warming up by lifting a light weight several times, you add a little weight at a time and lift it once each time. You rest a few minutes after each lift, and the heaviest weight you lift once with good form is written down as your 1RM. Measured this way, a retest a few days later gives almost the same value.
Calculating it without a max lift
Estimate 1RM from how many reps you did
There are several formulas that estimate 1RM from how many times you lifted a somewhat lighter weight, without lifting the maximum. A widely used one, the Brzycki formula, is 'weight × 36 ÷ (37 − reps)'. If you lifted 80 kg 5 times at most, 80 × 36 ÷ 32 = 90 kg.
Which formula fits?
For the bench press, the Brzycki formula came closest
The bench press means lying down and pushing a barbell (a long metal bar with plates on each end) up from the chest. When several formulas were compared with the real 1RM on this lift, the Brzycki formula came closest. The other formulas guessed too high or too low. Formulas worked better when used on results of 10 reps or fewer.
Setting weight as '% of 1RM'
Multiply your 1RM by a percentage to get your own weight
70% of 1RM is the 1RM multiplied by 0.7. For someone with a 100 kg 1RM, that is 70 kg. So people with different strength can each set a weight that fits them, using the same %.
How many reps at each %?
At 70% of 1RM, people usually lift about 11-15 reps
The higher the % of 1RM, that is, the heavier the weight, the fewer reps you can lift. At 70% of 1RM, people usually lift about 11-15 reps. So to build strength people train heavy with few reps, and to build the ability to keep going they train light with many reps.
A common misconception
Does everyone lift the same number of reps at the same %?
No. The rep numbers in the charts are averages of many people. Even at the same %, the number of reps differed from person to person and from exercise to exercise, and the lighter the weight, the bigger the gap between people.
Summary
1RM is the yardstick for measuring strength and setting training weight
1RM is the heaviest weight you can lift just once with good form. Training weight is set as a % of 1RM, but even at the same %, the number of reps differs from person to person. Related terms: progressive overload, RIR (reps in reserve).