Your one-rep max (1RM) is the heaviest weight you could lift once with good form. Lifting programmes use it to set loads: "work at 70% of your max". The catch is that finding it the direct way means piling on weight until you fail. That is a poor idea in your first months, when your technique is still forming and a missed rep costs more than it teaches.
The good news is that you can estimate it from a set you already did. Lift a weight for a handful of clean reps, and a simple formula tells you roughly what you could do for one. The same maths works backwards too: give it a target number of reps and it suggests a weight to load.
Reps left = how many more good reps you could have done. If unsure, use 2.
Two formulas are in common use. Epley says: 1RM = weight × (1 + reps ÷ 30). Brzycki says: 1RM = weight × 36 ÷ (37 − reps). They were fitted to how real lifters behave: the more reps you can do with a weight, the further it sits below your max. For example, 60 kg for 8 reps gives about 76 kg by Epley and 74 kg by Brzycki. The calculator averages the two and shows both ends so you can see how uncertain it is.
The "reps left in the tank" box matters. Most beginners stop a set well before true failure. If you did 8 reps but could have done 2 more, your body was really working at a 10-rep level, so the calculator treats it that way. Being honest here changes the answer more than the choice of formula does.
Most beginners don't need percentages at all. The simpler route is to pick a weight where the last rep is hard but clean, then add weight once you reach the top of your rep range. Your estimated max is useful for two things: sanity-checking a starting weight for a new lift, and seeing progress in a single figure over the weeks.
If you do want to train from percentages, a rough guide is that sets of 8 to 12 reps sit at roughly 65 to 75% of your max, and sets of 3 to 5 reps sit around 85 to 90%. The bars above show that spread for your own numbers. Leave a rep or two in reserve on most sets.
This is a planning aid, not a reason to attempt a max lift. Don't test a true one-rep max alone, without a spotter or safety bars, or when you're tired or in pain. If a movement hurts, or you have a heart or joint condition, or you're pregnant, check with a doctor or physio before loading heavier. Estimates from a set of 5 are plenty for a first year of training.