Your one-rep max — the most you can lift for a single rep — is the reference point most strength programs are built around. The good news is you do not have to actually attempt a true max, which is risky, to get a reliable number.
TL;DR — Enter a weight and the reps you managed in the one-rep max calculator to estimate your 1RM and see a percentage table.
Estimate instead of max out
Attempting a genuine one-rep max is hard on your joints and nervous system, and risky without a spotter. Instead, you can estimate it from a normal working set. Lift a weight for several reps to near failure, and a formula predicts what you could manage for one. This calculator averages two well-known formulas, Epley and Brzycki, for a balanced estimate.
Get the best estimate
Accuracy is highest when the set is heavy and the rep count is low — somewhere in the 3 to 8 range works well. Estimates from very high-rep sets drift, because endurance starts to matter more than pure strength. Pick a tough set you can complete with good form.
Use the percentage table
Once you have an estimated max, the percentage table turns it into working weights: 80% for strength sets, lighter loads for volume. Reassess every few weeks as you get stronger. Run your numbers in the one-rep max calculator and program from there — and always prioritize good form over chasing the number.