Attempting an actual 1RM carries injury risk even for experienced lifters. In most cases, estimating from a set of 3–8 reps is safe and accurate enough.
Here's a recommended procedure.
- After a thorough warm-up, choose a weight you can lift for about 5 reps.
- Perform the set with good form until you can't lift another rep (or stop with 1–2 reps in reserve).
- Enter the weight and reps into the 1RM calculator to get your estimate.
If you do need to attempt a true 1RM, always have a spotter and safety bars in place, and work through several progressive warm-up sets first. Your true max can vary day to day depending on condition, sleep, and nutrition.