Your one rep max (1RM) is the heaviest weight you can lift once with good form. Percentage-based programs are written around it: 5x5 at 75%, heavy triples at 85%, openers at 90%. You need the number, but grinding out a true max attempt every few weeks is risky and eats a training day.
The fix is a submaximal estimate. Lift a heavy weight for a few reps, plug the weight and reps into a proven formula, and you get a 1RM figure accurate to within a few pounds for most lifters. Safer, faster, repeatable.
Test when you are fresh. A 1RM estimate taken after a high-volume session reads low and will drag your training percentages down with it.
Three formulas cover the field, and each has a rep range where it shines:
No single formula wins everywhere, so BarbellMax averages all three. The average smooths out each formula's blind spots and lands closer to a tested max than any one alone. For the full research, see the science behind 1RM formulas.
One caution: every standard formula underestimates the deadlift. Grip fatigue and pulling from a dead stop change the math. Treat a deadlift estimate as a floor. Your true max is likely a little higher.
Once you have a 1RM, your program weights fall out of it. 70 to 80% builds strength with volume. 85 to 95% builds maximal strength in low reps. Retest every few weeks with the same rep-range method, log each result, and watch the trend rather than any single day.
The BarbellMax calculator keeps the history for you, shows every training percentage from your current max, and loads any of those weights straight into the plate calculator. When it is time to put the number on the bar, read How to Load a Barbell and Warm Up.