How I work it out
With the Epley formula, one of those compared by Reynolds et al.: the weight times 1 plus the reps divided by 30. With 1 rep, your 1RM is that weight. I round to the nearest half kilo.
The formulas work up to about 10 reps. Beyond that the error grows a lot, and Reynolds et al. advise against using them, so I don’t estimate above 10.
What it’s for
Many strength programmes are written as percentages of your 1RM. With the table you know what weight to use in each set without testing a true max.
Why not just go for it
A true max is a very demanding effort for your muscles, your joints and your blood pressure. If you’re a beginner, a minor or you have an injury or a blood pressure problem, stick with the estimate and don’t test it without supervision. Technique before weight.
Sources
It’s an estimate: your real max changes with technique, fatigue and the exercise. The data you type never leaves your browser: it isn’t stored or sent anywhere.