How your max is worked out
The calculator uses the Epley formula: max = weight × (1 + reps / 30). If you lift 100 kg five times, your estimated max is 100 × (1 + 5 / 30), which is about 116.7 kg. A set of a single rep is already a max, so the answer is the weight itself.
It is the same formula BIP uses for the estimated max in your progress, so the number here can be compared with the one you see in the app.
Why ten reps at most
The more reps a set has, the more it is about endurance and the less it tells you about what you can lift once. With 13 reps the formula can give a max far above what you can actually lift. That is why the calculator only works with sets of 1 to 10 reps, and the answer is most reliable with 3 to 6.
How to use the table
The table shows the weight at each share of your max and roughly how many reps it is good for. It is a handy way to pick a weight when a programme says "80 percent" or "8 reps". Heavy sets for strength are often at 80 to 90 percent, and sets for building muscle at 65 to 80. How to choose is in the guide to sets and reps, and most sets should end with a couple of reps in reserve, not at failure.
Don't test your max every week
A true max test is hard on the body, needs a proper warm-up and ideally someone to spot you. An estimated max from a normal set gives almost the same information without the risk. The formula works best on the main lifts such as the bench press, squat and deadlift, and less well on small exercises where technique matters more than strength.
The number is an estimate. Two people with the same set can have different maxes, and your own can vary by a few kilos from day to day. What matters is that it rises over time, and it does when you add a little at a time through progressive overload.
Let the log do the maths
If you log your workouts in BIP's training log, your estimated max is worked out from every set you do, and you see it grow week by week without doing the maths yourself.