It works by estimating with data you already know. You enter the reps and weight you were previously able to successfully complete and it will estimate your 1 rep max!
This app helps keep you safe by giving you an estimate on how much you can lift before actually attempting it yourself. This helps to prevent muscle and body damage so you can keep your health and continue to build your strength.
1RM also has a percentage chart for those who are doing workouts which require knowing the percentage of your 1 rep max.
This app works best when the reps used are less than 12. I recommend using the reps between 3 and 10 for highest accuracy. For this reason, I recommend you do not use this to calculate maxes for exercises such as push ups sit ups or crunches.
1RM will undoubtedly supplement any bodybuilders lifting regimen and can help you achieve more effect sets so you can build more muscle.
ALWAYS USE A SPOTTER. APPLICATIONS LIKE THIS ARE DESIGNED TO MAKE WEIGHT LIFTING SAFER BUT CANNOT GUARANTEE YOUR NOR ANYONE ELSE''S SAFETY. THESE CALCULATIONS ARE ONLY ESTIMATES AND CAN BE INACCURATE.