Lean Body Mass is everything except body fat. Your total weight minus weight of fat = Lean Body Mass. You can read a more in-depth explanation here.

Why track it?

For most people their goal is to lose body fat, but if you are looking to gain muscle then Lean Body Mass is the metric you should be tracking. Generally speaking, if a person is losing lean body mass as a percentage, they are likely gaining body fat, or if they are losing lean body mass in absolute terms, they could be losing muscle.

How is it measured?

Progress can automatically calculate your Lean Body Mass if you’re tracking your body fat percentage and have set your height. Just enable it under ‘What I Track’ on the ‘Me’ tab and you’re good to go!