When people say they want to “lose weight,” what they almost always mean is “lose fat.” The difference matters, because the other thing on the scale — your lean body mass — is exactly what you want to keep.
TL;DR — Enter your height, weight and sex in the lean body mass calculator to estimate your lean and fat mass.
What lean body mass is
Lean body mass is everything in your body that is not fat: muscle, bone, organs and water. The part you can change through training is muscle. It is what gives you strength, supports your joints, and keeps your metabolism ticking over — lean tissue burns more calories at rest than fat does.
Why it’s the real target
A diet that drops weight fast often strips muscle along with fat, which lowers your metabolism and leaves you weaker and “skinny-fat.” Tracking lean mass — not just the scale — tells you whether you are losing the right thing. The goal of a good plan is to lose fat while holding lean mass steady, or to build lean mass while keeping fat in check.
How to protect it
Three habits preserve lean mass: eat enough protein, keep your calorie deficit moderate rather than extreme, and lift weights or do resistance work. Together they tell your body to burn fat for energy while keeping its muscle. Estimate your starting point in the lean body mass calculator and track it as you go.