What is lean body mass?

Lean mass, fat-free mass and muscle

These three terms get used interchangeably, but they aren’t the same:

• Fat-free mass is everything except fat: muscle, bone, organs, connective tissue and water.
• Lean body mass is used almost synonymously in fitness contexts. Strictly, it includes the small amount of essential lipid in cell membranes and organs, which fat-free mass excludes. For practical tracking, the difference doesn’t matter.
• Skeletal muscle mass is only one component of lean mass. Most home methods can’t measure it separately.

How it’s estimated

Nobody weighs lean mass directly. Every method estimates fat and subtracts it from total weight:

Lean mass = weight × (1 − body fat % ÷ 100)

So lean mass inherits whatever error the body-fat estimate carries, whether that’s DEXA, bioelectrical impedance, skinfolds, a tape-measure equation or a photo. Hydration matters too: a large share of lean mass is water, so a salty meal, a high-carb day or dehydration can shift an estimate with no change in tissue. The methods comparison covers where each method goes wrong.

Why it’s worth tracking

Weight can’t tell you whether a change was fat, muscle or water. Lean mass can at least separate the two directions you care about:

• losing fat while keeping muscle
• gaining muscle without adding much fat

Scaled for height, lean mass becomes FFMI, which lets people of different heights compare fairly.

As with every estimate, the useful read is the trend, measured the same way over weeks. If you don’t have a body-fat number yet, the body fat calculator gives you a tape-measure or skinfold estimate to start from.