Lean body mass is everything you are made of except fat. Muscle, bone, organs, connective tissue and the water in all of it. If you weigh 80 kg at 20% body fat, then 16 kg of you is fat and the other 64 kg is your lean body mass.
Lean mass is not the same as muscle
This trips people up constantly. Skeletal muscle is a large part of lean body mass, but it is not all of it.
Your skeleton, liver, kidneys, brain and blood are all in there too, and so is a great deal of water. Which is why lean mass readings jump around from day to day on a bathroom scale. You did not gain a kilogram of muscle overnight. You drank more, ate more salt, or filled your muscles with glycogen after a big carbohydrate meal.
Fat-free mass is the stricter term, and the difference between the two is the small amount of essential fat inside organs and cell membranes. In everyday use the two get treated as the same thing.
Why the number is worth knowing
Two people can weigh exactly the same and be in completely different physical condition. Weight alone cannot tell them apart. Lean mass can.
It matters for what happens during a diet, too. Losing weight is easy; losing fat while keeping lean mass is the actual objective. If you drop 10 kg and 4 of them are lean tissue, you have made yourself smaller and weaker, and you have lowered your maintenance calories more than you needed to.
Lean tissue is metabolically active, so it drives a decent share of what you burn at rest. The effect is often oversold, since a kilogram of muscle does not burn hundreds of calories a day. But over the years, holding onto lean mass keeps your maintenance calories higher than they would otherwise be.
It matters more as you get older
From middle age onward, adults lose muscle steadily unless they do something about it. That loss is what turns into frailty, poor balance and broken hips in later life.
Strength training is the something. It is the most reliable intervention available for holding lean mass over decades, and the case for it gets stronger with every year that passes, not weaker.
Measuring it
Every method for measuring body fat gives you lean mass as the other half of the answer.
DEXA is the most detailed option available to normal people and reports fat, lean tissue and bone separately. Bioelectrical impedance, which is what smart scales use, is convenient and heavily affected by hydration. Skinfold calipers work well in consistent hands. Hydrostatic weighing is accurate and hard to find.
Whichever you pick, keep it constant and watch the trend rather than the number. Our guide to measuring body fat goes through the trade-offs in full.
How to build and keep it
Lift heavy things regularly. Compound movements like squats, deadlifts and presses recruit the most muscle for the time spent, and progressive overload is what makes the body adapt.
Eat enough protein, 1.6 to 2.2 grams per kilogram of body weight, spread through the day.
Do not diet so hard that you shed muscle along with the fat. A moderate deficit with high protein and continued lifting protects lean mass. An aggressive crash diet does not, and the strength you lose takes far longer to rebuild than the fat took to lose.
Sleep. Muscle is repaired while you are unconscious, and no amount of training makes up for a chronic shortage of it.
Setting expectations
Muscle accrues slowly. A beginner training well and eating enough might add a few kilograms in a year, and an experienced lifter far less. Anyone promising faster is selling something.
Which is why holding onto what you have is so valuable. It took years to build and it can be lost in a few months of a bad diet, and that trade is never worth it.