These two lines sit near each other on a body composition sheet, one is roughly twice the other, and almost nothing explains the difference. It matters more than it looks, because the change your sheet recommends is written against one of them and most people read it as the other.
Lean body mass is everything in you that is not fat. Skeletal muscle mass is only the muscle attached to your skeleton, the part you can actually train. Skeletal muscle is inside lean body mass, and it is roughly half of it.
Take your weight and subtract your fat. What is left is lean body mass: muscle, organs, bone, skin, blood, and all the water held in them. You will also see it called fat-free mass, which is the same thing and a clearer name for it.
Most of your lean mass is not trainable. Your liver and your skeleton are in this number and they are not going to respond to a training block.
The muscle that moves your bones. Not your heart, not the smooth muscle in your gut, not your organs. This is the number that answers "am I building muscle", and it is the one to want going up.
It moves slowly. Around a pound a month is a good rate, and a jump much faster than that is usually water or a different hydration state rather than new tissue.
Skeletal muscle is a subset of lean body mass, so lean mass is always the bigger number. The gap between them is your organs, your bone and the water around everything, and that gap does not shrink when you train.
Skeletal muscle comes to somewhere around 40 to 50% of lean body mass in a typical adult. That is a rough average, not a formal guideline, and it shifts with age, sex and training history.
Which is why the two numbers do not move together. A gain in lean mass can be entirely water. A gain in skeletal muscle is harder to fake.
Your sheet prints a recommended change, usually as a pair: lose this much fat, gain this much lean mass. That second figure is LEAN BODY MASS. It is not a muscle target. Read "+8 lb lean mass" as "+8 lb of muscle" and you have just set yourself something like double the goal the sheet intended, because part of that lean figure is water and tissue you do not build in the gym.
If you want a muscle target, set one deliberately against the skeletal muscle line. Do not copy it across from the lean mass recommendation.
Dry lean mass is lean body mass with the water taken out, so mostly protein and mineral. It is small and it barely moves, which is exactly what makes it useful: it is the cross-check. If your muscle number jumped but dry lean mass sat still, you gained water.
Nobody sets goals against this line. Use it to check the other two.
Skeletal muscle divided by your height squared, in kg/m². It is the muscle version of what BMI does for weight, and it exists so a tall person and a short person can be compared without the tall one winning by default.
Low-muscle cutoffs for this do exist in clinical use, but they differ by population, so a reading that is low against one set is normal against another. Any single cutoff you are quoted is one population's choice, not a settled standard.
Useful for watching your own trend. Not useful for deciding you are below or above some line, because which line depends on whose you pick.
Skeletal muscle mass, for whether training is working. Percent body fat, for whether the fat side is moving. Lean body mass is worth a glance because your metabolic rate is calculated from it, but it is too easily moved by water to be the number you judge a month by.
One reading is a data point. Three over three months is a trend, and the trend is the only part of this that is reliable.
Both get called "lean mass" in conversation, the sheet puts them a few lines apart, and the recommended change sits under the bigger one. It is an easy mistake to make and it gets made constantly, including by people reading the sheet back to you.
If someone quotes you a muscle goal off your sheet, ask which line it came from.
A body composition machine does not see muscle. It passes a small current through you and measures the resistance. Muscle holds a lot of water so it conducts well, fat holds little so it does not, and every figure on the sheet is worked back out of that resistance along with your height, age and sex.
Bodivio reads an InBody sheet from a photo and charts every reading, so you can see the year rather than the day. Every figure it shows says where it came from, the same way this page does.
Have a lookThis explains what the numbers on a body composition sheet mean. It is not medical advice, a diagnosis, or a substitute for talking to a doctor or a dietitian.