Lean Body Mass Calculator
Split your weight into fat mass and lean mass, the part scale weight alone cannot show.
Fill in the form and your result lands here.
Learn the details
Lean Body Mass (LBM) is everything in your body except fat: muscle, bone, organs, and water. It is calculated as LBM = weight × (1 − body_fat_percentage / 100).
Almost everyone who runs this calculator for the first time thinks the lean number looks too big. It is not a mistake, and understanding why changes what you do with it. Here is what the number contains, where it comes from, and why I care about it more than scale weight.
What lean mass really is
Lean body mass is everything you are made of that is not fat. Muscle is the part people picture, but it shares the total with bone, organs, connective tissue, blood, and the water held inside all of it. Water alone is a large share of your bodyweight, so the lean figure is always far bigger than the muscle you could flex in a mirror. If you expected a number that matched your idea of how muscular you are, you were expecting a different measurement.
Where the number comes from
This tool does the honest version of the arithmetic. It takes your bodyweight, uses your body fat percentage to work out how many pounds of that are fat, and calls the remainder lean. There is a second family of equations, the ones published by Hume in 1966, James in 1976, and Boer in 1984, that predict lean mass from height, weight, and sex alone. Those were built for clinical work like drug dosing and fluid volume estimates, and they have to guess at your composition because nobody measured it. I would rather use the body fat number you actually have, even though that makes this result only as good as the estimate you fed it.
A worked example
Say you weigh 200 pounds and your body fat estimate is 20 percent. Twenty percent of 200 is 40, so 40 pounds is fat mass. The other 160 pounds is lean mass, and the split bar shows it as 80 percent lean and 20 percent fat. Now lose 10 pounds and re-measure at 18 percent. Fat mass falls to about 34 pounds while lean mass sits near 156. You lost 6 pounds of fat and about 4 pounds of everything else, which is a useful thing to know and completely invisible on a bathroom scale.
Why it matters during a cut
Anyone can lose weight. Losing weight while holding lean mass is the part that takes work, and it is the entire reason I push protein and resistance training during a deficit. When lean mass slides down alongside fat mass, the deficit is too aggressive, protein is too low, training volume dropped, or all three. Weight alone cannot tell you which of those is happening. Recheck every two to four weeks rather than weekly, because the body fat estimate underneath carries enough noise to invent trends that are not there.
What to pair it with
This number is downstream of your body fat percentage, so measure that carefully and the same way every time. To judge whether your lean mass is a lot for your frame, run FFMI, which scales lean mass to your height the way BMI scales weight. And if you are trying to protect it, your daily protein target is the first lever to pull. If a reading concerns you, or lean mass keeps falling despite eating and training well, that is a conversation for a doctor rather than a calculator.
If LBM drops during a cut, increase protein or shrink your deficit. Recheck every 2-4 weeks alongside body fat percentage.
This split is one of five ways to read the same body. See all five against one set of measurements.