Lean Mass Calculator

Lean mass guide

What Is Lean Body Mass?

Lean body mass is your total weight minus all fat. Here is what it means, how it differs from muscle mass, a clear definition, and how to find your own number.

Last updated: · Reviewed by the Lean Mass Calculator editorial team

Who this page is for

Anyone looking up the meaning or definition of lean body mass and how to find their own value.

Start with the main lean mass calculator, then use the related tools below when you need body fat, FFMI, protein, or calorie context.

Lean body mass, defined

Lean body mass (LBM), also called lean mass or fat-free mass, is the total weight of everything in your body that is not fat. That includes your muscles, bones, organs, skin, connective tissue, and the water inside all of them. In plain terms: take your scale weight, remove every ounce of fat, and what remains is your lean body mass.

The simplest definition is a subtraction: Lean Body Mass = Total Body Weight − Fat Mass. If a 180 lb person carries 20% body fat, that is 36 lb of fat, leaving 144 lb of lean body mass. This is why LBM is a more useful number than scale weight alone — it tells you what your weight is actually made of.

Lean body mass is usually expressed either as an absolute figure (for example, 144 lb or 65 kg) or as a percentage of total weight (80% lean mass). Both describe the same thing from different angles, and the calculator on this site reports both.

What lean body mass is not

The most common mix-up is treating lean body mass and muscle mass as the same thing. They are not. Skeletal muscle is only one component of lean mass — bone, organs, and body water make up the rest. Your skeletal muscle mass is always a smaller number than your lean body mass, typically around 40-50% of it.

Lean body mass also is not a measure of how 'lean' you look. Someone can have a high absolute lean mass and still carry a lot of fat. Lean mass describes the fat-free portion of your body in weight terms, not your visible definition or body fat percentage.

Because roughly 70-75% of lean mass is water, LBM can shift by a pound or two day to day with hydration, sodium, and glycogen — so track the trend over weeks rather than reading a single measurement as gospel.

How to find your lean body mass

The most reliable way is the direct method: if you know your body fat percentage from a DEXA scan, calipers, or a consistent BIA scale, multiply your weight by the non-fat fraction. At 180 lb and 20% fat, that is 180 × 0.80 = 144 lb of lean mass.

If you do not have a body fat measurement, a validated formula estimates lean mass from height, weight, age, and sex. This site uses the Boer, James, and Hume equations. Enter your details in the calculator above and it returns your lean mass, fat mass, lean-mass percentage, and a protein target in one step.

For the muscle-only portion of your lean mass, use the muscle mass calculator, which applies the Lee (2000) equation to estimate skeletal muscle specifically rather than total fat-free mass.

Frequently asked questions

What does lean body mass mean?

Lean body mass means the total weight of your body minus all fat — your muscles, bones, organs, and body water combined. It is also called lean mass or fat-free mass, and it is calculated as total weight minus fat mass.

What is the definition of lean body mass?

Lean body mass is defined as body weight excluding fat mass. Formally, Lean Body Mass = Total Body Weight − Fat Mass. It includes skeletal muscle, bone, organs, connective tissue, and the water they contain.

Is lean body mass the same as muscle mass?

No. Muscle is only part of lean body mass. Lean mass also includes bone, organs, and water, so it is a larger number than skeletal muscle mass — muscle is typically around 40-50% of total lean mass.

What is a good lean body mass percentage?

Many healthy adult men fall around 75-90% lean mass and women around 65-80%, because women carry more essential fat. Age, training, and body fat goals all shift the context, so treat it as a reference rather than a fixed target.

How do I find my lean body mass?

If you know your body fat percentage, multiply your weight by (1 − body fat ÷ 100). Otherwise, enter your height, weight, age, and sex into the calculator on this page to get a formula-based estimate instantly.

Sources & references

The estimates on this page use published lean body mass equations and clinical reference ranges. See the full reference charts on the lean body mass chart hub.