Lean Body Mass (LBM) Calculator
Enter your weight and body fat percentage to calculate your lean body mass (LBM) and fat mass. Set a target body fat percentage to simulate the weight you would reach while keeping your lean mass unchanged.
What lean body mass means
Lean body mass (LBM) is what is left of your body weight once the fat is taken away. It is often mistaken for muscle alone, but it also includes bone, organs, blood and water. This tool takes your weight and body fat percentage, works out your lean body mass and fat mass, and — once you add a target body fat percentage — calculates the weight you would be at that target assuming your lean mass stays unchanged. The arithmetic is simply lean body mass = weight × (1 − body fat), and target weight = lean body mass ÷ (1 − target body fat).
Read the result with its assumption in mind. In a real diet some muscle goes as well, so the target weight here is closer to a best-case figure than to a floor. Body fat readings also move with the method used: household scales estimate it from electrical resistance, which shifts with how hydrated you are. When you pick a target, stay above essential fat, generally put at around 3–5% for men and 10–13% for women. Anything touching on a medical condition belongs with a clinician, not a calculator.
How to calculate lean body mass
- Enter your weight Readings settle down if you always measure under the same conditions — first thing in the morning after using the bathroom, for instance.
- Enter your current body fat percentage Use the figure from your scales or from a measurement service. Anything from 0% up to (but not including) 100% is accepted.
- Enter a target body fat percentage A lower figure than today gives you a cutting target, a higher one a bulking target. Do not enter a value below essential fat.
- Read the results You get lean body mass, fat mass, the target weight and the change required. A negative change means losing weight, a positive one means gaining.
Tips for getting more out of it
- Lean body mass (LBM) is the total weight of everything in your body except fat — muscle, bone, organs, and body water. It is commonly used as a goal to preserve while cutting fat.
- Enter a body fat percentage measured with a body composition scale or skinfold calipers. Different measurement methods can vary by several percentage points.
- Set a lower target than your current body fat to see your weight after fat loss, or a higher target to see your weight after a bulking phase.
- Tracking body fat percentage alongside weight makes it much easier to see how training and diet are changing your body composition over time.
Where lean body mass is useful
Setting a weight target for a diet
Instead of picking a number of kilos to lose, work backwards from the body fat percentage you want. Because it assumes muscle is preserved, the target it produces is rarely an extreme one.
Tracking progress in the gym
If the scales have not moved but your lean body mass has risen, you have gained muscle and lost fat at the same time. That is precisely the change a bathroom scale alone cannot show.
Setting a daily protein target
Nutrition guidance is frequently given per kilogram of lean body mass rather than per kilogram of body weight, and this gives you the figure to work from.
Turning a body fat reading into something concrete
"25% body fat" is hard to picture. Seeing how many kilograms of fat that actually is makes the size of the task much clearer.
Body composition terms explained
- Lean body mass (LBM)
- Body weight minus body fat. Because it covers bone, organs, blood and water as well as muscle, it is not the same thing as muscle mass.
- Fat mass
- Body weight minus lean body mass — the actual weight of the fat stored on your body, and the thing a diet is aimed at.
- Body fat percentage
- The share of your weight that is fat. Household scales estimate it from how easily a small current passes through you, so the reading shifts after eating, bathing or exercising.
- Essential fat
- The minimum body fat needed to protect organs and produce hormones. It is usually put at around 3–5% for men and 10–13% for women, and going below it damages health.
- Weight at the target body fat
- What you would weigh at the body fat percentage you specified, assuming lean mass is unchanged — that is, if you lost fat without losing muscle.
- Change required
- The gap between the target weight and your current weight. Negative means you need to lose, positive means you need to gain.
FAQ
Side Note — How measuring body fat has evolved
Estimating body fat percentage has a long history. In the early 20th century, hydrostatic weighing — based on Archimedes' principle — was considered the "gold standard." A person was fully submerged in a water tank so their body volume could be measured, and body fat percentage was calculated from the density difference between fat tissue and lean tissue. It was highly accurate but required bulky, specialized equipment.
Since the 1990s, bioelectrical impedance analysis (BIA), now built into many home body composition scales, has become the dominant everyday method. A weak electrical current is passed through the body, and body fat percentage is estimated from how easily the current flows (impedance) — fat tissue contains little water and resists current, while muscle and blood are water-rich and conduct more easily. It is convenient, but sensitive to your body's hydration level at the time of measurement (meals, fluids, recent exercise), so measuring at a consistent time and under consistent conditions is recommended.
In strength training circles, two phases are often discussed: "cutting," which aims to lose fat while preserving as much lean body mass as possible, and "bulking," which aims to increase lean body mass (mainly muscle). Tracking the breakdown between lean mass and fat mass — not just total body weight — makes it much easier to tell whether your body is actually changing in the direction you intend.