What is lean body mass?
Lean body mass (LBM), sometimes called fat-free mass, is your total body weight minus all of your body fat. It includes your muscle, bones, organs, body water, and connective tissue. The simple relationship is: lean body mass + fat mass = total body weight.
Men usually carry a larger share of lean mass than women, who have more essential fat. Because muscle and organ tissue — not fat — drive your metabolism and strength, LBM is often a more meaningful number than the scale alone.
The 3 lean body mass formulas
This calculator uses three published regression equations and averages them; the averaging is this site's choice, not part of any of the formulas. All use weight in kilograms and height in centimeters:
Boer Formula (1984)
Male: 0.407 × wt + 0.267 × ht − 19.2 | Female: 0.252 × wt + 0.473 × ht − 48.3
Boer P, American Journal of Physiology, 1984, from work on scaling body fluid volumes to lean mass.
James Formula (1976)
Male: 1.1 × wt − 128 × (wt ÷ ht)² | Female: 1.07 × wt − 148 × (wt ÷ ht)²
James WPT, Research on Obesity (HMSO), 1976. Because of its squared term the estimate peaks and then falls as weight rises: for a man 5 ft 10 in tall, above about 300 lb.
Hume Formula (1966)
Male: 0.3281 × wt + 0.33929 × ht − 29.5336 | Female: 0.29569 × wt + 0.41813 × ht − 43.2933
Hume R, Journal of Clinical Pathology, 1966. The oldest of the three.
Once LBM is known, your body-fat mass is simply total weight minus LBM, and your body-fat percentage is fat mass ÷ total weight × 100.
Lean mass vs. body fat
Your body weight is the sum of two parts. A worked example for a 180 lb adult at 20% body fat:
| Component | Weight | What it is |
|---|---|---|
| Lean body mass | 144 lb | Muscle, bone, organs, water |
| Fat mass | 36 lb | Stored body fat (20%) |
| Total weight | 180 lb | What the scale shows |
This is why the scale can mislead: gaining 5 lb of muscle while losing 5 lb of fat leaves your weight unchanged but improves your body composition. Tracking that needs a method that looks at body composition, since the formulas on this page see only height and weight.
Why lean body mass matters
- Metabolism. Lean tissue burns more calories at rest than fat, so LBM is a major input to your TDEE and daily calorie needs.
- Fitness tracking. Preserving or building LBM while losing fat is the goal of most body-recomposition programs — a better benchmark than weight alone.
- Clinical research. Estimated lean mass is used to scale body measurements; Boer's equation was published for normalizing body fluid volumes.
- Healthy aging. Adults lose muscle with age (sarcopenia); monitoring LBM helps you catch and counter that decline.
How to increase lean body mass
Building lean mass is mostly about training and protein:
- Resistance train on at least 2 days a week, CDC's minimum for adults' muscle-strengthening activity (CDC), progressively increasing load over time.
- Eat enough protein across the day to support training.
- Don't under-eat. Building muscle requires adequate total calories; an aggressive deficit stalls lean-mass gains.
- Prioritize sleep and recovery — muscle is built between workouts, not during them.
This calculator can't show that progress: its formulas use only height and weight, so any weight gain raises the estimate whether it was muscle or fat. A measured method, or a tape-based body fat estimate combined with your weight, says more.
Limitations
- Estimates, not measurements. Formula-based LBM is inferred from height and weight and is least reliable at the extremes of build.
- Doesn't account for unusual body composition. Very muscular or very lean individuals may get less reliable results.
- Adults only. These equations are not designed for children or adolescents.
- For a precise reading, use DEXA, hydrostatic weighing, or a bioelectrical-impedance scale.
FAQs
What is lean body mass?
It is what you would weigh with all body fat taken away: muscle, bone, organs, water and connective tissue together. Because the fat-free part of the body does most of the metabolic work, lean mass is often a more useful figure to follow than weight on its own.
How is lean body mass calculated?
Here it comes from three published regression equations that take height, weight and sex: Boer (1984), James (1976) and Hume (1966). Boer's version for men, for instance, is 0.407 × wt + 0.267 × ht − 19.2 with weight in kg and height in cm, so a man of 80 kg and 180 cm gets 61.4 kg. Showing all three, and their average, makes the spread between them visible.
What's the difference between the Boer, James, and Hume formulas?
Each was fitted to a different group of people, so they disagree, most of all for very light or very heavy people. James has a squared weight term, which makes its answer stop rising and then shrink as weight climbs; for a man of 5 ft 10 in that turn comes at about 300 lb. Hume's, from 1966, is the oldest. Taking the mean of the three is this site's choice, not a rule from any of the papers.
What is a normal lean body mass percentage?
No official body publishes a normal range for lean mass as a share of weight. It depends on height, frame, sex and training, and men usually have a larger share than women. Following how your own figure changes, measured the same way, tells you more than comparing it with other people.
Lean body mass vs. body fat — what's the difference?
Add them together and you get your weight. At 180 lb and 20% body fat, fat accounts for 36 lb and lean mass for the other 144 lb. Strength training aims to raise the lean part; losing fat shrinks the other part. The best outcome keeps lean mass while fat falls.
Why does lean body mass matter?
Muscle uses more energy at rest than fat does, so lean mass feeds into your daily calorie needs. It also explains why the scale can stall while your shape changes: putting on muscle while shedding fat can leave weight flat. Researchers use estimated lean mass too, for example to scale body fluid volumes, which is what Boer's equation was built for.
How can I increase my lean body mass?
Muscle-strengthening exercise is the main driver; CDC's guideline for adults is at least 2 days of it each week. Enough protein, enough total food and enough sleep support it. Aerobic exercise helps with fat loss but does little to build muscle by itself.
How accurate is a lean body mass calculator?
It gives an estimate, not a measurement. The equations know only your height, weight and sex, so they can't tell a muscular person from a heavier one with more fat, and any weight change moves the result whether it was muscle or fat. DEXA scans, underwater weighing and bioelectrical impedance look at body composition itself.