Lean body mass is everything you weigh that isn't fat — muscle, bone, organs, skin and body water. This calculator estimates it from height, weight and sex using the Boer formula, shows how two older formulas compare, and works out the body fat percentage and fat mass the estimate implies.
- What it estimates
- Body weight minus fat mass
- Primary formula
- Boer (1984)
- Compared with
- James (1976) and Hume (1966)
- Inputs
- Sex, weight, height
- Applies to
- Adults; not validated in children or pregnancy
Formula
Boer formula (1984)Men: LBM = 0.407 × W + 0.267 × H − 19.2 Women: LBM = 0.252 × W + 0.473 × H − 48.3
- LBM
- — Lean body mass, in kilograms
- W
- — Body weight in kilograms (pounds × 0.4536)
- H
- — Height in centimetres (inches × 2.54)
Boer is linear in both weight and height, so the estimate always rises with weight. The James formula, still common in older software, includes a weight-squared term that makes its estimate level off and then fall at high body weight — an artefact, not physiology. Hume is shown for comparison. Implied body fat is (1 − LBM ÷ W) × 100.
Lean Body Mass examples
| Input | Result | Category |
|---|---|---|
| Man, 5'10", 180 lb | 135.6 lb | Boer formula |
| Man, 6'0", 220 lb | 154.9 lb | Boer formula |
| Man, 5'9", 300 lb | 182.9 lb | Boer formula |
| Man, 180 cm, 75 kg | 59.4 kg | Boer formula |
| Woman, 5'5", 150 lb | 103.5 lb | Boer formula |
| Woman, 5'4", 125 lb | 94.5 lb | Boer formula |
| Woman, 5'5", 250 lb | 128.7 lb | Boer formula |
What is lean body mass?
Lean body mass (LBM) is your total weight minus the weight of your body fat. It includes skeletal muscle, bone, internal organs, skin, connective tissue and the water held in all of them. It is not the same as muscle mass: muscle is the largest single part of lean mass, but only one part.
The terms lean body mass and fat-free mass are often used interchangeably. Strictly, lean body mass includes the small amount of essential fat inside organs and cell membranes, while fat-free mass excludes all fat, but for height-and-weight estimates like these the difference is not meaningful.
Why does lean body mass matter?
- Drug dosing: many medications distribute mainly into lean tissue, so in people with obesity, dosing on total weight can overshoot. Pharmacologists use lean or fat-free mass estimates as a size descriptor for this reason.
- Protein targets: protein needs track lean tissue more closely than total weight, which matters most for people carrying a lot of fat mass.
- Body recomposition: if you are losing fat and gaining muscle at the same time, the scale can stay flat while lean mass rises. Tracking both tells you what is actually changing.
- Energy needs: lean tissue is the main determinant of resting energy expenditure, which is why some BMR equations use lean mass directly.
Which lean body mass formula is most accurate?
| Formula | Men | Women |
|---|---|---|
| Boer (1984) | 0.407W + 0.267H − 19.2 | 0.252W + 0.473H − 48.3 |
| James (1976) | 1.1W − 128(W/H)² | 1.07W − 148(W/H)² |
| Hume (1966) | 0.32810W + 0.33929H − 29.5336 | 0.29569W + 0.41813H − 43.2933 |
All three were fitted to small adult samples, and none is accurate for a given individual. This calculator leads with Boer because it behaves sensibly across the whole adult weight range. Hume, derived from body-water measurements in 56 adults, is shown alongside so you can see how much the methods disagree for your height and weight.
The James formula's (W/H)² term is the problem. As weight climbs, that subtracted term grows faster than the 1.1W or 1.07W term, so the curve flattens and then turns down. Working through the equation, the peak falls at a BMI of about 43 for men and about 36 for women regardless of height; past it, James predicts a heavier person has less lean mass. This calculator flags James at a BMI of 35 or more, or when it falls well below Boer.
How is lean body mass measured directly?
| Method | How it works | Practical notes |
|---|---|---|
| DEXA (dual-energy X-ray absorptiometry) | Low-dose X-rays at two energies separate fat, lean tissue and bone | Widely regarded as the clinical reference; low radiation; offered by many clinics |
| Bioelectrical impedance (BIA) | A small current through the body; lean tissue conducts better than fat | Cheap and quick; strongly affected by hydration, food and exercise |
| Hydrostatic (underwater) weighing | Body density from weight in air vs. under water | Accurate but cumbersome; now less common |
A height-and-weight formula cannot tell a muscular 90 kg person from a sedentary one of the same height. If the number matters — for example, to track a training programme — a measured method is the right tool, and repeating it on the same device under the same conditions matters more than the device chosen.
Who shouldn't use a lean body mass formula?
- Children and adolescents — the equations were derived in adults.
- Pregnancy — weight gain from the pregnancy is not lean or fat tissue in the usual sense.
- Very muscular people — the formulas assume average build and will underestimate lean mass.
- People with fluid retention, such as from heart, liver or kidney disease — extra water counts as lean mass and inflates the estimate.
How do you increase lean body mass?
Resistance training is the main driver of muscle gain, supported by adequate total protein and enough energy intake to build tissue. During weight loss, resistance training and sufficient protein help preserve lean mass so that more of the weight lost comes from fat.
Because these formulas only use height and weight, they cannot show recomposition: if your weight stays the same while you gain muscle and lose fat, the estimate won't change. Use a waist measurement or a measured method to track that.
Frequently asked questions
What is a normal lean body mass?
There is no single normal value, because lean mass scales with height, frame and sex. A more useful view is the implied body fat percentage, which this calculator shows. For adult men, lean mass is usually the large majority of body weight; women naturally carry proportionally more essential fat.
Is lean body mass the same as muscle mass?
No. Lean body mass is everything except fat — muscle, bone, organs, skin, connective tissue and water. Muscle is the biggest component, but lean mass will always be considerably higher than muscle mass alone, so the two numbers shouldn't be compared directly.
Which lean body mass formula should I use?
Boer is a sound default for adults and is the one this calculator leads with. James is widely used but unreliable at high BMI, where it can predict lean mass falling as weight rises. Hume gives somewhat different values. All are estimates; DEXA measures directly.
How accurate is a lean body mass calculator?
It estimates the lean mass of a typical person with your height, weight and sex. For an individual it can be off by several kilograms, more so if you are very muscular, very sedentary or carry extra fluid. Treat it as a ballpark and use DEXA for precision.
Why does the James formula give a lower result for me?
The James equation subtracts a term based on weight divided by height, squared. At higher body weights that term grows quickly and pulls the estimate down — eventually below what a lighter person would get. That's a limitation of the formula, not a finding about your body.
How do I calculate body fat from lean body mass?
Subtract lean body mass from total weight to get fat mass, then divide fat mass by total weight and multiply by 100. For example, a 180 lb man with an estimated lean mass of 135.6 lb has about 44.4 lb of fat, or roughly 24.7% body fat.
Sources
- Estimated lean body mass as an index for normalization of body fluid volumes in humans — Boer P, American Journal of Physiology, 1984
- Prediction of lean body mass from height and weight — Hume R, Journal of Clinical Pathology, 1966
- Quantification of lean bodyweight — Janmahasatian et al., Clinical Pharmacokinetics, 2005