What is lean body mass?
Lean body mass is the weight of everything in your body that is not storage fat. It includes your skeletal muscle, bones, organs, skin, blood, and body water. It is not the same as muscle mass, and it is not exactly fat-free mass — a small amount of essential fat that your body needs to function is typically included in the lean body mass figure produced by commonly used estimation equations.
Knowing your lean body mass can help you set realistic weight-management goals. If you are losing weight, tracking whether the loss is coming from fat or from lean tissue can tell you whether your approach is preserving the body components that support metabolism, strength, and day-to-day function. If you are gaining weight, a rising lean body mass alongside a stable or falling fat mass is a sign that your nutrition and training are working as intended.
How the Boer formula works
This calculator estimates lean body mass using the Boer equation, first published by Piet Boer in 1984. The Boer formula was developed from anthropometric data collected from a large cohort of adults and validated against reference measurements of total body water. It uses only three inputs — weight in kilograms, height in centimetres, and biological sex — to produce a lean body mass estimate in kilograms.
The equations are:
Male: 0.407 × weight (kg) + 0.267 × height (cm) − 19.2
Female: 0.252 × weight (kg) + 0.473 × height (cm) − 48.3
The Boer formula is widely cited in clinical and research settings because it is straightforward to apply and does not require skinfold calipers, bioelectrical impedance devices, or dual-energy X-ray absorptiometry (DXA) scans. It is one of several anthropometric equations used to estimate body composition; others include the Hume, James, and Peters formulas. Each produces slightly different results, and no single equation is universally superior.
Limitations of the inputs
The calculator requires positive values for both weight and height. If the calculated result is zero or negative, or if it exceeds the total body weight entered, the result is rejected — such outcomes are mathematically impossible for a living person and indicate that the input values fall outside the range for which the Boer formula is appropriate.
The equation was developed using data from adults with a relatively typical body composition. It may produce less reliable estimates for people who are very tall or very short, for older adults, for children and adolescents, for pregnant or breastfeeding individuals, and for highly trained athletes whose body composition differs substantially from the general population. If you fall into any of these categories, treat the estimate with additional caution and consider discussing it with a qualified health professional.
Worked example
Consider a male who weighs 70 kg and is 175 cm tall. Plugging these values into the male Boer equation:
Lean body mass = (0.407 × 70) + (0.267 × 175) − 19.2
Step by step:
- 0.407 × 70 = 28.49
- 0.267 × 175 = 46.725
- 28.49 + 46.725 = 75.215
- 75.215 − 19.2 = 56.015
The estimated lean body mass is approximately 56.02 kg. Subtracting this from the total body weight of 70 kg gives an estimated fat mass of roughly 13.98 kg, which represents about 20% body fat. This falls within the range that many health authorities consider typical for an adult male.
The same individual using a different estimation equation might obtain a lean body mass figure that differs by one or two kilograms. That is expected and does not mean one result is wrong — it reflects the inherent variability among anthropometric prediction formulas.
How to interpret the estimate
The number this calculator produces is a body composition estimate, not a direct measurement. It is derived from a population-level equation and should be treated as a rough approximation rather than a precise laboratory value.
Compare your lean body mass estimate to your total body weight to understand your body composition in broad terms. A higher lean body mass relative to total weight generally indicates a lower proportion of body fat, which is associated with a range of positive health outcomes. A lower lean body mass may suggest a higher proportion of body fat, though this is not a diagnosis and should not be taken as one.
Tracking changes in your lean body mass estimate over weeks or months — using the same formula and consistent measurement conditions — can be more informative than looking at a single number in isolation. Trends matter more than any one data point.
This estimate is not a diagnosis, is not medical advice, and is not a treatment recommendation. It is not suitable for dosing medications, planning surgical procedures, or making clinical decisions of any kind. If you have concerns about your body composition, weight, or metabolic health, speak with a doctor or a registered dietitian.
Accuracy and limitations
The Boer formula was validated against total body water measurements and has been shown to correlate well with reference methods in the population it was developed for. However, like all anthropometric equations, it has inherent limitations.
The equation assumes a relatively constant relationship between height, weight, and lean body mass across individuals. In reality, this relationship varies with age, ethnicity, fitness level, and body shape. The Boer formula was derived from a population of Dutch adults and may systematically overestimate or underestimate lean body mass in populations with different body composition characteristics.
The standard error of the estimate for the Boer formula is approximately 2 to 3 kg in the original validation studies. This means that about two-thirds of individual estimates fall within ±2–3 kg of the value that would be obtained from a reference method. For some individuals, the discrepancy can be larger.
Do not use this estimate as a substitute for a comprehensive body composition assessment. If you need a precise measurement for health or performance reasons, consider methods such as DXA scanning, air displacement plethysmography, or hydrostatic weighing — all of which should be administered and interpreted by qualified professionals.
Other factors that influence body composition
Lean body mass is influenced by factors beyond weight and height. Age-related muscle loss, known as sarcopenia, begins as early as the fourth decade of life and accelerates after age 60. Hormonal changes, chronic illness, medication use, and prolonged inactivity can all reduce lean tissue. Conversely, consistent resistance training, adequate protein intake, and sufficient sleep support the maintenance and growth of lean body mass.
The Boer formula does not account for any of these factors. Two people of the same sex, weight, and height will receive the same estimate regardless of whether one is sedentary and the other is a competitive athlete. Use the estimate as a starting point for understanding your body composition, not as a definitive statement about your health or fitness.
Sources
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. American Journal of Physiology. 1984;247(4):F632–F636. https://pubmed.ncbi.nlm.nih.gov/6496691/
- National Institute of Diabetes and Digestive and Kidney Diseases. Staying Active at Any Size. https://www.niddk.nih.gov/health-information/weight-management/staying-active-at-any-size
Editorial record
This article was written by the SoupCalc Editorial Team and draws on peer-reviewed anthropometric research and public health guidance from the National Institutes of Health. The Boer formula and its coefficients were verified against the original 1984 publication. The worked example was recalculated independently to confirm the arithmetic. All content is provided for general informational purposes and does not constitute medical advice, diagnosis, or treatment. Author: SoupCalc Editorial Team Last reviewed: August 11, 2026.