Lean Body Mass Calculator
Calculate lean body mass using Boer, James, and Hume formulas. Get fat mass, body fat percentage, and FFMI from your weight and height. Metric and imperial.
Enter weight, height, and sex to calculate lean body mass.
Lean Body Mass Calculator: LBM, Body Fat, and FFMI
Lean body mass (LBM) is the weight of everything in your body except fat, which includes muscle, bone, organs, connective tissue, and water. Knowing your LBM is essential for setting accurate protein targets, calculating precise caloric needs, and tracking true body composition changes over time. This calculator uses three validated clinical formulas (Boer, James, and Hume) and displays the average alongside individual results so you can see the range of estimates.
Boer (female): LBM = 0.252 x weight(kg) + 0.473 x height(cm) - 48.3
James (male): LBM = 1.1 x weight - 128 x (weight/height)^2
James (female): LBM = 1.07 x weight - 148 x (weight/height)^2
Hume (male): LBM = 0.3281 x weight + 0.3393 x height - 29.5336
Hume (female): LBM = 0.2969 x weight + 0.4165 x height - 43.2933
FFMI = LBM(kg) / height(m)^2
Body Fat % = (Total Weight - LBM) / Total Weight x 100
The Three Formulas Compared
The Boer formula (1984) is widely used in clinical pharmacokinetics for drug dosing calculations and performs consistently across a broad weight range. The James formula (1976) is one of the oldest and most cited but can produce inaccurate results in heavier individuals due to its quadratic weight term. The Hume formula was derived from cadaver studies and tends to give lower estimates. Averaging all three reduces the impact of any single formula's limitations.
Fat-Free Mass Index (FFMI)
FFMI normalises lean mass for height in the same way BMI normalises total weight. For men, an FFMI below 17 is below average, 17 to 20 is average, 20 to 22 is above average, 22 to 24 is excellent, and above 24 is elite. For women, the thresholds are approximately 3 points lower. Research suggests that natural muscular development rarely pushes FFMI above 25 in men, making it a useful benchmark for assessing physique progress.
Why LBM Matters for Nutrition
Many nutritional recommendations are based on LBM rather than total body weight. Protein requirements for muscle building are typically expressed as 1.6 to 2.2 grams per kilogram of LBM per day. Using total body weight inflates targets for people with higher body fat, while using LBM gives a more accurate reflection of actual muscle tissue that needs to be supported. Basal metabolic rate calculations also become more precise when LBM is known.
Limitations and Clinical Context
These formulas estimate LBM from weight and height alone and cannot account for individual variation in bone density, hydration status, or muscle distribution. The most accurate methods for measuring LBM are DEXA scanning, hydrostatic weighing, and air displacement plethysmography, all of which require clinical or laboratory equipment. Use this calculator as a starting point for tracking trends over time rather than as an absolute measure.
Tips & Recommendations
The three formulas can differ by 3 to 5 kg. Use the averaged result for nutrition planning rather than relying on any single formula.
Estimated LBM is most useful as a tracking tool. Measure at the same time of day, same hydration level, and same clothing to keep comparisons consistent.
For muscle building, aim for 1.6 to 2.2 g of protein per kg of LBM daily. This is more precise than basing protein on total body weight.
FFMI above 20 (men) or 17 (women) represents above-average muscularity. Use it alongside strength metrics to track long-term training progress.
Frequently Asked Questions
What is lean body mass?
Lean body mass (LBM) is the portion of your total body weight that is not fat. It includes muscle, bone, organs, water, and connective tissue. LBM is important for calculating caloric needs, protein targets, and tracking body composition changes during training or dieting.
Which LBM formula is most accurate?
No single formula is universally most accurate. The Boer formula performs well across a wide range of body weights. The James formula can produce unreliable results for heavier individuals. The Hume formula was derived from cadaver studies. This calculator shows all three and averages them for a balanced estimate.
What is FFMI and why does it matter?
Fat-Free Mass Index (FFMI) is lean body mass divided by height in meters squared. It normalises muscle mass for height, similar to how BMI normalises weight. An FFMI above 25 in men or 20 in women is considered exceptional. Values above 26 in men are rarely seen without performance-enhancing substances.
How can I increase my lean body mass?
Progressive resistance training (weightlifting) combined with adequate protein intake (1.6 to 2.2 g per kg of body weight per day) is the most evidence-based approach to increasing LBM. Sufficient sleep and a slight caloric surplus also support muscle growth. Consult a qualified professional for personalised advice.
Is lean body mass the same as muscle mass?
No. Lean body mass includes all non-fat tissue: muscle, bone, organs, connective tissue, and water. Skeletal muscle typically makes up about 40 to 50 percent of LBM in healthy adults. Muscle mass alone is a subset of LBM.
Can I calculate body fat percentage from LBM?
Yes. Body fat percentage equals fat mass divided by total weight, multiplied by 100. Fat mass equals total weight minus LBM. This calculator computes body fat percentage automatically from the average LBM of the three formulas.
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