Lean Body Mass Calculator
Enter your height, weight and sex to estimate your lean body mass (LBM) — the total weight of your body minus fat mass.
Reviewed by the ToolNestr Editorial Team — July 2026
Enter your height and weight to estimate lean body mass.
About lean body mass
Lean body mass (LBM) is one of the most informative metrics in body composition analysis. Unlike total body weight, which lumps fat and lean tissue together, LBM tells you how much of your weight is metabolically active. Your LBM is the primary driver of your Basal Metabolic Rate (BMR) — more lean mass means more calories burned at rest. It also correlates strongly with strength, bone density and longevity. Tracking LBM over time helps you see whether your weight changes are coming from fat loss (good) or muscle loss (bad). Pair this with Body Fat Calculator and BMI Calculator for a fuller picture.
⚕️ For general information only — not medical advice.
How it works
Lean body mass is calculated using anthropometric formulas that estimate fat-free mass from height, weight and sex. The Boer formula (published in 1984) is one of the most widely validated and is the primary method used here. We also display the James formula result so you can compare. These formulas assume a standard body composition distribution and are calibrated for the general adult population. They work best for people within a normal to moderately overweight BMI range and are less accurate for extremely muscular individuals or those with very high body fat.
The formula explained
Boer formula:
Men: LBM = 0.407 × weight (kg) + 0.267 × height (cm) − 19.2
Women: LBM = 0.252 × weight (kg) + 0.473 × height (cm) − 48.3
James formula (for comparison):
Men: LBM = 1.10 × weight − 128 × (weight/height)²
Women: LBM = 1.07 × weight − 148 × (weight/height)²
(weight in kg, height in cm)
Worked example
A 35-year-old man, 175 cm tall, weighing 78 kg, wants to know his lean body mass.
Strength athlete
Monitor lean mass gains during a bulk to ensure you are adding quality tissue, not just fat. Compare LBM changes month to month.
Weight-loss seeker
Track whether the weight you are losing is fat or muscle. If LBM drops too fast, increase protein and add resistance training to preserve it.
Older adult
Sarcopenia (age-related muscle loss) is a major health risk. Tracking LBM helps you catch muscle loss early and take action with strength training and protein.
Healthcare practitioner
Use LBM estimates to calculate medication dosages, assess nutritional status and monitor patients for muscle wasting conditions.
How to use this calculator
Enter your stats
Provide your sex, height and weight in either metric or imperial units. Accurate height measurement is important for the formula.
Read your LBM
See your fat-free mass in kilograms (or pounds). The estimated body fat is calculated by subtracting LBM from total weight.
Track over time
Recalculate every 2–4 weeks during a training or diet programme. A rising LBM means you are building muscle while losing fat is the ideal scenario.
Tips for maintaining lean mass
Eat enough protein
Aim for at least 1.6 g of protein per kg of body weight daily when you are active. This provides the amino acids needed to repair and build lean tissue.
Lift weights
Resistance training is the single most effective stimulus for building and preserving lean mass. Without it, any calorie deficit will include some muscle loss.
Don't cut calories too aggressively
Large calorie deficits (more than 500–750 below maintenance) accelerate muscle loss. A moderate deficit with high protein and strength training preserves LBM during weight loss.
Related tools
Frequently asked questions
What is lean body mass?
Lean body mass is everything in your body that is not fat — muscles, bones, organs, skin, blood and water. It is sometimes called fat-free mass and is a key metric for assessing body composition.
Which formula do you use?
We use the Boer formula (the most validated for general populations) and also show the James formula for comparison. Both use height, weight and sex to estimate fat-free mass.
Why does LBM matter?
LBM is more informative than total weight because it tells you how much metabolically active tissue you carry. A higher LBM means a higher BMR, better glucose regulation and greater physical strength.
How is LBM different from muscle mass?
Muscle mass is a component of LBM, but LBM also includes bones, organs, connective tissue and water. Muscle makes up roughly 40–50% of LBM in most people.
How accurate are these estimates?
The Boer formula gives estimates within ±3–4% of DEXA scan results for most adults. Accuracy drops for very muscular or very obese individuals. The James formula is a useful alternative comparison.
Sources & references
This tool uses standard formulas and reference values from:
- • NIH / National Institutes of Health and CDC reference values and formulas. nih.gov
- • Peer-reviewed equations (e.g. Mifflin-St Jeor, Harris-Benedict) as originally published.
- • ACOG for pregnancy, ovulation and due-date methodology. acog.org
Not medical advice. Estimates only — consult a qualified healthcare professional for any personal health decision.