Lean Body Mass Calculator
Estimate lean body mass with the Boer, James and Hume formulas, or exactly from your body fat percentage.
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Tool guide
What this tool does
The Lean Body Mass Calculator estimates fat-free mass from height, weight and sex using three published formulas, or computes it directly when you know your body fat percentage. It reports fat mass, lean mass share and the Katch-McArdle BMR that follows from it.
Lean body mass (LBM) is your total weight minus fat: muscle, bone, organs, skin and water. It drives your metabolic rate, is used to dose some medicines, including anaesthetic agents and certain antibiotics, and is what strength training aims to increase. Two people of the same weight can differ by 10 kg or more in lean mass.
Without a body composition scan, LBM is estimated from height and weight. This calculator shows three classic equations: Boer (1984), commonly used in pharmacology and the default here; James (1976), which includes a squared weight-to-height term and breaks down at high BMI; and Hume (1966). If you have a body fat percentage from DEXA, calipers or the US Navy method, the tool uses weight × (1 − body fat) instead, which reflects your own composition.
How to use Lean Body Mass Calculator
- Choose metric or imperial units and select your sex.
- Enter your weight and height.
- If you know your body fat percentage, enter it; otherwise leave it at 0.
- Read your lean body mass, lean share and fat mass.
- Compare the three formula estimates in the chart and table.
- Use the Katch-McArdle BMR figure as an input for calorie planning.
Tips for better results
- Track lean mass alongside weight during a diet to check you are losing mostly fat.
- Use the same body fat method each time; switching methods can create apparent changes that are not real.
- When formulas disagree a lot, your build is probably far from average, so a measured body fat figure is worth getting.
Common use cases
- Feeding lean mass into the Katch-McArdle BMR formula.
- Setting protein targets per kg of lean mass.
- Pharmacy and nursing students practising dose calculations.
- Monitoring muscle retention during a cutting phase.
Common mistakes to avoid
- Using the James formula at high BMI, where it can produce lean mass that falls as weight rises.
- Assuming lean mass equals muscle; it also includes bone, organs and water.
- Comparing morning and evening results, which can differ because of hydration.
Frequently asked questions
What is the Boer formula?
For men, LBM = 0.407 × weight (kg) + 0.267 × height (cm) − 19.2. For women, 0.252 × weight + 0.473 × height − 48.3.
How do I calculate lean body mass from body fat?
Multiply weight by (1 − body fat ÷ 100). An 80 kg person at 20 percent body fat has 80 × 0.8 = 64 kg of lean mass.
Why does the James formula say "out of range"?
Its squared term makes estimates fall at high weights, so for some people it returns negative or impossible values. The tool hides those results.
What is a normal lean body mass percentage?
Lean mass is usually about 75 to 85 percent of body weight in men and 65 to 75 percent in women, mirroring typical body fat ranges.
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