Lean Body Mass Calculator
Estimate your lean body mass (LBM) — the weight of everything in your body except fat — using the Boer, James, Hume, and Katch-McArdle formulas. Get your fat mass, body fat percentage, and a detailed breakdown — all for free.
What Is Lean Body Mass?
Lean body mass (LBM) — also called fat-free mass (FFM) — is the total weight of everything in your body except fat: muscles, bones, organs, water, skin, and connective tissue. It's a critical measurement for athletes, dieters, and anyone tracking body composition, because it reflects muscle mass and overall metabolic health more accurately than total weight alone.
This advanced calculator uses four well-established formulas to estimate your lean body mass:
- Boer Formula: Based on weight and height. Simple and reliable for most adults.
- James Formula: Uses a weight-to-height ratio, making it useful for individuals at extremes of size.
- Hume Formula: One of the oldest anthropometric formulas, widely used in clinical settings.
- Katch-McArdle Formula: The most accurate of all — it uses your actual body fat percentage (if you know it) to calculate lean mass directly.
The calculator averages the applicable formulas to give you a robust LBM estimate, then derives your fat mass, body fat percentage, and body composition category.
How Does the Lean Body Mass Calculator Work?
Each formula produces its own LBM estimate. The average of all available formulas becomes your main lean body mass result. Here are the formulas used:
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:
Men: LBM = 1.1 × weight(kg) − 128 × (weight ÷ height)²
Women: LBM = 1.07 × weight(kg) − 148 × (weight ÷ height)²
Hume Formula:
Men: LBM = 0.32810 × weight(kg) + 0.33929 × height(cm) − 29.5336
Women: LBM = 0.29569 × weight(kg) + 0.41813 × height(cm) − 43.2933
Katch-McArdle Formula (if body fat % is provided):
LBM = weight(kg) × (1 − body fat % ÷ 100)
Fat Mass = weight − LBM
Body Fat % = (fat mass ÷ weight) × 100
Lean Mass % = (LBM ÷ weight) × 100
If you enter your body fat percentage, the Katch-McArdle formula is included and tends to be the most accurate. Without it, the calculator averages Boer, James, and Hume.
Why Use This Lean Body Mass Calculator?
- Four validated formulas: Boer, James, Hume, and Katch-McArdle — with a robust average.
- Body fat percentage support: Enter your BF% for the most accurate LBM estimate.
- Complete body composition: See lean mass, fat mass, body fat %, and lean mass % side by side.
- Unit flexibility: Switch between metric and imperial units.
- Visual breakdown: A chart shows your body composition at a glance.
- Free & Private: No registration, no data storage.
Understanding Your Body Composition Category
- Essential Fat (men 2–5%, women 10–13%): The minimum fat needed for normal bodily function.
- Athletic (men 6–13%, women 14–20%): Typical for competitive athletes.
- Fitness (men 14–17%, women 21–24%): Good fitness level.
- Average (men 18–24%, women 25–31%): Average healthy range.
- Obese (men 25%+, women 32%+): Health risks increase significantly.
Note: These categories are general guidelines based on body fat percentage. Individual needs and healthy ranges vary based on genetics, age, activity level, and specific goals. Consult a healthcare professional or registered dietitian for personalized advice.
❓ Lean Body Mass Calculator FAQ
What is lean body mass?
Lean body mass (LBM) is everything in your body that isn't fat — muscle, bone, organs, water, skin, and connective tissue. It's also called fat-free mass (FFM). LBM is a key indicator of metabolic health and physical performance.
How is lean body mass different from muscle mass?
Lean body mass includes muscle and bones, organs, water, and other non-fat tissues. Muscle mass is just the muscle portion. LBM is therefore always larger than muscle mass — often by 20–30 kg or more.
Which formula is the most accurate for LBM?
Katch-McArdle is generally considered the most accurate because it uses your actual body fat percentage instead of relying on population averages. Without body fat data, Boer, James, and Hume are all solid estimates; averaging them (as this calculator does) reduces error.
Why do I need to enter my body fat percentage?
Body fat percentage isn't required, but entering it unlocks the Katch-McArdle formula — the most accurate LBM formula available. If you don't know your BF%, you can use our Body Fat Calculator to estimate it first.
Is lean body mass the same for men and women?
No. Men naturally have more lean mass (especially muscle) than women of the same weight and height. All formulas used in this calculator apply gender-specific coefficients to account for this difference.
How can I increase my lean body mass?
Increase LBM through resistance training (weightlifting) combined with adequate protein intake (1.6–2.2 g/kg of body weight per day). Progressive overload, sufficient sleep, and a slight calorie surplus are key for muscle growth.
Does lean body mass decrease with age?
Yes. Starting around age 30, adults lose about 3–8% of their muscle mass per decade (sarcopenia). Resistance training and adequate protein intake can significantly slow or reverse this decline.
What is a healthy lean body mass?
There's no single "healthy" LBM because it depends on your height, gender, and build. A better indicator of health is your body fat percentage. Aim for the athletic or fitness category (men 6–17%, women 14–24%).
Can I use lean body mass to calculate my calorie needs?
Yes. The Katch-McArdle BMR formula uses lean body mass directly: BMR = 370 + 21.6 × LBM (kg). For most people, this gives a more accurate BMR than formulas based on total weight. Try our BMR Calculator for a full breakdown.
Does lean body mass include water?
Yes. LBM includes all body water, which makes up about 60% of your total body weight. Hydration status can temporarily affect LBM estimates by a few percent.
How often should I recalculate my lean body mass?
Recalculate every 4–8 weeks if you're actively training, or whenever your weight changes significantly (more than 2–3 kg). Tracking LBM over time is a great way to monitor whether you're gaining muscle and losing fat.
Can I use this calculator if I'm pregnant?
This calculator is not designed for pregnant or breastfeeding women. Pregnancy significantly changes body composition and fluid distribution. Consult your healthcare provider for appropriate assessments.
How do I convert measurements between metric and imperial?
This calculator automatically converts between metric and imperial units. Simply select your preferred unit system, enter your measurements, and the calculator handles the conversions for you.