What is lean body mass?
Lean body mass (LBM) is your total body weight minus fat mass. It includes muscle, bone, water, organs, and other non-fat tissue. It is often used interchangeably with fat-free mass (FFM); FFM technically excludes essential body fat, but for fitness and nutrition the difference is negligible. Knowing your LBM helps with setting protein and calorie targets and tracking body composition over time.
How do I calculate lean body mass from body fat percentage?
Use the formula: Lean body mass = total weight × (1 − body fat % ÷ 100). For example, at 70 kg and 20% body fat, fat mass = 14 kg and lean mass = 56 kg. This calculator does that for you and supports both kg and lb. Accuracy depends on how accurate your body fat percentage is (from a scale, calipers, or other method).
Is lean body mass the same as muscle mass?
No. Lean body mass includes muscle, bone, water, and organs—not just muscle. Muscle mass is a part of LBM. So your LBM is always higher than your muscle mass. Formula-based LBM estimates do not separate muscle from other lean tissue.
What is the difference between lean body mass and fat-free mass?
In practice they are often used the same way. Technically, fat-free mass (FFM) excludes essential body fat, while lean body mass can be defined to include a small amount of essential fat. For most fitness and nutrition purposes the difference is negligible, and this calculator gives you an estimate of non-fat mass for planning purposes.
Which formula is best for estimating lean body mass without body fat %?
Boer, James, and Hume are all established equations; results usually differ by 1–3 kg. Boer is commonly used in clinical settings. We show all three plus an average so you can see the range. For a more accurate result, use the direct method if you have a body fat estimate from a scale or calipers.
Is this calculator accurate?
The math is correct. With a known body fat percentage, the direct method gives an exact result given that input. Formula-based (Boer/James/Hume) estimates are population averages and can be off for very muscular, very lean, obese, elderly, or under-18 users. Use results as estimates for fitness and nutrition planning, not as medical diagnosis.
Can I use lb and feet/inches?
Yes. Weight can be entered in kg or lb; height in estimate mode can be in cm or ft/in. When you switch units, the calculator converts your current value so you don't lose your number. Results are shown in both metric and imperial where relevant.
Does this tool store my data?
No. Everything runs in your browser. We don't save or send your weight, body fat, height, or any results. No signup, no server storage. Your data never leaves your device.
How is BMR calculated?
We use the Katch-McArdle formula: BMR = 370 + (21.6 × lean body mass in kg). It uses LBM instead of total weight, so it's often more accurate when you know body composition. The result is resting calories per day (kcal); multiply by an activity factor to estimate total daily energy expenditure.