Quick answer: lean body mass is your total weight minus fat mass, calculated using the Boer formula from your weight and height. Use our free Lean Body Mass Calculator for an instant estimate.
What Lean Body Mass Includes
Lean body mass includes muscle, bone, organs, and water — essentially everything except stored fat. It's a useful metric for tracking muscle gain independent of fat loss during a fitness program.
The Boer Formula
The Boer formula estimates LBM using weight and height, with separate equations for men and women, and is widely used in clinical and fitness settings for its balance of simplicity and accuracy.
Frequently Asked Questions
Why track lean body mass instead of just weight?
Weight alone doesn't distinguish muscle from fat — tracking LBM shows whether you're gaining muscle or losing fat during training.
Is this the same as fat-free mass?
Yes, lean body mass and fat-free mass are generally used interchangeably.
How accurate is a formula-based estimate?
It's a reasonable estimate; body composition scans (DEXA, bioelectrical impedance) are more precise but less accessible.
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Open the Tool →How to interpret the result
Lean body mass is an estimate of body weight excluding body fat. Depending on the method, it may include muscle, bone, organs, and body water. It is not the same as skeletal muscle mass, and it does not directly measure strength or health. Formula-based estimates use a small set of inputs such as height, weight, and sometimes sex; people with the same measurements can still have different body composition.
Use the calculator as a rough reference and keep the method consistent if you compare results over time. Hydration, recent exercise, meals, and measurement technique can affect some body-composition readings. Avoid treating a single number as a diagnosis or as proof that a diet or training plan is working.
When a more reliable assessment matters
For clinical decisions, a qualified health professional can explain which assessment method is appropriate and what its limitations are. A formula may be less reliable for people whose body proportions differ from the population used to develop it, including some athletes and older adults. If you are changing nutrition or exercise for a medical reason, use professional guidance rather than relying on an online estimate alone.
Track trends, not one reading
If you record an estimate over time, use the same formula and measurement units each time. A change in hydration or input measurements can shift the result without reflecting a meaningful change in body tissue. Avoid comparing two tools that use different equations as if they were measuring the same thing. If the number affects a treatment, nutrition plan, or athletic decision, ask a qualified professional to interpret it in context.