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Lean Body Mass Calculator

Lean body mass is everything you are made of that isn’t fat — muscle, bone, organs, and water. This calculator estimates yours from your height and weight, and uses your body fat percentage directly when you know it.

Assigned sex

The Lean Body Mass Calculator is only an estimate, based on your height, weight, assigned sex, and body fat percentage where you provide one. Predictive formulas are drawn from general adult populations, so the figures may be less accurate for people with high muscle mass, pregnant women, children, the elderly, or those with specific health conditions. It is not a body-composition scan and should not be used as a sole diagnostic tool.

The calculator does not determine eligibility for any weight-loss medications or treatments provided via &you. Consultation with a licensed healthcare provider is required to assess suitability for any medical treatment based on individual health and medical history.


What's the difference between lean mass and muscle mass?


Lean body mass includes muscle, bone, organs, and water — everything that isn't fat. Skeletal muscle typically makes up 40–50% of lean mass.

What's FFMI?

Fat-free mass index normalises lean mass against height, like BMI. Normalised FFMI of ~25 is a natural ceiling for men without performance-enhancing drugs; above that raises questions.


Which formula is most accurate?


If you know your body fat %, use that directly — most accurate. Boer is generally favoured for clinical use; James is older but still cited. Provide both for context.

Prescription weight loss treatments

Sustainable weight loss takes more than medication — it starts with the right plan for your body and lifestyle.

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