What is LBM — Lean Body Mass?
LBM is everything in the body except body fat. That sounds like a simple definition, but the key lies in the word : LBM includes the fat the body cannot do without — essential fat.
Essential fat is found in:
- Organs (heart, liver, kidneys)
- The brain and nervous system
- Cell membranes throughout the body
This fat is structurally and functionally necessary — it cannot be reduced without disrupting the body's basic processes. It is therefore counted as part of the "lean" mass in the LBM definition.
What is FFM — Fat Free Mass?
FFM is a stricter definition: completely fat-free tissue. No fat is included — not even essential fat. FFM consists of:
- Muscles
- Organs (without adipose tissue)
- Body fluid and water
- Skeleton and minerals
FFM is thus a purer biochemical definition — the mass you would have left if all fat were eliminated, including what the body actually needs.
The difference in practice
LBM — includes essential fat (organs, brain, cell membranes)
FFM — completely fat-free, no form of fat included
Difference: approx. 3–5% of body weight
For a person at 80 kg with 15% body fat, LBM is approx. 2–3 kg higher than FFM. In most practical contexts — calculating calorie needs, protein intake or training load — that difference is negligible. But it's important to know which measure a given formula expects.
Which measure should you use?
Most measurement methods — DEXA, bioimpedance and calipers — return FFM, not LBM. FFM is the standard measure in research and clinical practice.
Practical consequence: if a formula asks for "fat-free mass", it is FFM you should enter — that is, the value you get directly from your measurement. The Cunningham equation for RMR is a typical example: it is built on FFM and gives more accurate results for well-trained individuals than Mifflin-St Jeor, which only uses total weight.
If you don't know which measure your measurement method gives — assume FFM. It is the most common output and matches the formulas you are likely to use.