Glossary
Fat-Free Mass
Updated February 28, 2026
Fat-Free Mass includes lean tissue and water compartments excluding fat mass. Keep terminology clear so it is not confused with body mass or lean mass alone.
Terminology and interpretation
| Term | Definition |
|---|---|
| Body weight | total body mass |
| Fat mass | percentage-based fat estimate |
| Fat-free mass | everything not fat |
| Lean mass | often close to FFM, but context and source vary |
Use lean mass for training focus and body fat percentage for fat change interpretation.
Meaningful change by block
| Block length | Meaningful FFM shift |
|---|---|
| 2 weeks | small noise range, avoid hard conclusions |
| 6 weeks | possible directional signal with adherence support |
| 12 weeks | likely stable trend if measurement is reliable |
Benchmark examples
| Scenario | Setup | Interpretation |
|---|---|---|
| Beginner recomposition | progressive training + protein stable | visible gain can be measurable over several blocks |
| Dieting phase | reduced intake and training maintained | small loss can occur if intake and recovery dip |
| Long cut | aggressive deficit | prioritize fat loss and avoid over-interpreting FFM drops |
Formula remains straightforward.
`FFM = Body weight × (1 − Body fat fraction)`