Fat-free mass (FFM) is total body weight minus all fat mass. It's a close cousin of lean mass, and the two terms are often used interchangeably, but they aren't always the same number: on a DXA report, "lean mass" usually means lean soft tissue (muscle, organs, skin, water) with bone counted separately, while fat-free mass adds the bone back in. Some older definitions also count a small amount of essential fat inside "lean body mass" but not inside fat-free mass. When you compare results, check which one your scan or study is reporting.
On GLP-1 medications, fat-free mass matters because a share of the weight lost comes from it rather than from fat, and most of what you can influence in it is muscle. Trials often report FFM from DXA or bioimpedance, so knowing the definition helps you read those numbers. See our guides on DXA scans for tracking progress and why strength training matters. An international expert group's 2025 terminology guide sets out how fat-free mass, lean mass, and lean soft tissue differ (Prado et al., American Journal of Clinical Nutrition, 2025 (external link)).