Lean body mass: the part of you that isn't fat
Lean body mass (LBM) is your total body weight minus fat mass — muscle, bone, organs, connective tissue, and body water all included. It's a useful number because it's the metabolically active part of your body that drives most of your calorie burn, and it's what most people actually mean when they talk about wanting to "keep muscle" during a diet or "build mass" during a bulk.
This calculator uses the Boer formula:
Men: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
Women: LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3
Like every height-and-weight-only formula, it's a population-level estimate. It won't capture genuine individual variation in muscularity the way a DEXA scan would, but it's consistent and convenient enough to track meaningfully over time — which, for most people, matters more than lab-grade precision on any single day.
Why this number matters
Tracking lean mass alongside total weight gives a much better picture of progress than the scale alone. Two people can lose the same 5kg, but if one loses mostly fat and the other loses a meaningful chunk of lean mass, the health and aesthetic outcomes are very different. This is part of why slower, more moderate calorie deficits paired with resistance training tend to outperform aggressive crash diets for body composition, even when total weight loss looks similar on paper.