Lean Body Mass Calculator

Lean body mass (LBM), also called fat-free mass, is your total body weight minus your body fat. It includes muscle, bone, organs, water, and connective tissue. Tracking LBM helps you tell whether weight change is muscle or fat.

This tool uses two height-and-weight formulas. Boer, men: LBM = 0.407·weight(kg) + 0.267·height(cm) − 19.2; women: 0.252·weight + 0.473·height − 48.3. James, men: 1.1·weight − 128·(weight/height)²; women: 1.07·weight − 148·(weight/height)². Both estimate LBM without measuring body fat directly.

Lean body mass includes everything that isn't fat — muscle plus bone, organs, and water. Skeletal muscle is only part of LBM, typically a bit under half of it. So LBM is always larger than muscle mass alone.

The Boer formula is widely used in clinical settings and tends to be reliable across a broad range of body types. The James formula can read low for heavier individuals. Seeing both gives you a sensible range; for most people they're within a few pounds.

LBM drives your resting metabolic rate — more lean mass means you burn more calories at rest. Preserving LBM during weight loss (via protein and resistance training) keeps your metabolism higher and improves body composition.