Lean Body Mass Calculator
lbs
20%
Lean Body Mass144.0 lbs
Fat Mass36.0 lbs
Body Fat %20.0%
Lean body mass (LBM) is everything in your body that is not fat: muscle, bone, organs, connective tissue, and the water they hold. This tool splits your total weight into lean mass and fat mass from a single body-fat percentage, which is useful for tracking whether weight changes are coming from muscle or from fat. Knowing your lean mass also helps set realistic protein and calorie targets, since metabolically active tissue — not stored fat — drives most of your daily energy use.
Formula
LBM = weight × (1 − bodyFat% / 100)
- weight
- Total body weight (converted internally to kilograms)
- bodyFat%
- Estimated body-fat percentage, between 0 and 100
- LBM
- Lean body mass — the fat-free portion of your weight
How it works
- Enter your total body weight in pounds and your estimated body-fat percentage (from a scale, calipers, a DEXA scan, or a body-fat calculator).
- The weight is converted to kilograms, then multiplied by (1 − bodyfat/100) to give lean mass; the remainder is your fat mass.
- Both lean mass and fat mass are converted back to pounds and displayed, so you can watch how each part shifts as your training and diet change.
Worked example
A person weighs 180 lb with a measured body fat of 20%.
- Fat-free fraction: 1 − 20/100 = 0.80.
- Convert weight: 180 × 0.453592 = 81.65 kg.
- Lean mass: 81.65 × 0.80 = 65.32 kg → 65.32 × 2.20462 = 144.0 lb.
Lean body mass ≈ 144 lb and fat mass ≈ 36 lb.
Frequently asked questions
- How is lean body mass different from muscle mass?
- Lean body mass includes muscle but also bone, organs, skin, and body water. Skeletal muscle is only one part of it, so LBM is always larger than your muscle mass alone.
- Why does this calculator need my body-fat percentage?
- This method derives lean mass directly from body fat, so it is only as accurate as the percentage you supply. A measured value from a DEXA scan or careful caliper reading will give a far more reliable split than a rough guess.
- Should I aim to increase my lean body mass?
- For most people, gradually building lean mass through resistance training and adequate protein supports strength, metabolism, and healthy aging. The right target depends on your goals, so treat rising lean mass as a positive trend rather than chasing a fixed number.
- Does lean body mass change when I lose weight?
- It can. Rapid or very low-protein dieting tends to strip away lean tissue alongside fat, whereas a moderate deficit with enough protein and strength training helps preserve it. Tracking lean mass shows which is happening.