Lean Body Mass
Body mass excluding fat.
Formula
Variables
Example
80 kg at 20% BF = 64 kg
How it works
Lean body mass is the portion of your body weight that is not fat, so it includes muscles, organs, bones, water, and other non-fat tissues. The formula works by taking the total mass and subtracting the fat fraction, because the remaining fraction of the body is the lean portion. In other words, if body fat makes up BF% of the body, then 1 - BF% is the fraction left over as lean mass.
When to use it
Use this when estimating body composition from a known body fat percentage, such as in fitness tracking, nutrition planning, or adjusting training targets. It is also useful when comparing athletes or patients whose total body weight alone does not show how much of that weight is lean tissue.
Worked example, step by step
Step 1: Write down the total mass and body fat fraction. Total mass m = 72 kg Body fat BF = 25% = 0.25 Step 2: Substitute into the formula. LBM = m(1 - BF) LBM = 72(1 - 0.25) Step 3: Calculate the remaining fraction. 1 - 0.25 = 0.75 Step 4: Multiply by total mass. LBM = 72 × 0.75 = 54 kg Final answer: Lean body mass = 54 kg
Common mistakes to avoid
- Using BF% as 25 instead of converting it to 0.25 before calculating
- Subtracting the percentage from the mass directly, such as 72 - 25, instead of using the fraction of total mass
- Confusing lean body mass with muscle mass only, when it also includes bones, organs, and body water
Did You Know?
Strength training raises lean mass, which quietly increases the calories you burn even while resting.
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