Fat Mass
Total mass of body fat.
Formula
Variables
Example
80 kg at 20% ⇒ 16 kg of fat
How it works
This formula finds the actual mass of fat in the body by taking the total body mass and multiplying it by the fraction that is fat. Since body-fat percentage is just a percentage of the whole body, dividing by 100 converts it into a decimal proportion before multiplying. The result tells you how many kilograms of the total body weight come from fat tissue.
When to use it
Use this when you know someone’s body mass and body-fat percentage and want to estimate their fat mass in kilograms. It’s common in fitness tracking, health assessments, and body-composition calculations.
Worked example, step by step
Body mass, m = 68 kg Body-fat percentage, BF% = 25% FM = m × (BF% / 100) FM = 68 × (25 / 100) FM = 68 × 0.25 FM = 17 kg So the fat mass is 17 kg.
Common mistakes to avoid
- Using the percentage as 25 instead of 0.25 in the calculation
- Confusing fat mass with body mass or lean mass
- Forgetting that the result is in kilograms when the body mass is given in kilograms
Did You Know?
Where fat sits matters more than the total — fat around the organs is far riskier than fat under the skin.
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