FFMI Calculator
Fat-free mass index (FFMI) relates estimated lean mass to height.
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What this calculator estimates
Fat-free mass index (FFMI) relates estimated lean mass to height. It answers a different question from BMI: how much non-fat mass is present for a given height.
Inputs and calculation method
Enter weight in kilograms, height in centimetres and body fat as a percentage such as 20, not 0.20. The tool calculates lean mass first, then reports FFMI and a separate height-adjusted value.
Formula and units
Lean mass (kg) = weight × (1 − body fat % ÷ 100). FFMI = lean mass ÷ height in metres². This tool’s height adjustment is FFMI + 6.1 × (1.8 − height in metres). The two indices are rounded to one decimal place.
How to use it
- Enter weight in kg and height in cm.
- Use a body-fat estimate measured with a consistent method.
- Calculate and read both the raw and adjusted FFMI.
- When comparing results over time, keep the body-fat measurement method the same.
Worked example
At 70 kg, 165 cm and 28% body fat, estimated lean mass is 70 × 0.72 = 50.4 kg. FFMI is 50.4 ÷ 1.65² = 18.5. Adding 6.1 × (1.8 − 1.65) gives a height-adjusted FFMI of 19.4.
How to read the result
FFMI is an index, not kilograms of muscle. Lean mass includes water, bone and organs as well as muscle. The adjusted result is a mathematical comparison to 1.8 m, not a measurement of what you would weigh at another height.
Common mistakes
- Entering a decimal fraction instead of a percentage.
- Comparing tape, scale and laboratory body-fat estimates as if they were interchangeable.
- Using an FFMI threshold to claim someone uses performance-enhancing drugs.
Method limitations
Body-fat measurement error feeds directly into FFMI. The index cannot diagnose health, identify drug use or isolate muscle growth from changes in body water.
Supporting guides for this calculator
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FFMI Calculator FAQs
Is FFMI the same as BMI?
No. BMI uses total weight, whereas FFMI uses estimated fat-free mass.
What if body-fat percentage is uncertain?
Try the plausible upper and lower estimates. The FFMI difference shows the uncertainty introduced by that input.
Does a higher FFMI always mean better health?
No. This calculator does not assess health outcomes, movement ability or the reasons for a change in lean mass.
Using the estimate
Compare FFMI only alongside the input method and its uncertainty. A consistent body-fat measurement is more useful than treating the last decimal place as exact.