FFMI vs BMI: the difference is what you count

The two formulas answer different questions

BMI = body weight in kg ÷ height in metres squared. Its input is everything on the scale. The CDC describes BMI as a screening measure and explicitly notes that it cannot distinguish fat, muscle and bone. It also recommends considering other information when assessing an individual. Source: CDC BMI explanation.

FFMI = fat-free mass in kg ÷ height in metres squared. This is the definition used in the original Kouri study. Fat-free mass is the estimated non-fat portion of your body; it is broader than skeletal muscle. Source: Kouri et al.

The practical distinction is the numerator. A change in fat mass can change BMI while leaving FFMI unchanged in a simplified calculation. A change in fat-free mass affects both. For your own numbers, use the BMI calculator and FFMI calculator with the same height and weight.

A worked example: same BMI, different FFMI

Take two hypothetical adults who both weigh 80 kg and stand 1.80 m tall. Their height squared is 3.24 m², so both have a BMI of 80 ÷ 3.24 = 24.69. Now give the first person an estimated body fat of 15% and the second 25%.

The FFMI difference describes an 8 kg difference in estimated fat-free mass at this height. It does not prove an 8 kg difference in skeletal muscle. Nor do these values tell you whose training, appearance or health is better. This example demonstrates what the formulas can separate, without attaching a physique grade to either person.

FFMI needs an extra input, and that input matters

BMI requires height and weight. Calculating FFMI from those inputs also requires an estimate of body fat: fat-free mass = weight × (1 − body-fat percentage ÷ 100). That extra estimate creates another way for the result to move.

At 80 kg and 1.80 m, entering 15% body fat produces an FFMI of 20.99. Entering 18% produces 65.6 kg of fat-free mass and an FFMI of 20.25. The 0.74-point difference comes entirely from the input assumption; the hypothetical person has not changed.

Save the measurement method alongside the result. If an estimate seems uncertain, try a reasonable range of inputs and view the output as a range too. An exact-looking decimal does not make the body-fat estimate exact. For the terminology behind that numerator, read lean body mass versus muscle mass.

Choose the metric around your question

• Tracking total weight relative to height: BMI provides that calculation without needing a body-fat estimate.
• Comparing estimated fat-free mass relative to height: FFMI provides the more relevant calculation.
• Checking whether a cut changed your composition: compare weight, estimated fat mass and estimated fat-free mass, keeping the inputs and method visible.
• Understanding strength or appearance: record those outcomes directly instead of expecting either index to describe them.

Also check whether an FFMI value is raw or normalized. CutRank's calculator displays a height-adjusted value using its stated convention; mixing that with another person's raw result creates an avoidable comparison problem. The normalized FFMI guide shows exactly what changes. Neither version turns this comparison into a test for drug use or an individual medical diagnosis.

Common questions

Keep exploring

Sources & further reading

• CDC: About Body Mass Index
• Kouri et al. (1995): Fat-free mass index