FREQUENTLY ASKED QUESTIONS

FFMI stands for Fat-Free Mass Index. It's a way of expressing your fat-free mass (everything in your body that isn't fat — muscle, bone, organs, water, and connective tissue) relative to your height, similar in spirit to how BMI relates weight to height. It's calculated by dividing your fat-free mass in kilograms by your height in meters, squared.

First, we estimate your lean body mass: LBM = Weight × (1 − Body Fat % ÷ 100). Then raw FFMI = LBM ÷ Height² (with height in meters). You can see the exact formulas, with your own numbers plugged in, on the Calculator page, and the full derivation on our Methodology page.

Raw FFMI on its own still slightly favors taller or shorter people depending on their exact proportions. Normalized FFMI applies a small height-based adjustment — Normalized FFMI = FFMI + 6.1 × (1.80 − Height in meters) — so that people of different heights with proportionally similar builds land in a similar range. This adjustment technique comes from the original 1995 research paper that introduced FFMI (Kouri et al.), though that paper used a slightly different constant (6.3) than the commonly-used 6.1 variant implemented here.

Fat-free mass (also called lean body mass) is your total body weight minus your fat mass. It includes muscle, but also bone, organs, body water, and connective tissue — so it is not the same thing as "muscle mass," even though muscle usually makes up the largest share of it.

Because nearly every other metric on this Calculator — lean mass, fat mass, FFMI, and even the calorie and protein recommendations — is derived from your body fat percentage. If that number is off, everything calculated from it shifts too. This is exactly why body fat percentage is usually the single biggest source of uncertainty in the whole calculation, whether it's entered directly, measured with a tape (Navy method), or roughly estimated from age and gender when no other information is available.

FFMI is only as accurate as the body fat percentage used to calculate it, and body fat percentage itself is genuinely difficult to measure precisely outside a lab (a tape measurement, a smart scale, and a DEXA scan can all disagree for the same person). Treat your FFMI result as a reasonable estimate rather than an exact measurement, and don't read small differences of a point or two as meaningful — they're often just measurement noise.

No. BMI (Body Mass Index) is simply your total weight divided by height squared — it doesn't distinguish between fat mass and lean mass at all, which is why two people with the same BMI can look completely different. FFMI specifically isolates the fat-free portion of your weight, which is why we show it as the primary metric and BMI only as secondary context.

The biggest limitation is that body fat percentage — the input that everything else depends on — is itself an estimate unless you've had it measured by a clinical method like DEXA or hydrostatic weighing. FFMI also doesn't distinguish muscle from other fat-free tissue, doesn't account for bone density or frame size directly, and the descriptive reference bands (Below Average / Average / Above Average / High) are general reference points rather than a rigorous population percentile study. None of the results here are a medical assessment.

Both. The Calculator has a Metric (kg / cm) and Imperial (lb / ft-in) toggle for every measurement — weight, height, neck, waist, and hip. Switching units converts your already-entered values automatically, so you don't lose your inputs.

No. Every calculation runs locally in your browser — the measurements you enter aren't transmitted to our server as part of the calculation, and we don't ask for an account or login. See our Privacy Policy for the full details, including the third-party services (like analytics) this site uses.