FFMI Calculator

What is FFMI?

FFMI stands for Fat-Free Mass Index. It estimates how much lean mass you carry relative to your height. Unlike weight alone, FFMI tries to separate “how big you are” into lean mass versus fat mass, using your body fat % input.

Normalized FFMI adjusts the score to a standard height so taller or shorter people can compare more fairly. The most useful way to use FFMI is as a trend metric: keep inputs consistent and watch how the number changes over time.

What This FFMI Calculator Tells You

The FFMI calculator above estimates your Fat-Free Mass Index (FFMI) based on your height, weight, and body fat percentage. Unlike scale weight alone, FFMI focuses on how much lean mass you carry relative to your height.

After entering your stats, the calculator shows your FFMI, your height-adjusted (normalized) FFMI, and where you sit compared to typical ranges for men and women. This makes it easier to tell whether changes in your body are coming from real muscle gain or from fat.

If you want an absolute lean-mass estimate before normalizing for height, use the Lean Body Mass Calculator. For a skeletal-muscle-specific estimate from circumferences and skinfolds, use the Muscle Mass Calculator.

To understand how skeletal build can change visual appearance at the same body-fat level, check the Body Frame Size Calculator.

If your goal is estimating whether your current mass is still within common natural limits after frame adjustment, use the Natty or Not Calculator.

If you want an explicit lean-mass ceiling model and stage table, use the Muscular Potential Calculator.

If you want a combined score that blends structure leverage with projected muscular potential, use the Bodybuilding Genetics Calculator.

If you want a waist-centered roundness score, compare with the BRI Calculator. If you prefer a hip-height adiposity estimate, use the BAI Calculator.

For the most accurate results, use a realistic body fat estimate and your current body weight. FFMI works best as a trend metric — something you track over time as your training and nutrition evolve.

Why Measure FFMI?

Most people are familiar with BMI (Body Mass Index). It uses only height and weight, which makes it simple — but also deeply flawed for anyone who trains.

BMI does not distinguish between muscle and fat. A lean, muscular athlete and someone carrying a high amount of body fat can end up with the same BMI, even though their bodies — and health profiles — are completely different. That tradeoff is covered in BMI vs Body Fat % – Which Is Important?.

FFMI fixes this by stripping away fat mass and focusing on lean body mass relative to height. It answers a more useful question: “How much muscle am I actually carrying for my frame?”

That makes FFMI especially valuable if you lift weights, care about physique changes, or want to know whether your training is building muscle — not just adding scale weight. For appearance variability at the same percentages, see Why Two People at the Same Body Fat Percentage Look Different.

FFMI vs Normalized FFMI

Standard FFMI measures how much lean mass you carry relative to your height. However, FFMI naturally increases as height increases — even if two people have the same muscular development.

This is why normalized FFMI exists. It adjusts FFMI to a standard reference height, allowing for fairer comparisons between people who are taller or shorter than average.

In most research, normalized FFMI is calculated using an average height of roughly 5’10–5’11. This makes normalized FFMI the better metric when comparing yourself to population ranges or to other individuals.

If your goal is tracking personal progress, both numbers are useful. If your goal is comparison, normalized FFMI is the one to focus on.

Is FFMI Backed by Research?

Yes. FFMI has been studied in sports science and body composition research, including work examining FFMI in trained athletes and comparing typical ranges across groups.

In practice, FFMI is often used as a rough reference for muscularity (lean mass relative to height). It’s not a diagnosis — but it can be a useful benchmark when paired with consistent inputs and trend tracking over time.

If your FFMI looks unusually high, double-check the basics first: height, weight, and especially body fat percentage. Small errors in body fat % can move FFMI more than people expect.