FFMI Calculator

What does FFMI tell you?

Fat free mass index, or FFMI, measures how much lean mass you carry relative to your height. It is a close cousin of BMI, but where BMI counts all your weight, FFMI isolates muscle and other lean tissue, which makes it a far better snapshot of muscularity for people who train. Two people can share a BMI while having very different FFMIs depending on how much of their weight is muscle.

The formula in plain language

First we work out your lean mass by removing body fat from your total weight. Then FFMI equals your lean mass in kilograms divided by your height in metres squared. Because taller people naturally have a slightly different index, we also report a normalized FFMI, which adjusts the figure to a standard height of 1.8 metres so people of different heights can be compared fairly. This is why an accurate body fat percentage matters: if that input is off, your FFMI shifts with it. The formula and its source are documented on the methodology page.

How should you read the bands?

These are neutral, descriptive ranges, not judgments about anyone's body. A high FFMI simply means a lot of lean mass for your height. If you want to add lean mass, pair the body recomposition calculator or weight gain calculator with a high protein intake from the protein calculator.

When is FFMI not the right measure?

FFMI describes muscularity, not health or fitness, so read it as one number among many. It is unreliable if your body fat estimate is a guess, and it says nothing about central fat, which the waist to height ratio calculator screens far better. Its best use is tracking your own lean mass over time as you train, using the same body fat measurement method each time.

FFMI, lean body mass, and comparing men and women

FFMI is built directly on your lean body mass, so the same inputs give you both: lean mass is your weight minus fat, and the index simply scales it to height. That makes this a practical lean body mass calculator as well, since the panel shows the kilograms of lean tissue behind your score. FFMI works for men and women using the identical formula, but typical values differ because women naturally carry less muscle and more essential fat, so a strong female FFMI usually sits a few points below the male bands. Compare yourself to your own past numbers rather than across sexes.

Limitations of this calculator

FFMI is only as accurate as the body fat percentage you enter, and consumer methods like smart scales or calipers can be several points off, which shifts the index. It describes muscularity, not health, athletic performance, or fat distribution, and the interpretation bands are descriptive ranges from mostly male reference data, not medical cut-offs. Very tall or short people should lean on the normalized figure. Use it to track your own trend with a consistent measurement method, not to judge a body against a chart.

Factors that change your FFMI

• Body fat accuracy: the single biggest factor, since lean mass is weight minus fat; a caliper or scale error of a few points moves your FFMI.
• Measurement method: DEXA, calipers, and smart scales disagree, so use the same method each time to track a real trend.
• Height: the normalized figure adjusts to 1.8 m, which matters most for people who are notably tall or short.
• Training history: years of resistance training raise lean mass and FFMI; detraining lowers it.
• Sex: the same formula applies, but typical female values sit a few points below the male bands.
• Hydration and timing: big shifts in water or glycogen slightly change measured weight and body fat on the day.