FFMI Calculator

Fat-free mass index from height, weight, and body fat, with a height-normalized score.

What it does

Fat-free mass index turns lean mass into a height-adjusted number. It answers a simple question that weight alone cannot: how much muscle, bone, organ, and water mass does a person carry for their frame? Two people can weigh the same and score very differently once body fat and height enter the calculation.

The raw index is fat-free mass in kilograms divided by height in meters squared. The normalized version adds a small correction for height, because taller people naturally carry more lean mass and would otherwise look better on the raw score. The correction is 6.1 points for every meter away from 1.8 m.

The default example is a man at 180 cm, 80 kg, and 15 % body fat. His fat mass is 12.0 kg, his lean mass is 68.0 kg, and both FFMI values land at 21.0 kg/m2. The imperial mirror uses 70.9 in and 176.4 lb and returns the same rounded score.

FFMI was studied as a way to compare muscularity across body sizes. Typical untrained men often sit in the high teens or low twenties, while trained men can move into the low twenties and above. Values above 25 are uncommon in drug-free samples, but a single number cannot prove how anyone trains or what they use.

The result is an estimate because the body fat input is an estimate. Skinfold, scale, and tape methods all carry error, and that error passes straight into lean mass. Use the tool to track one person over time under similar conditions, not to rank strangers or make medical decisions.

How to use

1. Pick metric or imperial units.
2. Enter standing height without shoes.
3. Enter body weight at a consistent time of day.
4. Enter a body fat percentage from a measurement or a careful estimate.
5. Read the raw FFMI first, then the height-normalized value.
6. Repeat under the same conditions every few weeks and compare the trend.

Formula

FFM

Fat-free mass in kg.

W

Body weight in kg.

BF

Body fat percentage as a decimal.

H

Standing height in meters.

6.1

Height normalization slope in kg/m2 per meter.

The raw index favors taller people because lean mass grows faster than height squared across normal adult frames. The 6.1 slope shifts the score toward a common 1.8 m reference. The 1995 abstract prints 6.3, while later work and this tool use 6.1; the difference is 0.02 points for every 10 cm away from 1.8 m. It is a comparison aid, not a correction that makes every body type equivalent. The body fat estimate remains the largest source of error.

Worked example

Reference table

Limits

• The score inherits every error in the body fat estimate, which can easily be several percentage points.
• Height normalization is a simple linear correction, not a full model of frame size or limb length.
• Very lean, very muscular, older, and fluid-shifted bodies can all sit away from the reference sample.
• The index is not a diagnostic test for steroid use, health status, or athletic eligibility.

Common mistakes

• Entering body fat as a whole number when the field expects a percentage, such as 0.15 instead of 15.
• Using a scale body fat reading from a different time of day than the weight reading.
• Comparing raw FFMI between a 160 cm and a 195 cm person without checking the normalized column.
• Treating a score above 25 as proof of drug use, or a score below 20 as proof of no training.
• Chasing a higher score by cutting weight instead of preserving lean mass.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• Kouri et al., fat-free mass index in users and nonusers of anabolic-androgenic steroids (PubMed)
• Trexler et al., fat-free mass index in collegiate football players (PMC)

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