How this FFMI calculator works
The fat-free mass index (FFMI) describes how much lean tissue you carry relative to your height. It works like BMI, but instead of using total weight it uses only your fat-free mass — muscle, bone, organs and water — so it is not skewed by body fat. That makes it a popular, muscle-aware alternative to BMI for tracking body composition.
The calculation runs in three steps. First it finds fat-free mass: FFM = weight × (1 − body fat % ÷ 100). Then it divides by height in metres squared: FFMI = FFM ÷ height². Finally it computes the normalized FFMI, which adjusts the result to a standard 1.8 m height: normalized FFMI = FFMI + 6.1 × (1.8 − height in metres). The adjustment lets taller and shorter people be compared on a common scale.
Reference note: imperial inputs are converted first (lb × 0.453592, in × 2.54). FFMI results depend heavily on the accuracy of the body fat percentage you enter, and the interpretation bands are population generalisations that differ between men and women.
Frequently asked questions
- What is FFMI?
- FFMI stands for fat-free mass index — your fat-free mass (muscle, bone, organs and water) divided by your height in metres squared. Like BMI it scales body mass to height, but because it removes fat first it reflects how much lean tissue you carry rather than total weight.
- How is FFMI calculated?
- First find fat-free mass: FFM = weight × (1 − body fat % ÷ 100). Then divide by height in metres squared: FFMI = FFM ÷ height². The normalized FFMI adjusts to a standard 1.8 m height by adding 6.1 × (1.8 − height in metres).
- What is a good FFMI?
- As a general guide for men, around 18–20 is average, 20–22 is fit and athletic, and 22–23 is very muscular. Ranges for women typically run a few points lower. These are broad reference bands, not a diagnosis.
- What is the natural FFMI limit?
- Research often cites a normalized FFMI of about 25 as an approximate ceiling for drug-free men, with most natural athletes below it. It is a population observation rather than a hard rule, and the figure is lower for women.
- What is the difference between FFMI and BMI?
- BMI uses total weight, so it cannot tell muscle from fat and a muscular person can register as overweight. FFMI uses only fat-free mass, isolating lean tissue, but it needs a body fat percentage as input whereas BMI needs only weight and height.
- Is FFMI accurate for women?
- The same formula works for women, but the typical ranges and natural-limit figure are lower, so male bands should not be applied directly. This calculator is an educational estimate and not medical advice; results depend on how accurately body fat was measured. For a tailored assessment, consult a qualified professional.