Fat Free Mass Index (FFMI) Calculator

Fat Free Mass Index (FFMI) Calculator for Endocrinology. Fat-free mass is not identical to skeletal muscle. The source reference population is not a universal set of normal values. The component therefore does not label muscle quality or infer anabolic steroid use.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

Fat-free mass = weight x (1 - body-fat percentage/100); FFMI = fat-free mass / height in metres squared. This is unnormalized FFMI, consistent with the definition in Schutz et al.: https://pubmed.ncbi.nlm.nih.gov/12080449/.

When to use this calculator

Understand how a body-composition estimate differs from BMI. Record the method used to estimate body fat and use the same method for comparisons.

Inputs used

  • Weight in kg or lb
  • Height in cm or inches
  • Estimated body fat as a percentage

Clinical interpretation

Fat-free mass is not identical to skeletal muscle. The source reference population is not a universal set of normal values. The component therefore does not label muscle quality or infer anabolic steroid use.

Worked example

At 80 kg, 180 cm and 20% body fat, fat-free mass is 64 kg and FFMI = 64/1.8² = 19.753 kg/m². If the body-fat estimate is instead 25%, FFMI is 60/3.24 = 18.519. The 1.235 difference arises without any change in weight or height.

Limitations and safety notes

Measurement method and hydration can influence body-composition estimates. Input bounds are arithmetic safeguards, not plausible healthy ranges. The earlier height-normalization adjustment and generic muscle grades have been removed; no normalized or athlete-specific variant is calculated.

Frequently asked questions

Should 20% be entered as 0.20?

No. Enter 20. Entering 0.20 means 0.20% body fat and produces a substantially different result.

References

  • Schutz et al., 2002, original cross-sectional FFMI reference study. https://pubmed.ncbi.nlm.nih.gov/12080449/

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 9, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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