FFMI Calculator (Fat-Free Mass Index)

Written by Great Medico

Updated September 24, 2026 •2 min read

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One body-size number rarely tells the whole story, but it can answer a precise question. FFMI Calculator (Fat-Free Mass Index) is built for that narrower job.

What this calculator does

Fat-Free Mass Index, or FFMI, scales lean mass to height so muscularity can be compared more fairly across different body sizes. This calculator starts with body weight and body-fat percentage, then normalizes the result for height. Anthropometric equations are convenient because they use simple measurements, but they describe body size or proportion rather than directly measuring tissue composition or health.

How to use it

Enter or select Sex, Height, Weight, and Body fat. Use the units offered by the calculator and keep measurements consistent when you plan to compare results over time.

How the calculation works

Fat-free mass equals body weight × (1 − body-fat fraction). FFMI is fat-free mass divided by height squared. The calculator also reports a normalized FFMI using FFMI + 6.1 × (1.8 − height in meters).

Example

For the sample values prefilled in this calculator, the primary result is 20.99 kg/m² (Fat-free mass index), Normalized FFMI is 20.99, and Fat-free mass is 68 kg. This example is there to show how the inputs flow through the implemented equation; replace the sample values with your own measurements before using the result.

How to interpret the result

The result describes estimated lean mass relative to height. Tracking FFMI can be more informative than scale weight alone when body-fat estimates are measured consistently.

Limitations and notes

The biggest source of error is usually the entered body-fat percentage. Different body-fat methods can shift FFMI substantially. The sex field is present in the interface but does not change this calculator’s current FFMI equations, and FFMI should not be used alone to judge health or athletic performance. For repeat measurements, consistency usually matters more than false precision: use the same unit system, the same tape or scale when possible, and the same anatomical landmarks. A small change near a category boundary should be viewed in context rather than treated as a sudden biological change.