CALCULATE WITH CONFIDENCE

Health · Body Measurements & Composition

Fat Mass, Fat-Free Mass, FFMI and FMI Calculator

Turn an entered body-fat measurement into fat mass, fat-free mass, FFMI, and FMI while reconciling them with BMI.

Health · Body Measurements & Composition

Enter your measurements

Private in-browser calculation · no account or saved health record
Enter the percentage from the measurement method you are using.

Health values stay in this browser tab. They are not added to the URL, analytics, storage, or a cloud record.

Health result

—

Enter valid values to see the result.

Your entries are calculated in this browser and are not submitted to 365CALCS.COM.

Quick guide

How to use this calculator

  1. Use the labels and unit notes to enter the requested measurement, date, time, or observation exactly as recorded.
  2. Check the visible method, population, and unit basis before using the result; switch modes rather than converting values mentally.
  3. Read the calculation audit and limitations together. The result supports measurement or tracking and does not replace diagnosis, treatment, or urgent medical assessment.

Calculation method

How the fat mass, fat-free mass, ffmi and fmi calculator works

Fat mass = weight × body-fat percentage; fat-free mass = weight - fat mass; FFMI and FMI divide those masses by height in metres squared.

FFMI and FMI separate the two components whose sum equals BMI. They inherit the error and assumptions of the entered body-fat measurement method.

Worked example

Fat Mass, Fat-Free Mass, FFMI and FMI example

At 80 kg, 180 cm, and 25% measured body fat: fat mass is 20 kg, fat-free mass is 60 kg, FFMI is 18.52, and FMI is 6.17 kg/m².

Fat mass = weight × body-fat percentage; fat-free mass = weight - fat mass; FFMI and FMI divide those masses by height in metres squared.

Supported inputs

Precision and limits

Supported purpose

Enter a measured or independently estimated body-fat percentage and preserve its method. This page does not estimate body fat itself and does not apply universal lean, muscularity, or adiposity labels.

Measurement quality

The output is only as reliable as the entered measurement, device, timing, units, and applicable reference population. Recheck unexpected values against the original record.

Clinical boundary

This calculator does not diagnose a condition, choose treatment, change medication, or determine that an urgent symptom is safe to monitor at home. Contact an appropriate health professional or local emergency service when the situation may be urgent.

Privacy

Entered health values and results stay in this browser tab. They are not placed in URLs, analytics, logs, local storage, session replay, or a cloud record by this calculator.

Continue calculating

Related calculators