Calculating your fat-free mass index (ffmi) is straightforward once you know the Fat Free Mass Index formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Fat Free Mass Index Calculator.
What is Fat Free Mass Index?
The Fat Free Mass Index calculation tells you your fat-free mass index (ffmi) from a few simple inputs. The figure you are solving for here is the fat-free mass index (ffmi).
The Fat Free Mass Index formula
The core formula is:
Fat-free mass index (FFMI) = (Weight × (1 - Body fat percentage ÷ 100)) ÷ ((Height ÷ 100) ^ 2)Here is what each input means:
- Weight — a value measured in kg. Example: 70 kg.
- Body fat percentage — a percentage, such as an annual rate. Example: 15%.
- Height — a value measured in cm. Example: 175 cm.
How to calculate it step by step
- Write down the weight (for example, 70 kg).
- Write down the body fat percentage (for example, 15%).
- Write down the height (for example, 175 cm).
- Apply the formula above to get your fat-free mass index (ffmi).
- Double-check the result with the Fat Free Mass Index Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Weight | 70 kg |
| Body fat percentage | 15% |
| Height | 175 cm |
| Fat-free mass index (FFMI) | 19.43 |
| Fat-free mass | 59.50 |
With weight of 70 kg, body fat percentage of 15% and height of 175 cm, the fat-free mass index (ffmi) works out to 19.43.
Example 2
With weight of 140 kg, body fat percentage of 15% and height of 175 cm, the fat-free mass index (ffmi) works out to 38.86.
| Result | Value |
|---|---|
| Fat-free mass index (FFMI) | 38.86 |
| Fat-free mass | 119.00 |
Example 3
With weight of 35 kg, body fat percentage of 15% and height of 175 cm, the fat-free mass index (ffmi) works out to 9.71.
| Result | Value |
|---|---|
| Fat-free mass index (FFMI) | 9.71 |
| Fat-free mass | 29.75 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
- These figures are general estimates, not medical advice — check with a qualified professional before acting on them.
Prefer not to do the maths by hand? — the Fat Free Mass Index Calculator does it instantly, for free, with the formula and a worked example built in.
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