A step-by-step walkthrough of the FFMI formula, with fully worked metric and imperial examples so you can calculate it by hand or check a calculator's result.
You need three inputs: your body weight, your height, and your body fat percentage. If you don't already know your body fat percentage, get an estimate first from our Body Fat Calculator or another method (DEXA, BIA scale, skinfold, or tape measurement) — FFMI can't be calculated without it.
Enter your details once and get FFMI, normalized FFMI, fat-free mass and fat mass instantly.
Open the FFMI Calculator →Let's calculate FFMI for someone who is 80 kg, 178 cm tall, at 15% body fat.
| Step | Calculation | Result |
|---|---|---|
| 1. Fat-free mass | 80 × (1 − 0.15) | 68.0 kg |
| 2. Height in metres | 178 cm ÷ 100 | 1.78 m |
| 3. Height squared | 1.78 × 1.78 | 3.168 m² |
| 4. FFMI | 68.0 ÷ 3.168 | 21.5 |
| 5. Normalized FFMI | 21.5 + 6.3 × (1.80 − 1.78) | 21.6 |
Now the same calculation for someone who is 176 lb and 5 ft 10 in tall, also at 15% body fat — converting to metric units before applying the formula.
| Step | Calculation | Result |
|---|---|---|
| Convert weight to kg | 176 × 0.453592 | 79.83 kg |
| Convert height to inches | (5 × 12) + 10 | 70 in |
| Convert height to metres | 70 × 0.0254 | 1.778 m |
| 1. Fat-free mass | 79.83 × (1 − 0.15) | 67.85 kg |
| 3. Height squared | 1.778 × 1.778 | 3.161 m² |
| 4. FFMI | 67.85 ÷ 3.161 | 21.5 |
| 5. Normalized FFMI | 21.5 + 6.3 × (1.80 − 1.778) | 21.6 |
Normalized FFMI exists because raw FFMI slightly disadvantages taller people — dividing by a larger height-squared figure produces a lower score for the same underlying muscularity. The correction, from Kouri et al. (1995), adds 6.3 × (1.80 − your height in metres) to your raw FFMI. If you're shorter than 1.80 m, this adds a small amount to your score; if you're taller, it subtracts a small amount. It's a comparison tool, not a "more accurate" version of your FFMI — both numbers describe the same underlying body composition.
FFMI is a five-step calculation: work out fat-free mass, convert height to metres, square it, divide fat-free mass by that figure, and optionally apply the height-normalization adjustment. The accuracy of the whole calculation rests on your body fat percentage estimate, so start there if you're unsure.
What is the FFMI formula?
FFMI = fat-free mass (kg) ÷ height (m)². Fat-free mass is calculated first as body weight × (1 − body fat % ÷ 100). Normalized FFMI adds 6.3 × (1.80 − height in metres) to adjust for height.
Can I calculate FFMI in pounds and inches?
Yes, but convert to kilograms and metres first, since the formula is defined in metric units. Multiply pounds by 0.453592 for kg, and multiply total height in inches by 0.0254 for metres, before applying the formula.
Do I need my body fat percentage to calculate FFMI?
Yes. FFMI cannot be calculated from height and weight alone — it requires fat-free mass, which requires a body fat percentage estimate from a method like DEXA, a BIA scale, skinfold calipers, or a tape method.