How this FFMI calculator works
You enter your height, weight, and body fat percentage. We work it out in three steps.
- Fat-free mass: weight × (1 − body fat % ÷ 100). This is everything that isn't fat.
- FFMI: fat-free mass in kg ÷ height in meters squared.
- Normalized FFMI: FFMI + 6.3 × (1.8 − height in meters).
Here's an example. Someone who is 5 ft 11 in (180 cm), weighs 176 lb (80 kg), and has 15% body fat has about 150 lb (68 kg) of fat-free mass. Their FFMI is 68 ÷ (1.8 × 1.8), or about 21.0. Since they are exactly 1.8 m tall, their normalized FFMI is also 21.0.
These formulas come from a 1995 study by Kouri and colleagues. It compared male athletes who used steroids with those who didn't.
What your result means
In that study, the men who didn't use steroids topped out at a normalized FFMI of about 25. That's where the idea of a "natural limit" comes from. It's a rough line from one study, not a hard rule.
A higher FFMI means more muscle for your height. If you lift weights, you can use FFMI to track progress. If your FFMI goes up while your body fat stays the same or drops, you're building muscle.
Get the most accurate result
Your FFMI depends a lot on your body fat number. In the example above, a body fat reading that is 5 points off would move FFMI by about 1.2. Use the same method each time you measure. Our body fat calculator gives a free estimate with a tape measure.
Your fat-free mass is the same as the lean mass from the body fat method in our lean body mass calculator.
How to raise your FFMI
The main way to raise FFMI is to build muscle. That means regular strength training and enough protein. Our protein calculator gives you a daily target, and our protein guide for gym-goers shows how to spread it through the day.
The research was done on men. Women can use this tool to track changes, but the natural limit doesn't apply to them in the same way.