If you lift weights long enough, a couple questions will inevitably enter your mind:

“How big can I get?”
“How big should I get?”

Glancing in the mirror or hopping on a scale isn’t going to get to the root of these questions- we need hard data to help guide us toward some quantifiable answers. Luckily, there is a metric called fat-free mass index (FFMI) that we can use as a general estimate of overall muscularity. Originally, it was used to help researchers and medical professionals identify protein malnutrition at the lower end of the FFMI spectrum[1], but the information we’re interested is located toward the higher end. If you know your height, weight, and have a measurement (or pretty good estimate) of body fat percentage (BF%), you’ve got all the information you need.

 

How is FFMI calculated?

You don’t need a PhD in mathematics to crunch these numbers. FFMI is basically BMI, but we remove your fat mass from the equation. The result is a measurement of your fat-free mass, scaled to your height. The following equation shows just how simple this calculation can be:

 
FFMI eq 1[Eq 1.]

You may not have a direct measurement of “fat-free mass” available, so here is a slightly longer version of the equation that assumes you are using your weight, and a measured (or estimated) BF% value:

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