Lean Body Mass and FFMI Calculator

Estimates only. Talk to a doctor or registered dietitian before acting on these numbers for medical reasons.

What is lean body mass?

Lean body mass is everything in your body that is not fat: muscle, bone, organs, water and stored carbohydrate. It is the part of you that burns most of your calories at rest.

Lean mass = weight × (1 − body fat % ÷ 100)

At 176 lb and 20% body fat, you carry about 35 lb of fat and 141 lb of lean mass. Two people at 176 lb can be far apart: at 12% body fat the lean mass is 155 lb, at 30% it is 123 lb.

How to calculate lean body mass without a body fat number

If you do not know your body fat, the calculator lets you pick the body type that looks most like you, or you can measure it with the body fat calculator.

Another way uses only height and weight. The Boer formula, from a 1984 study on body fluid volumes, is common in medicine:

Men: 0.407 × kg + 0.267 × cm − 19.2
Women: 0.252 × kg + 0.473 × cm − 48.3

It does not know your body fat, so it gives the same answer to two people at the same height and weight. For someone who lifts, that is the number you want to know, so a body fat estimate is the better start.

What is FFMI?

Lean mass grows with height, so a number on its own does not tell you much. FFMI, fat-free mass index, divides lean mass in kg by height in meters squared. It works like BMI, but it counts only the lean part.

FFMI = lean kg ÷ (height m)²

In a 1995 study of male athletes, the ones who did not use steroids reached an FFMI of 25 at most. That limit is on a version adjusted to a height of 1.8 m, because taller people carry a little less lean mass per square meter: adjusted FFMI = FFMI + 6.3 × (1.8 − height in m). The calculator shows the adjusted number first and marks it when it is above 25.

What is a good FFMI?

In a study of 5,635 Swiss adults, the median FFMI at ages 18 to 34 was 18.9 for men and 15.4 for women. Half of people sit below those numbers and half above. For men who did not use steroids, the Kouri study found a height-adjusted FFMI of 25 at most. The study did not include women.

Why lean mass matters in a cut

Your resting burn comes from lean mass, so the calculators on this site start there instead of from total weight. Protein does too once body fat is high: above 25% for men or 32% for women, the targets scale to lean mass, because fat does not need protein.

A cut is meant to take off fat while you keep lean mass. The scale cannot tell the two apart, so measure body fat once a week and watch the trend of both numbers.