Lean Body Mass & FFMI Calculator

How it works

Lean body mass (LBM), also called fat-free mass, is your total weight minus your fat. Once you know your body-fat percentage, the split is exact:

Lean body mass = weight × (1 − body fat)

Fat mass = weight × body fat

Because lean mass alone doesn't account for height — a tall and a short person can carry the same muscle very differently — the fat-free mass index (FFMI) normalizes it, exactly as BMI normalizes weight (VanItallie 1990):

FFMI = lean body mass (kg) ÷ height (m)²

FFMI is popular in fitness because it tracks muscularity independent of fat. As a rough guide, untrained adults sit around 18–20, well-trained lifters reach the low-to-mid 20s, and values much above ~25 are uncommon without exceptional genetics. This calculator takes a body-fat percentage you already know — from a smart scale, calipers or a DEXA scan — and reports lean mass, fat mass and FFMI. If you don't know your body fat, estimate it first with the body-fat calculator.

Worked example

For an 80 kg person, 1.80 m tall, at 20% body fat:

• Lean body mass = 80 × (1 − 0.20) = 64 kg
• Fat mass = 80 × 0.20 = 16 kg
• FFMI = 64 ÷ 1.80² = 64 ÷ 3.24 ≈ 19.8

Drop the body fat to 12% at the same weight and lean mass rises to 70.4 kg, lifting FFMI to about 21.7 — the same scale weight, a very different physique.

Frequently asked questions

What is lean body mass?

Lean body mass is your body weight minus fat mass — the combined weight of muscle, bone, organs, connective tissue and body water. It equals weight × (1 − body-fat fraction).

How is lean body mass different from muscle mass?

Lean mass includes muscle but also bone, organs and water, so it is larger than muscle mass alone. Muscle is one component of lean body mass, not the whole of it.

What is FFMI?

The fat-free mass index normalizes lean (fat-free) mass for height: FFMI = lean mass in kg divided by height in metres squared. It measures muscularity the way BMI measures weight, so it is comparable across people of different heights.

What is a good FFMI?

Roughly, untrained adults are around 18–20, trained individuals reach the low-to-mid 20s, and values much above 25 are rare without unusual genetics. FFMI is a guide, not a target, and interpretation differs by sex and training history.

Do I need my body-fat percentage?

Yes — the split into lean and fat mass depends on it. Enter a body-fat percentage from a scale, calipers or DEXA. If you don't have one, estimate it first, since the results are only as accurate as that input.

Why use FFMI instead of BMI?

BMI can't tell muscle from fat, so a muscular person may register as "overweight." FFMI looks only at fat-free mass, making it a better height-adjusted gauge of muscularity for athletes and lifters.