Body Composition

Lean Body Mass (LBM) & FFMI Calculator: How It Works + How to Use It

Learn what lean body mass (LBM) and FFMI mean, how they’re calculated, and how to use our free LBM & FFMI calculator with body fat % or the Boer estimate.

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  • UpdatedJan 3, 2026
  • Reading time7 min read

Lean body mass (LBM) & FFMI: a practical guide

Lean body mass (LBM) is your body weight minus fat mass. It’s often used as a practical way to talk about “how much of you is not fat,” especially when tracking training or recomposition progress.

This guide explains:

  • what LBM and FFMI are (and what they’re not),
  • how the numbers are calculated,
  • how to use our tool for consistent tracking.

Use the free calculator

What is LBM?

At a high level:

LBM = body weight − fat mass

If you know (or estimate) your body fat %, you can estimate LBM as:

LBM ≈ weight × (1 − body fat%)

This is why your LBM estimate depends heavily on how you estimate body fat.

What is FFMI (and normalized FFMI)?

FFMI stands for Fat-Free Mass Index. It scales lean mass to height:

FFMI = LBM(kg) ÷ height(m)²

Some calculators also show normalized FFMI (a rough adjustment to compare people of different heights). It’s still an estimate—use it as a trend marker, not a diagnosis.

How our LBM calculator works

Our tool supports two modes:

  1. Recommended: use your body fat % (from tape, skinfold, DEXA, etc.) to derive LBM.
  2. Fallback: if you don’t have body fat %, we estimate LBM using the Boer formula (height/weight/gender).

Try it here:

Where to get body fat % (so LBM is more meaningful)

Body fat % methods often disagree. Pick one method and stay consistent:

How to use LBM/FFMI for long-term progress

LBM and FFMI are most useful when you:

  • re-measure on a consistent schedule (e.g., every 2–4 weeks),
  • keep the method consistent (same tape location / same caliper technique),
  • track trends instead of single readings.

For trend tracking:

Common mistakes (and how to avoid them)

  1. Switching methods (BIA one week, tape the next) and expecting the same absolute %
  2. Unit mix-ups (cm/in or kg/lb)
  3. Comparing FFMI across very different body fat estimates
  4. Treating one reading as “truth” instead of a directionally useful estimate

FAQ

Is LBM the same as “fat-free mass” (FFM)?

They’re used similarly in consumer tools. In strict terms, “fat-free mass” excludes all fat tissue; many tools use LBM/FFM interchangeably for practical tracking.

What’s a “good” FFMI?

There’s no universal “good” score. Height, genetics, training age, and measurement method all matter. Use FFMI to compare you vs you over time.

What should I do after I know my LBM?

Connect the number to a plan: