Fitness & Sports

FFMI Calculator

Calculate your Fat-Free Mass Index (FFMI) and normalized FFMI from your weight, height and body fat percentage, or estimate lean body mass with the Boer, James or Hume formula. Free, no sign-up needed.

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What is FFMI Calculator?

The FFMI Calculator estimates your Fat-Free Mass Index and normalized FFMI from your weight, height, and either a known body fat percentage or an estimate from the Boer, James or Hume lean-body-mass formula. It also shows lean body mass, fat mass, a natural-muscularity category, and how your normalized FFMI would change at a target body fat percentage.

When to Use This Calculator

  • When tracking muscle mass changes during a training block
  • When comparing your muscularity to natural-limit research benchmarks
  • When you want an LBM estimate without a body fat test

Steps:

  1. Enter your weight and height
  2. Choose whether to enter your body fat % or estimate lean body mass with a formula
  3. Select your sex
  4. View your FFMI, normalized FFMI, lean body mass and category

Formula

LBM = weight × (1 − body fat %), or estimated via Boer/James/Hume from weight and height alone; FFMI = LBM / height² (height in meters); normalized FFMI = FFMI + 6.1 × (1.8 − height in meters)

Use Cases

  • Compare muscularity fairly between lifters of different heights
  • Track changes in lean body mass across a bulk or cut
  • Estimate lean body mass without a body fat measurement
  • See how a target body fat percentage would affect your FFMI

Key Benefits

  • Compares muscularity independently of height using the normalized value
  • Works even without a body fat measurement via the Boer, James or Hume formulas
  • Shows fat mass and lean body mass alongside the FFMI score
  • Includes a target body fat what-if to preview your FFMI after a cut

Pro Tips

  • Use a consistent body fat measurement method over time so your FFMI trend is meaningful even if the absolute number has some error
  • Prefer the normalized FFMI value when comparing yourself to published natural-limit benchmarks
  • Re-run the calculation after significant weight changes, since both weight and body fat % affect the result

Common Mistakes to Avoid

  • Using an inaccurate body fat percentage from a low-quality consumer scale and treating the FFMI result as precise
  • Comparing raw (non-normalized) FFMI between people of very different heights
  • Assuming a high FFMI alone indicates good health or performance
  • Forgetting that LBM-estimation formulas assume average body proportions and can be less accurate for very lean or very tall/short individuals

Key Terms Explained

FFMI
Fat-Free Mass Index — lean body mass divided by height squared
Normalized FFMI
FFMI adjusted to a reference height of 1.80 m for fair comparison
Lean Body Mass (LBM)
Total body weight minus fat mass
Boer formula
A weight-and-height-only lean body mass estimate (Boer, 1984)

Example

80 kg, 15% body fat, 1.80 m → lean body mass 68 kg → FFMI 20.99, normalized FFMI 20.99

Frequently Asked Questions

What is FFMI?
FFMI (Fat-Free Mass Index) measures how much muscle mass you carry relative to your height, similar to how BMI measures total weight relative to height. It's calculated as lean body mass divided by height squared, and is widely used to compare muscularity between people of different heights.
What is normalized FFMI and why use it instead of raw FFMI?
Normalized FFMI adjusts your FFMI to what it would be at a reference height of 1.80 m (5'11"), using the formula FFMI + 6.1 × (1.80 − height in meters). This correction lets you compare muscularity fairly between people of different heights, since raw FFMI is naturally higher for shorter people at the same relative muscularity.
How is lean body mass calculated?
If you know your body fat percentage, lean body mass (LBM) = weight × (1 − body fat %). If you don't know your body fat %, you can estimate LBM directly from weight and height using the Boer, James or Hume formula, which are all cited equations from body-composition research.
Which lean body mass formula should I use?
If you have a reasonably accurate body fat percentage (from DEXA, BIA scale, calipers, or the Navy method), use it directly — it's generally more accurate than a formula. If you don't know your body fat %, the Boer formula is the most widely cited weight-and-height-only estimate for FFMI calculations.
What is considered a 'natural limit' FFMI?
Research by Kouri et al. (1995) on drug-tested bodybuilders found that a normalized FFMI around 25 was rarely exceeded without performance-enhancing drug use, making it a commonly cited (though not absolute) reference point for the upper range of natural muscularity.
What does 'FFMI at target body fat' show?
This shows what your normalized FFMI would become if you kept your current lean body mass but reduced your body fat percentage to a target value — useful for seeing how a cut would affect your FFMI once you're leaner, assuming no muscle loss.
Is a high FFMI always a good thing?
A higher FFMI generally reflects more muscle mass relative to height, which is desirable for most strength athletes, but FFMI alone doesn't account for body fat distribution, health markers, or performance. It's one data point among several, not a complete measure of fitness.
How accurate is FFMI?
FFMI is only as accurate as the body fat percentage or LBM-estimation formula behind it. Body fat measurements from calipers or consumer BIA scales can be off by several percentage points, which shifts both LBM and FFMI. Treat the result as a useful estimate, not a precise clinical measurement.
Can FFMI be used for women?
Yes, the underlying FFMI and normalized FFMI formulas apply the same way to women. The LBM-estimation formulas (Boer, James, Hume) each use different coefficients for men and women, which this calculator applies automatically based on the sex you select.

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