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Last updated: June 4, 2026

FFMI Calculator

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FFMI Calculator — Fat‑Free Mass Index, Inputs and Worked Result

Quick start: Enter body weight (lb), height (in), and body fat (%). The calculator outputs FFMI, a height‑normalized index of lean mass.

Definition

Fat‑Free Mass Index (FFMI) quantifies lean mass relative to height. It removes fat mass and scales by stature, enabling like‑for‑like comparisons.

How It Works

  1. Inputs (imperial): body weight (lb), height (in), body fat (%).
  2. Lean mass is computed from weight and body fat percentage.
  3. Height is converted to meters; lean mass to kilograms.
  4. FFMI is calculated as kg of lean mass per square meter.

Formula / Method

Equations (units noted):

  • leanMass = weight × (1 − bodyFat / 100) [lb]
  • heightMeters = height × 0.0254 [m]
  • ffmi = (leanMass / 2.20462) / (heightMeters × heightMeters) [kg/m²]

Variables: weight (lb), height (in), bodyFat (%). Constants: 1 in = 0.0254 m; 1 kg = 2.20462 lb.

Worked Example

Sample inputs: weight = 180 lb, height = 70 in, body fat = 15.0%.

  1. Lean mass = 180 × (1 − 15/100) = 180 × 0.85 = 153.0 lb.
  2. Height (m) = 70 × 0.0254 = 1.778 m.
  3. Lean mass (kg) = 153.0 / 2.20462 ≈ 69.40 kg.
  4. FFMI = 69.40 / (1.778²) = 69.40 / 3.162 ≈ 21.95.

Rounded calculator output: 21.95. Note: Display precision is typically two decimals.

Applications / Use Cases

  • Track lean mass development during resistance training.
  • Compare current physique to past values at different body fats.
  • Plan cutting/bulking phases while monitoring lean mass retention.

Assumptions & Limitations

  • Body fat input depends on method (BIA, calipers, DEXA). Typical error: ±2–5% body fat.
  • No height normalization beyond m²; no age/sex adjustment here.
  • Hydration, glycogen, and measurement timing can shift weight and body fat estimates.

Tips / Common Mistakes

  • Do not mix units; keep lb and in as specified.
  • Enter body fat with a decimal if needed (e.g., 15.5%), not as 0.155.
  • Measure height accurately; small errors affect m² and FFMI noticeably.

Non‑advisory note: This tool is informational and not a medical diagnosis. Use consistent methods for trend accuracy.

Frequently Asked Questions

What inputs do I need for FFMI?

Body weight in pounds, height in inches, and body fat percentage.

How is lean mass calculated here?

Lean mass = body weight × (1 − body fat%/100), returned in pounds.

What unit is the final FFMI value?

FFMI is in kg/m² after converting lean mass to kg and height to meters.

Can I enter body fat as a decimal like 0.15?

No. Enter body fat as a percent, e.g., 15 or 15.0.

How sensitive is FFMI to measurement error?

A ±2% body fat error can shift FFMI by about ±0.5–1.0 in many cases.

Does this calculator adjust for age or sex?

No. It strictly follows the FFMI formula without demographic adjustments.

What if my height or weight is in metric units?

Convert first to inches and pounds, or use the given conversion factors within the method.

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