Health July 13, 2026 · 8 Min Read

Lean Mass Calculator – Guide & Formulas

Calculate your Lean Body Mass (LBM) and Fat-Free Mass Index (FFMI) using the Boer, James, or Hume formulas, or body fat percentage.

Lean Body Mass (LBM) is the metric representing the weight of your muscles, bones, nervous system, organs, skin, and body water. Calculating LBM is highly useful for determining protein guidelines, structuring caloric deficits, and calculating the Fat-Free Mass Index (FFMI). This calculator utilizes the established Boer, James, and Hume clinical formulas to provide precise comparative LBM models.

Key Takeaway

Use the free Lean Mass Calculator to calculate your lean body mass (lbm) and fat-free mass index (ffmi) using the boer, james, or hume formulas, or body fat percentage. Get instant results with step-by-step explanations.

How to Use the Lean Mass Calculator

  1. Enter your biological sex, height, and body weight.
  2. Provide your body fat percentage if known for the most accurate calculation.
  3. Analyze your LBM across multiple medical formulas.
  4. Check your Fat-Free Mass Index (FFMI) to evaluate your muscular potential.

The Formula

Boer (Male): LBM = 0.407 * Weight (kg) + 0.267 * Height (cm) - 19.2 | Boer (Female): LBM = 0.252 * Weight (kg) + 0.473 * Height (cm) - 48.3

Variable Definitions

  • W: Total physical body weight in kilograms
  • H: Stature height in centimeters
  • LBM: Lean Body Mass (excluding adipose fat tissue)

Comparing LBM Formulas

A male is 180 cm tall (5'11") and weighs 85 kg (187 lbs).

  1. Step 1: Apply Boer Formula: LBM = 0.407 * 85 + 0.267 * 180 - 19.2 = 34.595 + 48.06 - 19.2 = 63.45 kg.
  2. Step 2: Apply James Formula: LBM = 1.1 * 85 - 128 * (85 / 180)^2 = 93.5 - 128 * 0.222 = 65.0 kg.
  3. Step 3: Average LBM output is approximately 64.2 kg (141.5 lbs) of lean tissue, leaving 20.8 kg (45.5 lbs) of fat mass.

Frequently Asked Questions

Why is Lean Body Mass important for weight loss?

Your muscle tissue is highly active metabolically. The more Lean Body Mass you have, the higher your Resting Metabolic Rate (RMR). When in a calorie deficit, tracking LBM helps ensure you are primarily losing body fat while preserving lean muscle mass through adequate protein intake and resistance training.

What is the Fat-Free Mass Index (FFMI)?

FFMI is a metric used to evaluate an individual's muscularity relative to their height. Unlike BMI, it accounts for body fat, making it a favorite for natural bodybuilders to measure muscular potential and track year-over-year gains.