FFMI Formula & Calculation Guide
Learn how to calculate your Fat-Free Mass Index with step-by-step formulas, examples, and accurate body fat estimation methods.
The Complete FFMI Formula
Step 1: Calculate Fat-Free Mass
FFM = Weight × (1 - Body Fat% / 100)
Step 2: Calculate FFMI
FFMI = FFM / Height²
Step 3: Normalize for Height
Normalized FFMI = FFMI + 6.3 × (1.8 - Height)
Step-by-Step Calculation
Determine Body Fat Percentage
Body fat percentage is crucial for accurate FFMI. We use the Deurenberg Formula, a scientifically validated method:
BF% = (1.2 × BMI + 0.23 × Age - 5.4) - 10.8 × Sex
Where: Sex = 1 for male, 0 for female
BMI = Weight(kg) / Height(m)²
Example:
Male, 30 years old, 178cm (1.78m), 80kg
BMI = 80 / (1.78)² = 25.2
BF% = (1.2 × 25.2 + 0.23 × 30 - 5.4) - 10.8 × 1
BF% = (30.24 + 6.9 - 5.4) - 10.8 = 20.9%
Calculate Fat-Free Mass (FFM)
FFM is your total body weight minus fat mass:
FFM = Weight × (1 - BF% / 100)
This gives you your lean body mass in kg or lbs
Example: (continuing from above):
Weight = 80kg, BF% = 20.9%
FFM = 80 × (1 - 20.9/100)
FFM = 80 × 0.791 = 63.3kg
Calculate Raw FFMI
Divide FFM by height squared (in meters):
FFMI = FFM / (Height in meters)²
Height must be in meters. Convert cm to m by dividing by 100.
Example: (continuing):
FFM = 63.3kg, Height = 1.78m
FFMI = 63.3 / (1.78)²
FFMI = 63.3 / 3.1684 = 20.0
Normalize for Height (Optional but Recommended)
Normalized FFMI adjusts for height differences, allowing fair comparisons:
Normalized FFMI = FFMI + 6.3 × (1.8 - Height)
This adjusts your FFMI to what it would be if you were 1.8m (5'11") tall
Example: (final step):
FFMI = 20.0, Height = 1.78m
Normalized FFMI = 20.0 + 6.3 × (1.8 - 1.78)
Normalized FFMI = 20.0 + 6.3 × 0.02
Normalized FFMI = 20.0 + 0.126 = 20.1
Complete Worked Example
Male Example
Age: 25 years
Height: 175cm (1.75m)
Weight: 75kg
Sex: Male (1)
Calculations:
BMI = 75 / (1.75)² = 24.5
BF% = (1.2×24.5 + 0.23×25 - 5.4) - 10.8×1 = 13.8%
FFM = 75 × (1 - 13.8/100) = 64.7kg
FFMI = 64.7 / (1.75)² = 21.1
Normalized FFMI = 21.1 + 6.3×(1.8-1.75) = 21.4
Female Example
Age: 28 years
Height: 165cm (1.65m)
Weight: 60kg
Sex: Female (0)
Calculations:
BMI = 60 / (1.65)² = 22.0
BF% = (1.2×22.0 + 0.23×28 - 5.4) - 10.8×0 = 26.5%
FFM = 60 × (1 - 26.5/100) = 44.1kg
FFMI = 44.1 / (1.65)² = 16.2
Normalized FFMI = 16.2 + 6.3×(1.8-1.65) = 17.1
Interpretation:
Male (21.4 FFMI): Excellent muscle development, representing dedicated training.
Female (17.1 FFMI): Very good muscle mass for a female, above average development.
Imperial Units (lbs / inches)
If you use imperial measurements, convert to metric first:
Weight Conversion
Weight (kg) = Weight (lbs) × 0.453592
Example: 180 lbs = 81.6 kg
Height Conversion
Height (m) = Height (inches) × 0.0254
Example: 72 inches (6'0") = 1.83 m
Body Fat Estimation Methods
The Deurenberg formula is convenient but has limitations. Here are other methods ranked by accuracy:
DEXA Scan (Most Accurate)
Accuracy: ±1-2% | Cost: $50-150 | Gold standard
Hydrostatic Weighing
Accuracy: ±2-3% | Cost: $40-75 | Very reliable
Bod Pod (Air Displacement)
Accuracy: ±2-4% | Cost: $45-100 | Good option
Deurenberg Formula (This Calculator)
Accuracy: ±4-5% | Cost: Free | Convenient estimate
Skinfold Calipers
Accuracy: ±3-8% (varies by skill) | Cost: $5-50
Bioelectrical Impedance (BIA Scales)
Accuracy: ±5-10% | Cost: $20-200 | Least reliable
Common Calculation Mistakes
❌ Using BMI Instead of FFMI
BMI doesn't account for muscle. A muscular person will have high BMI but that doesn't mean they're overfat.
❌ Inaccurate Body Fat Measurement
A 5% error in BF% can change FFMI by 2-3 points. Use professional methods for accuracy.
❌ Forgetting to Square Height
The formula is FFM / Height², not FFM / Height. Height must be multiplied by itself.
❌ Mixing Metric and Imperial
Don't mix units. Convert everything to metric (kg and meters) or use a calculator.
Skip the Math - Use Our Calculator
Our free calculator handles all these formulas automatically, including the Deurenberg body fat estimation. Get your FFMI in seconds.
Calculate My FFMI Now