Katch-McArdle Calculator: The BMR Formula for People Who Know Their Body Fat
The Katch-McArdle formula calculates basal metabolic rate (BMR) using lean body mass rather than total body weight. This makes it more accurate than Mifflin-St Jeor or Harris-Benedict for athletes, lifters, and anyone with a higher-than-average muscle-to-fat ratio — because muscle is metabolically more expensive than fat, and standard formulas that ignore body composition underestimate BMR for these individuals.
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BMR = 370 + (21.6 × Lean Body Mass in kg)
Unlike Mifflin-St Jeor, there is no separate formula for men and women — lean body mass already accounts for the compositional differences between sexes that drive metabolic variation. A 70 kg man with 15% body fat and a 70 kg woman with 30% body fat have very different lean body masses, and therefore very different BMR estimates.
How to Calculate Lean Body Mass
You need lean body mass (LBM) to use the Katch-McArdle formula. LBM is total body weight minus fat mass.
Method 1: If you know your body fat percentage
LBM (kg) = Body weight (kg) × (1 − Body Fat Fraction)
Example: 80 kg, 18% body fat
LBM = 80 × (1 − 0.18) = 80 × 0.82 = 65.6 kg
BMR = 370 + (21.6 × 65.6) = 370 + 1,416.96 = 1,787 kcal/day
Method 2: Boer formula estimate (weight and height only)
If body fat percentage is not available, LBM can be estimated:
Men: LBM = (0.407 × weight kg) + (0.267 × height cm) − 19.2
Women: LBM = (0.252 × weight kg) + (0.473 × height cm) − 48.3
Example: Woman, 65 kg, 168 cm
LBM = (0.252 × 65) + (0.473 × 168) − 48.3 = 16.38 + 79.46 − 48.3 = 47.5 kg
BMR = 370 + (21.6 × 47.5) = 370 + 1,026 = 1,396 kcal/day
The Boer estimation uses the same inputs as Mifflin-St Jeor, so accuracy gains are marginal when using estimated LBM. The full advantage of Katch-McArdle only materializes when LBM is measured directly.
Methods for Measuring Body Fat Percentage
Katch-McArdle’s accuracy depends entirely on the accuracy of your body fat measurement. Common methods, ranked by accuracy:
| Method | Accuracy | Notes |
|---|---|---|
| DEXA scan | Highest (±1–2%) | Gold standard, available at many sports medicine clinics |
| Bod Pod (air displacement) | High (±2–3%) | University and sports lab settings |
| Hydrostatic weighing | High (±2–3%) | Less commonly available |
| Skinfold calipers (professional) | Moderate (±3–5%) | Requires trained technician; cheap and accessible |
| BIA (bioelectrical impedance) | Variable (±3–8%) | Common in scales and handheld devices; affected by hydration |
| Visual estimate | Low (±5–10%+) | Highly subjective; not recommended for formula input |
Katch-McArdle vs Mifflin-St Jeor: When to Use Each
| Situation | Better Formula | Why |
|---|---|---|
| General population, unknown body fat | Mifflin-St Jeor | Validated across large populations, requires no extra measurement |
| Athlete with known body fat % | Katch-McArdle | Directly accounts for high muscle mass |
| High body fat percentage | Katch-McArdle | Avoids BMR overestimation from high body weight |
| Low body fat (under 15% men, 22% women) | Katch-McArdle | Most accurate when lean mass is disproportionately high |
| Older adults, modest muscle mass | Mifflin-St Jeor | Convenient and adequately accurate for average body composition |
Step-by-Step: Two Athletes, Same Weight, Different Results
This example shows why body composition matters for BMR calculation.
Athlete A: 85 kg, 10% body fat (highly muscular powerlifter)
LBM = 85 × 0.90 = 76.5 kg
Katch-McArdle BMR = 370 + (21.6 × 76.5) = 370 + 1,652.4 = 2,022 kcal/day
Athlete B: 85 kg, 28% body fat (recreational gym-goer)
LBM = 85 × 0.72 = 61.2 kg
Katch-McArdle BMR = 370 + (21.6 × 61.2) = 370 + 1,321.9 = 1,692 kcal/day
Same total weight. 330 calorie difference in resting metabolic rate. Mifflin-St Jeor would give both the same BMR estimate (approximately 1,900 kcal for an average 35-year-old male at 85 kg and 178 cm). It underestimates Athlete A’s needs and overestimates Athlete B’s needs.
From Katch-McArdle BMR to TDEE
Once you have your Katch-McArdle BMR, calculate TDEE the same way as any other formula — multiply by your activity factor:
- Sedentary: × 1.2
- Lightly active: × 1.375
- Moderately active: × 1.55
- Active: × 1.725
- Very active: × 1.9
Using Athlete A (BMR 2,022) who trains 5 days per week:
TDEE = 2,022 × 1.55 = 3,134 kcal/day
This is the athlete’s maintenance calorie intake. To cut body fat, reduce by 300–500 calories daily while maintaining protein at approximately 2.0–2.2 g/kg of bodyweight to preserve lean mass during the deficit.
Calculate Your Daily Calorie Target
Use the TDEE calculator to convert your BMR into a full daily calorie target at your activity level.
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