Last updated: May 2026

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.

The Katch-McArdle Formula

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.