Key Takeaways
- The Katch-McArdle equation estimates resting metabolic rate (BMR) as 370 + 21.6 × LBM_kg.
- It requires lean body mass (LBM) as the primary input, which can be derived from body weight and body fat percentage.
- TDEE (Total Daily Energy Expenditure) is calculated by multiplying BMR by an activity factor (1.2–1.9).
- Body fat percentage can be estimated through several methods with varying levels of precision.
- This is the same formula as the 1991 revised Cunningham equation — both produce identical BMR outputs.
- All values are informational estimates for personal energy tracking and journaling.
What Is the Katch-McArdle Equation?
The Katch-McArdle equation is a formula for estimating basal metabolic rate (BMR) using lean body mass (LBM) rather than total body weight. The rationale is that lean tissue (muscle, organs, bone) is the primary driver of resting energy expenditure, while fat tissue contributes relatively less.
The formula is:
BMR = 370 + (21.6 × LBM_kg)
Where LBM (lean body mass) is calculated as: Weight × (1 − Body Fat % / 100).
How It Differs from Weight-Based Equations
Equations like Mifflin-St Jeor and Harris-Benedict use total body weight, height, age, and gender as inputs. They produce the same estimate for two people of the same weight, height, age, and gender — regardless of body composition.
The Katch-McArdle equation instead uses lean body mass, so two individuals at the same total weight but different body compositions will receive different BMR estimates. The person with more lean mass relative to fat mass will have a higher estimated BMR.
The trade-off is that Katch-McArdle requires a body fat percentage input, which introduces its own estimation uncertainty depending on the method used to measure it.
Body Fat Estimation Methods
Since the Katch-McArdle equation requires lean body mass, knowing or estimating your body fat percentage is necessary. Common approaches include:
- DEXA Scan: X-ray-based body composition analysis. Considered a reference standard but requires a clinical facility.
- Skinfold Calipers: Measures subcutaneous fat at specific body sites. Accuracy depends on technician skill and protocol used.
- Bioelectrical Impedance (BIA): Found in consumer scales and handheld devices. Results are affected by hydration and meal timing.
- Navy Body Fat Formula: Uses waist, neck, and hip circumference. Requires only a tape measure.
- Hydrostatic Weighing: Underwater weighing to measure body density. High accuracy but requires specialized equipment.
- Bod Pod (ADP): Air displacement method. Comparable accuracy to hydrostatic weighing.
Each method has trade-offs between accessibility, cost, and consistency. For tracking changes over time, using the same method under consistent conditions is more important than the absolute accuracy of any single measurement.
Understanding TDEE Activity Multipliers
BMR represents resting energy expenditure over 24 hours. To estimate total daily energy expenditure (TDEE), BMR is multiplied by an activity factor:
| Level | Factor | Description |
|---|---|---|
| Sedentary | ×1.2 | Little or no regular exercise |
| Lightly Active | ×1.375 | Light activity 1–3 days/week |
| Moderately Active | ×1.55 | Moderate activity 3–5 days/week |
| Very Active | ×1.725 | Hard activity 6–7 days/week |
| Extremely Active | ×1.9 | Very hard daily activity or physical occupation |
Activity multipliers are broad categories. Actual energy expenditure depends on the specific type, duration, and intensity of activity. TDEE estimates serve as starting reference points for personal tracking.
Comparison with the Cunningham Equation
The Katch-McArdle equation (BMR = 370 + 21.6 × LBM) is mathematically identical to the 1991 revised Cunningham equation. Both produce the same BMR output for the same lean body mass input.
Cunningham also published an original 1980 formula (BMR = 500 + 22 × LBM) which uses slightly different coefficients and produces a somewhat higher estimate. The Cunningham equation calculator on this site shows both versions for comparison.
The distinction is primarily historical rather than practical. Both approaches use lean body mass as the sole predictor of resting metabolic rate.
Practical Considerations
When using any BMR/TDEE estimate:
- Treat the output as a reference starting point, not a precise measurement
- Individual metabolism varies based on genetics, hormonal factors, organ mass, and other variables
- Activity multipliers are broad categories — track actual activity for more specific estimates
- Consistency in measurement method matters more than absolute accuracy for tracking changes over time
- All estimates from this tool are for informational and journaling purposes
Frequently Asked Questions
Sources & References
- Exercise Physiology: Energy, Nutrition, and Human Performance (5th ed.). McArdle WD, Katch FI, Katch VL. Lippincott Williams & Wilkins (2001)
- The Katch-McArdle Formula for TDEE Estimation: Applications in Sports Science and Bodybuilding Research. Various. ResearchGate (2024)
- Predictive equations for resting energy expenditure. Various. Nutrition Research Reviews (2021)
- Comparison of predictive equations for resting metabolic rate. Frankenfield et al.. Journal of the American Dietetic Association (2005)
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This tool is for informational and journaling purposes only.
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