Katch-McArdle Calculator
Calculate your estimated resting metabolic rate using lean body mass with the Katch-McArdle formula.
What Is the Katch-McArdle Formula?
The Katch-McArdle formula estimates resting metabolic rate (RMR) based on lean body mass rather than total body weight. Unlike formulas such as Mifflin-St Jeor or Harris-Benedict, which rely on weight, height, age, and sex, Katch-McArdle uses only lean mass. This makes it more accurate for individuals with known body composition, particularly those who are athletic or have lower body fat percentages.
The formula is:
RMR = 370 + (21.6 × Lean Body Mass in kg)
Lean body mass is your total weight minus body fat weight. If you know your body fat percentage, you can calculate lean mass directly.
How to Use This Calculator
- Enter your total body weight in kilograms or pounds.
- Enter your body fat percentage. This must be an estimate from a reliable method such as DEXA scan, calipers, or bioelectrical impedance.
- The calculator computes your lean body mass and applies the Katch-McArdle formula to estimate your resting metabolic rate in calories per day.
No age, height, or sex inputs are required because the formula assumes lean mass is the primary driver of metabolic rate.
Understanding Your Results
Your RMR is the number of calories your body burns at complete rest to maintain basic functions like breathing, circulation, and cell repair. This is not your total daily energy expenditure (TDEE). To estimate TDEE, multiply your RMR by an activity factor:
- Sedentary (little or no exercise): RMR × 1.2
- Lightly active (1–3 days per week): RMR × 1.375
- Moderately active (3–5 days per week): RMR × 1.55
- Very active (6–7 days per week): RMR × 1.725
- Extremely active (physical job + intense training): RMR × 1.9
For example, if your RMR is 1,600 calories and you are moderately active, your estimated TDEE is approximately 2,480 calories per day.
Who Should Use the Katch-McArdle Formula
This formula is most useful for:
- Athletes and bodybuilders with known low body fat percentages
- Individuals who have had body composition measured accurately
- People who find standard formulas underestimate or overestimate their calorie needs
For individuals with average or unknown body composition, the Mifflin-St Jeor formula is generally recommended as a starting point.
Limitations
- Requires an accurate body fat percentage estimate. Inaccurate body fat input leads to misleading RMR results.
- Not validated for all populations. The formula was developed from a relatively small sample and may be less accurate for older adults or individuals with very high body fat percentages.
- Does not account for hormonal, genetic, or medical factors that influence metabolic rate.
- Provides an estimate, not a precise measurement. Indirect calorimetry remains the gold standard for RMR assessment.
Common Mistakes
- Using estimated body fat from visual assessment or unreliable scales. This introduces significant error.
- Confusing RMR with TDEE. RMR is only one component of total calorie burn.
- Assuming the result is exact. Metabolic rate varies day to day and between individuals with identical body composition.
FAQ
Is the Katch-McArdle formula more accurate than other formulas?
For individuals with known lean body mass, Katch-McArdle tends to be more accurate than weight-based formulas. For the general population, Mifflin-St Jeor is often preferred because it does not require body fat data and has been validated across a broader range of body types.
How do I find my lean body mass?
Lean body mass = total body weight × (1 − body fat percentage). For example, if you weigh 80 kg and have 15% body fat, your lean mass is 80 × 0.85 = 68 kg.
Can I use this calculator if I don't know my body fat percentage?
No. The Katch-McArdle formula requires body fat percentage as an input. Without it, the calculation cannot be performed. Consider using a different RMR calculator that uses weight, height, age, and sex instead.
Does the formula work for women?
Yes. The formula uses only lean body mass and does not differentiate by sex. However, women typically have higher body fat percentages than men at the same weight, so accurate body fat measurement is especially important.
Should I use this for weight loss planning?
You can use the RMR estimate as a baseline for calorie planning, but always combine it with an activity multiplier to estimate total needs. Individual results vary, and adjustments based on real-world progress are necessary.