BMR Calculator — Katch-McArdle Formula

About the BMR Calculator — Katch-McArdle Formula

The Katch-McArdle formula estimates Basal Metabolic Rate (BMR) based on lean body mass rather than total body weight. Developed by Frank Katch and William McArdle, this equation recognizes that metabolically active tissue — primarily skeletal muscle — is the main driver of resting energy expenditure. By factoring in body composition, the Katch-McArdle formula provides a more individualized estimate for people who know their body fat percentage.

Unlike the Harris-Benedict and Mifflin-St Jeor equations, which rely on total weight, height, age, and sex, the Katch-McArdle formula uses a single predictor variable: lean body mass (total weight minus fat mass). This makes it particularly useful for athletes, bodybuilders, and anyone who has had their body composition measured through methods like DEXA, skinfold calipers, or bioelectrical impedance analysis.

This calculator computes your lean body mass from your weight and body fat percentage, applies the Katch-McArdle equation, and provides Total Daily Energy Expenditure (TDEE) estimates across five activity levels to help you plan your nutrition.

When This Page Helps

The Katch-McArdle formula is often useful for individuals with body compositions that differ from the general population average. Standard height-and-weight-based equations can overestimate BMR for some people with high body fat and underestimate it for some lean, muscular individuals. If you know your body fat percentage — whether from a DEXA scan, skinfold measurement, or body composition scale — the Katch-McArdle formula can provide a lean-mass-based calorie baseline to compare with population-averaged equations.

How to Use the Inputs

1. Enter your total body weight in kilograms or pounds.
2. Enter your body fat percentage. Use a body composition measurement method for best accuracy.
3. Choose your preferred unit system — metric or imperial.
4. The calculator automatically computes your lean body mass (weight × (1 − body fat %)).
5. View your estimated BMR in kilocalories per day.
6. Review the TDEE table to find your estimated daily calorie needs based on activity level.
7. Compare results with the Cunningham equation shown alongside for reference.

Example Calculation

Tips & Best Practices

• DEXA scans and hydrostatic weighing are among the stronger body-fat reference methods; visual estimates can be off by 5-10% or more.
• If you don't know your body fat percentage, estimate it using our Navy Method or skinfold calculators first.
• The Katch-McArdle formula is sex-neutral — it uses lean mass directly, which already accounts for sex-related body composition differences.
• As you lose fat and maintain muscle, your Katch-McArdle BMR may stay relatively stable, which can make it a useful comparison point as body composition changes.
• Athletes with very low body fat (under 8% for men, 14% for women) may find this formula slightly overestimates BMR.
• Recalculate after significant body composition changes, not just scale weight changes.

Understanding Lean Body Mass and Metabolism

Lean body mass (LBM) includes everything in your body except stored fat: skeletal muscle, organs, bones, water, and connective tissue. Skeletal muscle is the largest component and is more metabolically active at rest than fat tissue, which is why lean mass is such a strong predictor of resting metabolic rate.

When to Choose Katch-McArdle Over Other Formulas

Consider using the Katch-McArdle formula when you have a reasonably reliable body fat percentage measurement, your body composition differs significantly from the average population, you are tracking body composition changes over time, or you want to compare BMR across different body fat levels. If you do not have body fat data, the Mifflin-St Jeor equation is a practical alternative.

Practical Nutrition Planning

Once you have your Katch-McArdle BMR, multiply by your activity factor to get TDEE. From there, calorie intake can be adjusted upward or downward depending on whether the goal is weight maintenance, fat loss, or muscle gain. Protein intake is also commonly adjusted during dieting phases or mass-gain phases, and body fat can be reassessed periodically as composition changes.

Limitations of the Katch-McArdle Formula

The primary limitation is its dependency on accurate body fat measurement. A 5% error in body fat estimation translates to roughly 50-80 kcal/day error in BMR. The formula also does not account for metabolic adaptation from prolonged dieting, thyroid conditions, or genetic variation in metabolic efficiency. For clinical precision, indirect calorimetry remains the direct-measurement reference.

Frequently Asked Questions

• What makes Katch-McArdle different from other BMR formulas?

  Katch-McArdle is the only common BMR equation that uses lean body mass as its sole predictor variable. Other equations like Harris-Benedict and Mifflin-St Jeor use total weight, height, age, and sex. Because muscle tissue is a primary driver of resting metabolism, using lean mass directly can be more informative for people whose body composition differs significantly from population averages.

• Do I need to know my exact body fat percentage?

  The accuracy of the Katch-McArdle result depends directly on the accuracy of your body fat input. A DEXA scan provides a strong reference measurement (±1-2% error). Skinfold calipers performed by experienced technicians are reasonably accurate (±3-4%). Bioelectrical impedance scales vary widely in accuracy. Even an estimate is better than nothing, but if your body fat input is off by 5 percentage points, your BMR estimate could be off by 50-70 kcal/day.

• Is Katch-McArdle better than Mifflin-St Jeor?

  It depends on your situation. For the general population without body composition data, Mifflin-St Jeor is one of the most studied equations. For individuals who know their body fat percentage and have atypical body compositions — very lean, very muscular, or very high body fat — Katch-McArdle can be more informative because it accounts for the actual amount of lean tissue.

• Why doesn't Katch-McArdle include age or sex?

  Age-related metabolic decline is largely driven by age-related muscle loss (sarcopenia). Sex differences in BMR are also largely explained by differences in lean mass. By using lean body mass directly, the Katch-McArdle formula implicitly accounts for both age and sex effects through their impact on muscle mass, making separate variables unnecessary.

• How does Katch-McArdle compare to Cunningham?

  The Cunningham equation (500 + 22 × LBM) was developed for athletes and gives slightly higher BMR estimates than Katch-McArdle (370 + 21.6 × LBM). For a person with 70 kg of lean mass, Cunningham predicts about 2,040 kcal/day versus Katch-McArdle's 1,882 kcal/day. Cunningham is often used as a comparison point in highly trained athletes.

• Can I use this calculator during a cutting phase?

  Yes, it can be useful during a cut. As you lose fat while preserving muscle, your Katch-McArdle BMR may remain relatively stable compared with formulas that rely on total body weight alone. Many people start with a moderate deficit below TDEE and adjust based on actual weight change, training performance, and recovery.

• What body fat percentage should I aim for?

  Healthy body fat ranges are 10-20% for men and 18-28% for women. Essential fat minimums are approximately 3-5% for men and 10-13% for women. Athletes typically maintain 6-13% (men) or 14-20% (women). Going below essential fat levels is dangerous and unsustainable.

• Is this formula accurate for obese individuals?

  The Katch-McArdle formula can underestimate BMR in obese individuals because excess fat tissue, while less metabolically active than muscle, does contribute to total energy expenditure. For individuals with body fat above 35-40%, the Mifflin-St Jeor equation or adjusted body weight formulas may provide more practical estimates.