BMR & TDEE Formulas: Mifflin-St Jeor, Katch-McArdle, and Harris-Benedict
The calculator supports three methods to calculate your basal metabolic rate (BMR).The Katch-McArdle Formula
This method is considered the most accurate if you know your current body fat level with a great deal of certainty and are relatively lean.
It's recommended to use this formula only if you have a reliable way of measuring your body fat level. Acceptable methods include DEXA scan, InBody scan, or skinfold measurements done by someone experienced with using calipers. Most bathroom and fitness scales are grossly inaccurate and are known to be up to 10% off.
If you're a man with a body fat percentage less than 14-15% or a woman below 19-20% and know your body fat percentage accurately, use this formula for the best accuracy.
How the Katch-McArdle Formula Works
The Katch-McArdle formula calculates your Basal Metabolic Rate (BMR) based on your lean body mass (LBM), which is your total body weight minus your fat mass. The formula is:
BMR = 370 + (21.6 × lean body mass in kg)
This approach is superior to weight-based formulas for lean individuals because muscle tissue burns significantly more calories at rest than fat tissue. Two people of the same weight and height can have vastly different energy needs depending on their body composition.
When to Use Katch-McArdle
This formula is ideal for:
- Athletes and regular weight trainers with known body fat percentages
- Individuals who have had professional body composition testing (DEXA, hydrostatic weighing)
- People with lower body fat percentages (men under 15%, women under 20%)
- Anyone with significantly above-average muscle mass for their weight
Important: Do not use estimated body fat percentages from bathroom scales or basic online calculators with this formula. Inaccurate body fat measurements will produce unreliable results. If you don't have a professionally measured body fat percentage, the Mifflin-St Jeor formula will likely give you better results.
The Mifflin-St Jeor Formula
The Mifflin-St Jeor equation is the default formula used by this calculator and is recommended by the Academy of Nutrition and Dietetics for most people. Published in 1990, it has been extensively validated and is considered the most accurate formula for the general population.
How the Mifflin-St Jeor Formula Works
This formula calculates BMR using your weight, height, age, and biological sex:
For men: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age) + 5
For women: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161
Why Mifflin-St Jeor is the Default
Research has shown that Mifflin-St Jeor is approximately 5% more accurate than the older Harris-Benedict equation and produces reliable results for the majority of people without requiring body fat measurements. It accounts for the metabolic differences between sexes and the gradual decline in metabolic rate that occurs with age.
When to Use Mifflin-St Jeor
This formula is recommended for:
- Most people who don't have accurate body fat measurements
- Individuals with average to above-average body fat percentages
- Anyone starting their weight loss or fitness journey without previous body composition testing
- General population BMR estimation where high precision isn't critical
The Mifflin-St Jeor formula tends to slightly overestimate BMR for very obese individuals and underestimate it for very muscular individuals, but for the vast majority of users, it provides an excellent starting point.
The Harris-Benedict Formula
The Harris-Benedict equation was originally developed in 1919 and revised in 1984 by Roza and Shizgal. While it's one of the most well-known BMR formulas, it has largely been superseded by the Mifflin-St Jeor equation for general use.
How the Harris-Benedict Formula Works
The revised Harris-Benedict equation calculates BMR as follows:
For men: BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) - (5.677 × age)
For women: BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) - (4.330 × age)
When to Use Harris-Benedict
This formula may be preferred when:
- Comparing results to older research or clinical data that used Harris-Benedict
- Working with healthcare providers who specifically use this equation
- Cross-referencing calculations from multiple sources
For most modern applications, the Mifflin-St Jeor equation is preferred due to its improved accuracy. Studies have shown that Harris-Benedict tends to overestimate caloric needs by approximately 5% compared to measured expenditure.
Comparing the Three Formulas
Here's a quick comparison to help you choose the right formula:
| Formula | Best For | Requires Body Fat %? | Relative Accuracy |
|---|---|---|---|
| Katch-McArdle | Lean, athletic individuals with known body composition | Yes (professional measurement required) | Highest (when BF% is accurate) |
| Mifflin-St Jeor | General population, most people | No | High (recommended default) |
| Harris-Benedict | Legacy comparisons, clinical settings | No | Moderate (tends to overestimate) |
Understanding the Limitations
It's crucial to understand that all BMR formulas provide estimates, not exact measurements. Your actual metabolic rate can vary by 10-20% from calculated values due to factors including:
- Genetics: Some people naturally have faster or slower metabolisms
- Hormonal factors: Thyroid function, insulin sensitivity, and other hormones affect metabolism
- Metabolic adaptation: Extended calorie restriction can lower metabolic rate
- Muscle mass distribution: Where you carry muscle affects energy expenditure
- Non-exercise activity thermogenesis (NEAT): Unconscious movements and fidgeting vary widely
- Diet history: Previous dieting can temporarily suppress metabolism
These formulas should be used as a starting point. Monitor your actual weight changes over 2-4 weeks and adjust your calorie intake based on real-world results rather than relying solely on the calculated number.
Activity Multipliers: From BMR to TDEE
Your BMR represents the calories you burn at complete rest. Your Total Daily Energy Expenditure (TDEE) accounts for all activity throughout the day. After calculating BMR with one of the formulas above, it's multiplied by an activity factor:
- Sedentary (1.2): Office job, minimal movement, no intentional exercise
- Lightly active (1.375): Light exercise 1-3 days/week or active job
- Moderately active (1.55): Moderate exercise 3-5 days/week
- Very active (1.725): Intense exercise 6-7 days/week
- Extremely active (1.9): Physical job + intense daily training
Most people overestimate their activity level. Unless you have a physically demanding job or train intensely most days, start with "sedentary" or "lightly active" and adjust based on results.
Practical Example
Let's calculate TDEE for a 30-year-old man who weighs 80 kg (176 lbs), is 180 cm (5'11") tall, works a desk job, and exercises 3 times per week:
Using Mifflin-St Jeor:
BMR = (10 × 80) + (6.25 × 180) - (5 × 30) + 5
BMR = 800 + 1,125 - 150 + 5 = 1,780 calories/day
Applying activity multiplier (lightly active = 1.375):
TDEE = 1,780 × 1.375 = 2,447 calories/day
To lose weight at a moderate pace (about 0.5 kg or 1 lb per week), this person would target a 500 calorie deficit, eating approximately 1,950 calories per day.
Getting the Most Accurate Results
To improve the accuracy of your TDEE calculation:
- Be honest about activity level - When in doubt, choose the lower option
- Track your weight consistently - Weigh yourself at the same time daily, calculate weekly averages
- Give it time - Weight fluctuates daily from water, food volume, and hormones. Look at 2-4 week trends
- Adjust based on results - If you're not losing/gaining as expected, adjust calories by 10-15%
- Get professional body composition testing - If you're serious about precision, invest in a DEXA scan
Remember, the best formula is the one that gives you results you can act on. Start with Mifflin-St Jeor, track your progress, and adjust as needed. The calculator is a tool to guide you, not a rigid prescription.