BMR Calculator – Mifflin St Jeor, Harris Benedict, Katch McArdle & TDEE
This BMR Calculator is a professional-grade tool designed to help you estimate your Basal Metabolic Rate (BMR), Total Daily Energy Expenditure (TDEE), and optimal calorie intake for fat loss, maintenance, and muscle gain. It includes three of the most widely validated metabolic formulas used in clinical nutrition, sports science, and modern fitness programming: Mifflin St Jeor, Harris Benedict, and Katch McArdle.
Whether you are a fitness professional, athlete, coach, or someone tracking your nutrition with precision, this guide provides a clear and evidence-based explanation of each formula, its advantages, and when to use it. By the end, you’ll know exactly how to interpret your results and how to apply them to real-world diet planning.
What Is BMR (Basal Metabolic Rate)?
BMR represents the minimum number of calories your body requires at complete rest to maintain essential physiological functions. These functions include hormone production, cell repair, breathing, circulation, nutrient processing, and maintaining body temperature.
BMR is often described as your “resting calorie requirement,” and it accounts for the largest portion of your daily energy expenditure—typically 60–70% for most individuals. Accurate BMR estimation is essential for determining how many calories you should consume based on your fitness goals.
What Influences BMR?
Your basal metabolic rate is influenced by several factors:
- Sex: Men typically have higher BMR due to higher lean mass and lower body fat percentage.
- Age: BMR decreases gradually with age due to muscle loss and metabolic changes.
- Height & Weight: Larger body size requires more energy.
- Lean Body Mass (LBM): Muscle tissue is metabolically active, significantly boosting BMR.
- Genetics: Natural metabolic variations can affect daily calorie expenditure.
- Hormonal Factors: Thyroid levels, stress hormones, and reproductive hormones impact BMR.
An accurate BMR calculation allows you to estimate your TDEE and fine-tune your nutrition program with precision.
What Is TDEE?
Total Daily Energy Expenditure (TDEE) is the total number of calories you burn per day when BMR is combined with additional daily activities. TDEE includes:
- BMR: Resting metabolic activity.
- NEAT: Non-exercise activity (walking, chores, posture, fidgeting).
- TEF: Thermic effect of food (calories burned digesting food).
- EAT: Exercise activity (workouts, sports, training).
The calculator automatically multiplies your BMR by an activity factor to produce your TDEE. This tells you how many calories you need per day to maintain your weight.
Standard Activity Multipliers
- 1.2: Sedentary – little or no exercise
- 1.375: Lightly active – training 1–3 days/week
- 1.55: Moderately active – training 3–5 days/week
- 1.725: Very active – training 6–7 days/week
- 1.9: Extra active – intense daily training
Once your TDEE is known, you can determine calorie targets for fat loss, muscle gain, or weight maintenance.
Formulas Used in This BMR Calculator
This calculator allows you to evaluate your energy needs using three separate metabolic models. Each serves a specific purpose depending on body composition and measurement accuracy.
1. Mifflin St Jeor Equation
The Mifflin St Jeor formula, published in 1990, is widely regarded as the most accurate BMR equation for modern populations. Numerous clinical studies have demonstrated itsiability, making it the preferred formula for nutrition professionals.
Best For:
- General population
- Most fitness and weight-loss cases
- Individuals without known body fat percentage
Why It’s Preferred:
Mifflin St Jeor adjusts efficiently for modern body composition patterns and tends to align closely with metabolic testing (indirect calorimetry) for most people.
2. Harris Benedict Equation (Revised)
The Harris Benedict equation is one of the oldest BMR formulas, originally published in 1918 and later updated to reflect modern averages. While not as precise as Mifflin St Jeor, it remains a valid formula with decades of use in nutrition science.
Best For:
- Comparing modern vs. classic metabolic values
- Individuals following older nutrition programs
- General estimation and educational reference
Why It’s Useful:
Theised version provides a perspective on how body size influences metabolic rate across different eras, making it a valuable comparative tool.
3. Katch McArdle Equation
The Katch McArdle formula calculates BMR based on lean body mass (LBM). This makes it exceptionally accurate for athletes, trained individuals, and anyone who knows their body fat percentage.
Best For:
- Athletes
- Bodybuilders
- Individuals with above-average muscle mass
- Anyone with a recent body fat test
Why It’s Highly Accurate:
Since muscle is more metabolically active than fat, incorporating lean mass directly yields a more individualized metabolic estimate—especially in people with higher muscle proportions.
BMR vs. RMR – What’s the Difference?
Although commonly used interchangeably, BMR (Basal Metabolic Rate) and RMR (Resting Metabolic Rate) are measured differently:
- BMR: Measured under strict clinical conditions (fasting, controlled environment).
- RMR: Measured underaxed conditions (less strict, more practical real-world value).
Most equations (including those used here) estimate BMR but are functionally closer to RMR for everyday use. For nutrition planning, the difference is negligible.
How This BMR Calculator Determines Calories
Once BMR is calculated, the tool automatically produces:
- TDEE (Maintenance Calories): Calories needed to maintain current weight.
- Weight Loss Calories: A controlled deficit—large enough for fat loss but moderate enough to protect muscle.
- Muscle Gain Calories: A slight surplus for steady lean mass gain.
- Suggested Protein, Carbs, and Fats: Based on balanced or performance-oriented macro ratios.
Choosing the Right Energy Formula
Not every formula will fit every user. Here’s a professional guideline for formula selection:
Use Mifflin St Jeor if:
- You want the mostiable estimate for general use
- You do not know your body fat percentage
- You follow a traditional weight-loss or maintenance plan
Use Harris Benedict if:
- You want a comparative or historical reference
- You’re following older calorie guidelines
- You want a slightly higher BMR estimate
Use Katch McArdle if:
- You know your body fat percentage
- You are muscular, athletic, or active in strength training
- You want a lean-body-mass-based calculation
How to Apply Your BMR & TDEE to Real-World Nutrition
Understanding your numbers is the first step. Applying them correctly is what drives actual results.
1. Fat Loss (Calorie Deficit)
A moderate calorie deficit—usually 300–500 kcal/day below TDEE—supports sustainable fat loss while preserving muscle mass.
2. Weight Maintenance
Simply consume calories equal to your TDEE. This is your daily energy balance point.
3. Muscle Gain (Calorie Surplus)
A controlled surplus of 250–350 kcal/day provides enough energy to support hypertrophy without excessive fat gain.
4. Macro Distribution
Typical macro ratios for general fitness include:
- 30% protein – muscle repair and metabolic efficiency
- 40% carbohydrates – training fuel and daily activity energy
- 30% fats – hormone production and nutritional balance
You can refine macros further when combining this tool with the Macro Calculator or Calorie Calculator.
How Accurate Are BMR Formulas?
All BMR equations provide estimates. Actual metabolic rate may differ based on genetics, hormones, stress, sleep, and training load. However:
- Mifflin St Jeor accuracy: ~82–88%
- Harris Benedict accuracy: ~70–75%
- Katch McArdle accuracy: very high when body fat % is accurate
Indirect calorimetry (clinical metabolic testing) is the gold standard, but BMR formulas remain highly practical and sufficiently accurate for nutrition programming.
Why Your BMR Changes Over Time
BMR is not static. It evolves based on internal and external factors:
- Muscle gain increases BMR
- Fat loss may decrease BMR slightly
- Aging reduces metabolic rate over time
- Hormonal changes alter energy needs
- Chronic dieting can temporarily reduce metabolic rate
Tracking your BMR over time—especially when training or modifying body composition—is essential for accurate calorie planning.
BMR Calculator FAQs
Frequently Asked Questions
Professional answers to common questions BMR, TDEE, metabolic formulas, and how to use your results effectively.
Mifflin St Jeor is generally regarded as the most accurate for the general population. Katch McArdle is most accurate for individuals who know their body fat percentage and have above-average lean mass.
Every 4–8 weeks, or whenever you experience noticeable changes in body weight or composition. Athletes may recalculate more frequently.
Yes. In everyday nutrition planning, BMR functions interchangeably with resting calorie needs and serves as your baseline for determining TDEE.
Yes. Increasing lean muscle mass, resistance training, adequate protein intake, and proper sleep all contribute to higher metabolic activity.
Calorie targets are estimates but highly effective when paired with consistent tracking. Adjust caloric intake every 2–3 weeks based on progress.
Each formula weighs height, weight, age, and body composition differently. Variations are normal, and comparing values can provide insight into your metabolic range.