Body Mass Index (BMI) categorizes weight into four main groups:

• Underweight: Below 18.5
• Normal Weight: 18.5 – 24.9
• Overweight: 25.0 – 29.9
• Obese: 30.0 and above

Limitations of BMI: While useful for general population analysis, BMI does not account for muscle mass, bone density, or fat distribution. Athletes may have high BMIs despite low body fat percentages, while older adults may have normal BMIs despite having excess body fat.

These acronyms represent distinct but related metrics for your health:

• BMI (Body Mass Index): A ratio of your weight to your height. It tells you if your overall mass is healthy for your frame.
• BMR (Basal Metabolic Rate): The exact number of calories your body burns at rest just to keep you alive (breathing, blood circulation, cellular repair).
• TDEE (Total Daily Energy Expenditure): Your BMR multiplied by your physical activity multiplier. This is the ultimate "maintenance" number of calories you burn on an average day.

Think of your TDEE as a baseline estimate. Here is the actionable workflow used by fitness professionals:

1. Eat exactly at your calculated TDEE for 2-3 weeks.
2. Monitor your weight daily and look at the weekly average.
3. If your weight remains stable, your TDEE estimate is highly accurate.
4. If your weight goes up or down, adjust your daily calorie intake by 100–200 calories and reassess.

To lose fat, aim for a sustainable deficit of 250–500 calories per day below your confirmed TDEE. To gain muscle (bulk), aim for a moderate surplus of 250–500 calories above your TDEE.

The Thermic Effect of Food (TEF) is the amount of energy your body uses to digest, absorb, and metabolize the food you eat. It generally accounts for about 10% of your Total Daily Energy Expenditure (TDEE).

Different macronutrients have different thermic effects: protein requires the most energy to digest (burning up to 30% of the calories it contains), while fats and carbohydrates require significantly less (around 3-10%).

The Mifflin-St Jeor equation used in this calculator is widely considered by health and fitness professionals to be the most accurate standard formula for estimating BMR for the general population. It uses weight, height, age, and gender.

The Katch-McArdle formula is another highly accurate equation. It completely ignores age and gender, and instead uses your exact Lean Body Mass. While Katch-McArdle is superior for advanced athletes with known body fat percentages, it requires specialized equipment (like DEXA scans) to use accurately, which is why Mifflin-St Jeor remains the gold standard for everyday use.