What is BMR?
Your Basal Metabolic Rate is the energy your body burns at complete rest to keep you alive. Even when you are doing nothing, your body is working: your heart beats, your lungs breathe, your cells repair themselves, and your brain consumes about 20% of your total energy. BMR accounts for roughly 60–75% of the calories you burn each day, which means it is the largest single component of your energy expenditure. Factors that raise BMR include greater muscle mass, younger age, male sex, and certain medical conditions like an overactive thyroid.
From BMR to TDEE
Total Daily Energy Expenditure adds the calories you burn through activity to your BMR. It is calculated by multiplying BMR by an activity factor: 1.2 for a sedentary lifestyle, 1.375 for light activity, 1.55 for moderate exercise, 1.725 for very active, and 1.9 for athletes. TDEE gives you a realistic daily calorie target. Eat above it and you gain weight; eat below it and you lose weight; match it and you maintain. The Mifflin-St Jeor equation, used by most modern calculators, is the most accurate predictive formula for healthy adults, though individual variation means your true figure may differ by 5–10%.
How to use these numbers
For fat loss, a moderate deficit of 500 calories below TDEE produces about 0.5 kg of loss per week — a sustainable rate that preserves muscle and is easier to maintain. For muscle gain, a surplus of 300–500 calories combined with resistance training supports growth. Crash diets that drop below your BMR are counterproductive: they slow your metabolism, reduce muscle, and are difficult to sustain. As you lose weight, your BMR decreases, so you will need to recalculate periodically to keep making progress.
Beyond the numbers
Calorie targets are estimates, not commandments. Food quality matters as much as quantity — 2,000 calories of whole foods will leave you far healthier and more satisfied than 2,000 calories of ultra-processed snacks. Sleep, stress, and hormones also affect how your body uses energy. Use BMR and TDEE as a starting framework, adjust based on your results over a few weeks, and remember that consistency beats precision. A plan you can stick to is always better than a perfect plan you abandon.
