BMI and Basal Metabolic Rate Calculator

🔬 What is the basal metabolic rate?

The basal metabolic rate (BMR) is the minimum amount of energy the body needs to sustain vital functions at complete rest: heartbeat, breathing, body temperature, protein synthesis and continuous cell repair.

It accounts for 60–75% of the total energy expenditure of a sedentary person. Factors such as muscle mass, age and thyroid hormones directly influence this value.

⚡ ATP — The Cell’s Energy Currency

ATP (adenosine triphosphate) is the molecule that carries energy inside every cell. When a phosphate bond is broken (ATP → ADP + Pᵢ), it releases ~30.5 kJ/mol that powers molecular motors, ion pumps and protein synthesis.

ATP is not stored in large amounts — it is continuously regenerated by the mitochondria from the nutrients you eat. At rest, the body recycles an amount of ATP equal to its own body weight per day.

🍎 Macronutrients and Cellular Energy

Each macronutrient enters cellular respiration through a different route:

Carbohydrates are converted into glucose and go straight into glycolysis — the fastest way to produce ATP.

Fats go through β-oxidation, generating Acetyl-CoA that feeds the Krebs cycle. They yield ~9 kcal/g (double the carbohydrates), which is why they are the main fuel in low-intensity activity.

Proteins are used for energy only as a last resort: their amino acids are converted into Krebs cycle intermediates or into glucose (gluconeogenesis), but their primary role is structural.

📊 Why is every BMR different?

Muscle mass: muscle burns ~13 kcal/kg/day at rest, while fat burns only ~4.5 kcal/kg/day. More muscle means a higher BMR.

Age: BMR drops ~2% per decade after age 20, reflecting the gradual loss of muscle mass and reduced mitochondrial activity.

Hormones: T3 and T4 (thyroid hormones) directly regulate the rate of metabolic reactions in every cell. Hypothyroidism can lower BMR by up to 40%.

Body temperature: a 1 °C fever raises basal metabolism by ~10–13% — cells speed up to fight the infection.