Basal Metabolic Rate (BMR) is the amount of energy needed to maintain basic bodily functions at rest in a neutral environment and an inactive digestive state. It’s similar to measuring how much fuel an idle car uses when parked. In this state, energy is used only to sustain essential organs like the heart, brain, kidneys, and lungs, among others. BMR accounts for approximately 70% of total daily energy expenditure, while physical activity makes up about 20%, and around 10% is dedicated to digesting food (thermogenesis)

BMR is measured under controlled conditions when a person is fully rested, with a quiet sympathetic nervous system. Typically, BMR constitutes the largest part of daily calorie needs, which can be estimated by multiplying BMR by an activity level factor (1.2–1.9).