The human body constantly expends energy to perform life-sustaining functions, even during complete inactivity. This continuous energy burn, known as passive calorie burning, accounts for the largest portion of total daily calories used. Understanding this baseline expenditure answers how many calories are burned when a person is seemingly “doing nothing.” This metabolic process fuels the automatic work of the body, such as circulating blood, regulating temperature, and supporting organ function. The number of calories burned passively is highly individualized and forms the foundation for total daily energy needs.
Defining the Baseline: RMR and BMR
The core measurement of passive calorie burning is the Basal Metabolic Rate (BMR), which represents the absolute minimum calories required to keep the body functioning at complete rest. Obtaining a true BMR requires highly controlled conditions: the person must be completely rested, in a thermally neutral environment, and fasted for at least 12 hours. These strict conditions ensure no energy is spent on digestion or physical activity.
Because BMR measurement conditions are impractical for most people, the Resting Metabolic Rate (RMR) is the more commonly used metric. RMR measures energy expended while at rest but under less restrictive conditions, typically after a short period of quiet rest and a few hours without food or exercise. The RMR is usually about 10% higher than the BMR because it includes the minor energy costs of being awake and minimal recent digestion.
The RMR provides the most realistic estimate for the “doing nothing” calorie burn, accounting for approximately 60% to 75% of total daily energy expenditure. For an average adult, this rate generally falls between 1,200 and 2,500 calories per day, depending heavily on physical characteristics. Generalized estimates often place RMR around 1,400 calories for women and 1,800 calories for men, but these are only starting points.
Key Physiological Factors Influencing the Rate
The specific number of calories burned at rest is determined by several biological variables, with body composition being the most influential factor. Muscle tissue, or lean mass, is metabolically demanding and requires significant energy to maintain itself. In contrast, fat tissue is far less active and burns substantially fewer calories at rest.
Individuals with a higher proportion of muscle mass relative to fat mass will naturally have a higher RMR. Even a small increase in muscle mass can lead to a measurable rise in resting energy expenditure, making resistance training a practical way to affect this rate. Conversely, the loss of lean muscle tissue, which often occurs with aging or restrictive dieting, will reduce the RMR.
Body size and weight also play a significant role, as larger individuals have more metabolizing tissue and a greater volume of organs to maintain. This larger mass requires more energy to sustain basic life functions, leading to a higher resting energy expenditure. The RMR also tends to be higher in men than in women, largely attributed to men generally having greater average body size and a higher percentage of lean muscle mass.
Metabolic rate naturally slows down as a person ages, typically beginning around age 20. This age-related decrease is primarily due to a gradual loss of muscle mass over time, compounded by changes in hormonal and neurological processes. Studies suggest that basal metabolism can decrease by 1–2% per decade after age 20.
Non-Resting Energy Components
While RMR provides the baseline for “doing nothing,” the total daily calorie burn is always higher due to two other non-conscious energy components. The first is the Thermic Effect of Food (TEF), which is the energy required to digest, absorb, and process consumed nutrients. This process accounts for roughly 10% of the total calories consumed, though the exact percentage varies depending on the types of macronutrients eaten.
The second component is Non-Exercise Activity Thermogenesis (NEAT), which is the energy expended for all physical activity that is not deliberate exercise. This includes spontaneous movements like fidgeting, standing, and maintaining posture. Although individual NEAT levels can vary significantly, it can account for 15% to 30% of total daily energy expenditure in active individuals. For people with largely sedentary lifestyles, NEAT contributes a smaller, but still meaningful, portion of the total energy burn.