Daily Calorie Intake Calculator (TDEE-Based)

Daily energy requirement is built up in layers. Basal metabolic rate is the energy spent at complete rest, awake, thermally neutral, and 12 or more hours after eating, and it accounts for roughly 60 to 70 percent of total expenditure in a sedentary adult. The thermic effect of food adds about 10 percent, and physical activity supplies the remainder. The Mifflin-St Jeor equation predicts the basal layer from four variables: 10 kcal per kilogram of body mass, 6.25 kcal per centimetre of height, minus 5 kcal per year of age, plus 5 for men or minus 161 for women. The weight and height terms are proxies for fat-free mass, which is the tissue that actually consumes energy; the age term reflects the loss of muscle that begins around the third decade. Total daily energy expenditure is then the basal figure multiplied by a physical activity level, conventionally 1.2 for sedentary work up to about 1.9 for heavy daily training. The multipliers are population averages and are the largest source of error in the estimate, since most people place themselves a category too high. Deficit targets rest on the older approximation that one pound of body fat stores about 3,500 kcal, so a 500 kcal daily deficit predicts roughly one pound per week; in practice loss slows as body mass and therefore expenditure fall, so the linear projection overstates results beyond a few months. Common floors are 1,200 kcal for women and 1,500 for men without supervision, and the Acceptable Macronutrient Distribution Ranges are 45 to 65 percent of energy from carbohydrate, 10 to 35 percent from protein, and 20 to 35 percent from fat.

The measurement of human energy expenditure began with calorimetry. Antoine Lavoisier and Pierre-Simon Laplace showed in the 1780s that a guinea pig in an ice calorimeter melted ice in proportion to the carbon dioxide it produced, establishing respiration as a form of combustion. A century later Wilbur Atwater and Edward Rosa built a room-sized respiration calorimeter at Wesleyan University in the 1890s and measured whole-body heat production directly, work that produced the Atwater factors still used on food labels: 4 kcal per gram for carbohydrate and protein, 9 for fat. In 1919 James Harris and Francis Benedict published standard basal metabolism equations from Carnegie Institution data, and those formulas dominated clinical dietetics for the next seventy years. By the late twentieth century the Harris-Benedict equations were overestimating measured resting expenditure in contemporary populations, partly because body composition had shifted. Mark Mifflin, Sachiko St Jeor, and their colleagues addressed this in a 1990 paper in the American Journal of Clinical Nutrition, using indirect calorimetry on several hundred healthy adults, roughly half of them obese, to derive a simpler and better-calibrated formula. A systematic evidence review published by Frankenfield and colleagues for the American Dietetic Association in 2005 compared the available predictive equations and found Mifflin-St Jeor the most reliable in both non-obese and obese adults, which is why it became the default in clinical and consumer tools.