Calories
A calorie is a unit of energy. In nutrition, the word usually means a kilocalorie (kcal): the energy needed to heat one kilogram of water by one degree Celsius. One kilocalorie equals 4.184 kilojoules (kJ). On US food labels, a capital-C "Calorie" also means one kilocalorie, not one small calorie.
The calorie value of a food describes its estimated usable energy, not its nutritional quality. The main energy-providing macronutrients are protein and carbohydrates, at roughly 4 kcal per gram, and fat, at roughly 9 kcal per gram; alcohol provides about 7 kcal per gram. These conventional values are averages, so label values and database entries do not measure exactly what one individual absorbs.
Calories can describe both energy intake from food and energy expenditure by the body. Whether stored energy tends to rise, fall, or remain broadly stable depends on energy balance over time, not on one food or one day. A person's total daily energy expenditure also changes with body size, movement, training, food intake, and other factors.
For example, 30 grams of carbohydrate contributes about 120 kcal before label rounding. That calculation says how much energy the carbohydrate provides; it does not tell you how filling the food is, how much fiber or micronutrients it contains, or how it affects training. Those questions require more than a calorie total.
Calories are therefore useful for planning and comparing energy intake, but they should be treated as estimates. There is no fixed calorie number that guarantees a particular weekly weight change: body weight also reflects water, glycogen, digestive contents, and dynamic changes in expenditure. The NIDDK Body Weight Planner illustrates this dynamic approach rather than assuming that one static conversion applies indefinitely.