There are 1000 calories in a kilocalorie (kcal). In physics and chemistry, a calorie is a very small unit: it is the quantity of heat required to raise the temperature of 1 gram of water 1 degree Celsius.
When it comes to diet/nutrition, people use the kilocalorie (that’s why the abbreviation ”kcal” is used on food labels) but call it just “calorie” because it’s shorter and easier to say than “kilocalorie”.
If we had to use the ‘real’ calorie unit that is used by scientists, we would have to say for example “1,500,000 daily calories” to talk about a daily caloric intake of 1,500 calories (kcal).
Kilocaloric measurements (kcal) are shorter and easier to work with. The calories we’re talking about when it comes to nutrition are thus usually kilocalories.