What is a calorie?

In one line

A calorie is a unit that measures how much energy food gives us and how much energy the body uses.

What is a calorie?
The number on every food label and treadmill screen
You see 'kcal' on the back of a snack bag and on the treadmill screen. It comes up every time people talk about dieting. What exactly does it mean?
In one line
A unit for measuring energy
A calorie is a unit for measuring an amount of energy. Just as we measure length in cm and weight in kg, we measure the energy food gives us and the energy the body uses in calories. In Korean it is also called 'yeollyang' (amount of heat).
The original meaning
The heat that warms 1 g of water by 1°C
One calorie is the amount of heat needed to raise the temperature of 1 g of water by 1°C. Heat is energy that moves between two things at different temperatures. So 'how much water can it warm?' is a way to show how big an amount of energy is.
Calories in food
They are really kilocalories (kcal)
When we talk about food, we use a unit 1,000 times bigger: the kilocalorie (kcal). One kcal is the heat that raises 1 kg of water by 1°C. What people usually call a 'calorie', and the number on food labels, are both this kcal.
Where calories come from
Carbs, protein and fat
The energy in food comes from carbohydrates, protein and fat. Per gram, carbohydrate and protein give about 4 kcal, and fat gives about 9 kcal. The alcohol in drinks also gives about 7 kcal per gram.
How it is measured
Burn the food and measure the heat
Food is dried, ground into powder, and burned completely inside a strong steel container sitting in water. The energy is worked out from how many degrees the surrounding water warms up. This device is called a calorimeter, and numbers like the 4 and 9 above are averages of such measurements, minus the part that never gets into the body or cannot be fully used.
Try the math
Weight of each nutrient × 4, 4, 9
Label calories are usually found by multiplying each nutrient's weight by these numbers and adding them up. With 5 g protein, 31 g carbohydrate and 12 g fat: 20 + 124 + 108 = 252 kcal. Fiber, which the body barely digests, is left out of the carbohydrate count or counted lower.
In the body
Leftover calories are stored as fat
The body uses the calories it eats to keep the heart beating, breathe, hold body temperature and move. The basic energy used even while resting is called the basal metabolic rate. Leftover energy is stored mostly as fat, and when you burn more calories than you eat, body weight goes down.
Common myth
Does one workout let you eat a lot more?
No. In one experiment, people guessed they had burned three to four times the calories they actually burned, and when asked to eat that much, they ate far more than they had burned. Running 1.6 km (1 mile) uses only about 100 kcal.
Summary
A calorie is a unit of energy
A calorie is a unit that measures the energy food gives and the energy the body uses; for food we use kcal. Exercise burns fewer calories than most people think. Related terms: basal metabolic rate, TDEE, macronutrients.