A British thermal unit is the amount of energy required to heat a pound of water by 1 degree Fahrenheit. This can vary slightly depending on the temperature of the water, therefore different estimates for the BTU exist. The International Standards Organization defines a BTU as 1055.056 joules.
A nutritional Calorie, or kilocalorie, is commonly used in food labeling. It is equal to 4.184 kilojoules. The average American consumes about 2,700 kilocalories per day.