How does the calorie compare to the metric unit the joule? how do you calculate a calorie? is it the same for all types of food?

Calorie (cal) or small calorie, is the amount of energy needed to heat one gram of water by one degree Celsius.

One small calorie is approximately 4.2 joules.

A calorie is a unit of energy.

For example:

Q = 150000 J; released energy.

Convert Jules in calories:

Q = 150000 J ÷ 4.186 J/calorie.

Q = 35833.73 calorie.

The large calorie or nutritional calorie (symbols: Cal, kcal) is the heat energy needed to raise the temperature of one kilogramof water by one degree Celsius.  

A nutritional calorie, or kilocalorie, is equal to 1000 calories.

One large calorie is 4.184 kilojoules: 1 Cal = 4.184 kJ.

For example:

E = 2500 Cal; energy burnt.

E = 2500 Cal · 4.184 kJ/Cal.

E = 10460 kJ.