Crisps in a machine

Measuring energy

How do we measure the energy in food?

One way to measure all the different things you eat in a week is by the amount of energy the food contains. The International System of Units (SI) unit for energy is the joule (J); often, energy in food is discussed in kilojoules (kJ, 103 J) or megajoules (MJ, 106 J). Some food packaging and other nutritional information uses the kilocalorie (kcal). One kcal is equal to 4.184 kJ. The energy content of food is estimated by burning it in a bomb calorimeter, which measures heat production. Some foods yield more energy than others – sweet or fatty foods are likely to be more energy-dense.

Bomb calorimeter diagram

A bomb calorimeter.


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About this resource

This resource was first published in ‘Food and Diet’ in June 2011 and reviewed and updated in August 2016.

Statistics and maths, Physiology, Biotechnology and engineering
Food and Diet
Education levels:
16–19, Continuing professional development