Quick answer
A kilojoule (kJ) is 1,000 joules, and the joule is the SI unit of energy. 1 kJ = 0.239 kcal, so 1 kcal is 4.184 kJ.
Kilojoule definition
The joule (symbol J) is the international unit of energy. It is named after the English physicist James Prescott Joule, who studied the relationship between heat and mechanical work. One joule is the energy transferred when a force of one newton moves something one metre. The prefix "kilo" means one thousand, so 1 kilojoule = 1,000 joules.
A note on writing it: the symbol is kJ, with a lowercase k and an uppercase J, because the J honours a person's name. People often type "kj" when searching, and this site understands both.
How big is a kilojoule?
A single joule is tiny: roughly the energy needed to lift a 100 g apple one metre. A kilojoule is a thousand times larger, and everyday comparisons help:
- Lifting a 1 kg bag of sugar about 100 metres takes roughly 1 kJ (1 kg × 9.81 m/s² × 100 m = 981 J).
- A 100-watt light bulb uses 0.1 kJ every second, which is 360 kJ in an hour.
- One kilowatt-hour of electricity is 3,600 kJ, or about 860 kcal.
Kilojoules in food
Food energy is the energy your body can release from carbohydrate, protein, fat, alcohol and fibre. Labels estimate it from the amount of each nutrient:
| Nutrient | Energy per gram |
|---|---|
| Carbohydrate | 17 kJ |
| Protein | 17 kJ |
| Fat | 37 kJ |
| Alcohol | 29 kJ |
| Dietary fibre | 8 kJ |
Australian and New Zealand labels give kJ per serving and per 100 g, plus a percentage of an 8,700 kJ reference day. That reference equals 2,079.35 kcal. Learn to read those figures in our guide to reading nutrition labels in kJ and kcal.
Kilo joule to calories
To convert kilo joule to calories, divide by 4.184. So 1 kJ is 0.239 kcal, 100 kJ is 23.901 kcal and 1,000 kJ is 239.01 kcal. Try any amount in the KJ to Cal converter or scan the kJ to calories chart.
Kilojoule vs megajoule
A megajoule (MJ) is 1,000 kJ. Large energy amounts are easier to read this way: an 8,700 kJ day is 8.7 MJ, and 10,000 kJ is 10 MJ, or 2,390.06 kcal. In nutrition, kJ is the everyday scale, and MJ appears in fuel, electricity and engineering.
Frequently asked questions
What is a kilojoule in simple terms?
A kilojoule is a unit for measuring energy, equal to 1,000 joules. On food labels it tells you how much energy a food provides.
How many calories are in a kilojoule?
One kilojoule is 0.239 kcal (food calories).
Why do some countries use kilojoules?
The joule is the official SI unit of energy, so countries that use metric labelling, such as Australia and New Zealand, list food energy in kilojoules.
Is kJ the same as kj?
The correct symbol is kJ. People type kj in searches, and both refer to kilojoules.
Last updated 11 Oct 2026. How we calculate.