Joules to Kilojoules Converter

A decimal shift

Kilo means a thousand, so a kilojoule is a thousand joules. The conversion moves the decimal point three places and nothing else.

1 kJ = 1,000 J

The joule is a small unit — roughly the energy to lift an apple a metre — so most real quantities are more readable in kilojoules or megajoules. Food energy, reaction enthalpies and mechanical work all sit comfortably in kJ.

The joule

The SI unit of energy: the work done when a force of one newton moves something one metre. It is a small unit — lifting an apple a metre off the table takes about one joule.

Everything else on this page is the same quantity counted differently. A joule is also one watt for one second, which is the link between energy and power.

The kilojoule

A thousand joules, and the unit food energy is labelled in across Europe, Australia and much of Asia. A slice of bread is about 300 kJ; a daily intake of 2,000 food Calories is 8,368 kJ.

Labels in Europe usually print both kJ and kcal, which is why the same biscuit carries two numbers roughly four apart.

The working, step by step

Every line updates with whatever you type above. The point is the method, which is the same for any pair of units in this family.

Copy the working

The same quantity, two different numbers

One physical amount, read off two scales. The tick marks do not line up, and the gap between them is the conversion factor made visible.

How much is that?

A number is easier to judge next to something familiar.

Your turn

Reading a method and being able to use it are different things. Answer within half a per cent and it counts as correct.

How to use this converter

1. Type into either box. The other side updates as you go; there is no button to press.
2. Swap the direction with the button between the boxes, carrying your value across.
3. Set the decimals if the default is more or less precision than you need.
4. Copy the answer with its unit, ready to paste.
5. Use the shortcut buttons for the values that come up most often.
6. Share the result. The address bar tracks what you type, so the link you copy opens on the same number.

Common uses and tips

Where this comes up

• Food labels. Europe and Australia print kilojoules; the United States prints Calories. The same packet often carries both.
• Nutrition and exercise tracking, where one app reports kJ and another kcal.
• Chemistry and physics coursework, where reaction enthalpies come in kJ/mol and older texts use calories.
• Physics at atomic scale, where electronvolts are the only sensible size.

The trap: calorie and Calorie

The word means two things a thousand apart.

• calorie (small c) — the scientific unit, exactly 4.184 joules. Enough to warm a gram of water by one degree.
• Calorie (capital C), or kcal — a thousand of those, 4,184 joules. This is what every food label means.

So a 2,000-Calorie daily intake is 2,000 kcal, which is 2,000,000 small calories, or 8.37 megajoules. If a conversion comes out a thousand away from what you expected, this is why.

Tips worth knowing

• 4.184 is the number to remember. It works at every scale: cal to J, kcal to kJ, Mcal to MJ.
• Energy is not power. Joules are an amount; watts are a rate. They cannot be converted without a time.
• A kilowatt-hour is energy, not power: 3.6 MJ, or 860 kcal.

Values people look up most

Each one opens the converter with the number already in it.

• 100 J to kJ
• 500 J to kJ
• 1,000 J to kJ
• 2,000 J to kJ
• 5,000 J to kJ
• 10,000 J to kJ
• 20,000 J to kJ
• 50,000 J to kJ
• 100,000 J to kJ
• 250,000 J to kJ
• 500,000 J to kJ
• 1,000,000 J to kJ

Quick reference

Joules	Kilojoules
100 J	0.1 kJ
1,000 J	1 kJ
5,000 J	5 kJ
10,000 J	10 kJ
100,000 J	100 kJ
1,000,000 J	1,000 kJ
10,000,000 J	10,000 kJ

Questions people ask

How many joules in a kilojoule?

Exactly 1,000.

What is 5,000 J in kJ?

5 kJ.

Why do food labels use kJ?

Because the joule is too small to be readable for food energy, and EU regulations require kilojoules on packaging.