Kilojoules to joules:
multiply by 1,000

How many joules are in a kilojoule?

One kilojoule is exactly 1,000 joules. The prefix kilo means one thousand, so converting kJ to J only means moving the decimal point three places to the right.

The joule is the SI unit of energy. One joule is the energy used by a one watt device in one second, so it is a small amount, roughly what it takes to lift a small apple one metre. Larger amounts such as food energy or heating are normally given in kilojoules.

The kJ to joules formula

A second example uses a food label. A snack that lists 1,500 kJ per 100 g holds 1,500 × 1,000 = 1,500,000 J of energy in each 100 g. A 35 g portion would contain about 525 kJ, which is 525,000 J.

Chart: kJ to joules

For other values, shift the decimal. 7.5 kJ is 7,500 J, and 0.04 kJ is 40 J. For 12 kJ add 10 kJ and 2 kJ, which is 10,000 + 2,000 = 12,000 J.

Where kJ to joules is used

• Food labels: countries such as Australia, New Zealand and across Europe list energy in kJ, often alongside kcal.
• Physics and chemistry classes: formulas such as kinetic energy or heat use joules, so data in kJ must be converted before substituting.
• Heating: the energy to warm water is often given in kJ, while formulas for specific heat use J per kilogram per kelvin.
• Electricity: one kilowatt-hour equals 3,600 kJ, or 3,600,000 J.

kJ, kcal and joules together

Food energy is the place most people meet kilojoules. One kilocalorie (kcal), the unit often called a Calorie, is 4.184 kJ, which is 4,184 J. So a 250 kcal meal is 250 × 4.184 = 1,046 kJ, or 1,046,000 J.

If a label shows only kJ and you want kcal, divide by 4.184. A label showing 1,000 kJ is about 239 kcal. The calculators on this site convert between these units, so you can check both directions.

A short worked check with heating water

Heating 1 kg of water by 50 degrees Celsius takes about 4,180 × 50 = 209,000 J, which is 209 kJ. Going from the kJ answer back to joules, 209 kJ × 1,000 gives 209,000 J, matching the start. Doing the conversion both ways is a simple check that no zero has been dropped along the way.

Common mistakes with kJ and joules

• Dividing instead of multiplying. Joules are the smaller unit, so the joule number is always larger than the kJ number.
• Mixing up kJ and kW. A kilojoule is energy and a kilowatt is power, so the two cannot be converted directly.
• Confusing kJ with kcal on a food label. A kJ number is about four times bigger than the kcal number for the same food.
• Forgetting that capital J is joules, while lower case j is not a valid unit symbol.