KiloJoules to Joules Converter

How many joules are in a kilojoule?

One kilojoule (kJ) is exactly 1,000 joules (J). The prefix kilo means a thousand in the metric system, so the relationship involves no rounding and no approximation. Going the other way, one joule is 0.001 kJ.

joules = kilojoules × 1,000To go from joules to kilojoules, divide by 1,000.

Where each unit is used

The joule is the SI unit of energy and work. It equals the work done when a force of one newton moves something one meter, written 1 J = 1 N·m, and it is also the energy delivered by one watt in one second. Because a joule is small, it suits physics problems, small electronics and mechanical impacts.

The kilojoule is the practical size for everyday heat and chemistry. Food packaging in many countries lists energy in kJ, textbook reaction enthalpies are given in kJ per mole, and heating or fuel figures are often a few thousand kilojoules. Using kJ keeps those numbers short.

How to convert kJ to J

Multiply by 1,000, which is the same as moving the decimal point three places to the right.

• 2.5 kJ × 1,000 = 2,500 J
• 0.075 kJ × 1,000 = 75 J
• 450 kJ × 1,000 = 450,000 J

In the converter the top box holds joules and the second box holds kilojoules. Enter a number in the joule box to see the kilojoule value, or enter a number in the kilojoule box to get the joules. The result is rounded to six decimal places.

Example: food energy

Nutrition labels often show both kilojoules and the older unit, the kilocalorie (kcal, usually just called a Calorie). One kilocalorie is 4.184 kJ. A 2,000 kcal daily intake is therefore 2,000 × 4.184 = 8,368 kJ, which is 8,368,000 J.

A snack labelled 450 kJ is 450,000 J. In kilocalories that is 450 ÷ 4.184 ≈ 107.6 kcal.

Food energy figures here are plain unit conversions, not dietary advice. See the disclaimer for how to treat results from our tools.

kJ to J conversion table

Common mistakes

• Mixing kJ with kcal. The two differ by a factor of 4.184, so a label in kJ must be divided by 4.184 before comparing it with a calorie count.
• Confusing energy with power. Kilojoules measure energy, kilowatts measure power. Use the joules to watts converter or the watts to joules converter when time is involved.
• Writing the symbol in capitals. The correct symbol is kJ, with a lowercase k. Capital K would stand for kelvin.

For the reverse direction on its own page, see the joules to kilojoules converter. More energy and power tools are collected under physics converters.