Joules to kilojoules:
divide by 1,000

How many kilojoules is a joule?

One joule is 0.001 kilojoules, so converting joules to kJ means dividing by 1,000. Equivalently, 1,000 J equals exactly 1 kJ. The prefix kilo means one thousand, so no special factor is needed.

The joule is the SI unit of energy and is quite small. A 60 W bulb uses 60 joules every second, so you quickly reach thousands of joules in everyday situations, and kilojoules are easier to read.

The joules to kJ formula

A second example uses a bulb. A 60 W bulb uses 60 J each second, which is 3,600 J per minute, or 3.6 kJ. In one hour that is 216,000 J, which is 216 kJ.

Chart: joules to kJ

For other values, shift the decimal point left three places. 750 J is 0.75 kJ, 45 J is 0.045 kJ and 12,000 J is 12 kJ.

Where joules to kJ is used

• Physics problems: kinetic energy, work and heat answers often come out in joules and are reported in kJ once they reach four digits.
• Heating water: raising 1 kg of water by 1 degree Celsius takes about 4,180 J, which is roughly 4.18 kJ.
• Impact and safety: ratings for helmets, fences or crash barriers may be stated in kJ.
• Appliances: the energy used by a device in a short time is often calculated in joules first.

Joules, watts and time

A joule is a watt-second, so joules, watts and time are linked by one formula: energy in joules equals power in watts times time in seconds. A 1,500 W heater running for 20 seconds uses 30,000 J, which is 30 kJ.

To go further, one kilowatt-hour equals 3,600 kJ, so a 2 kW heater running for 30 minutes uses 1 kWh, or 3,600 kJ, which is 3,600,000 J. This is the link between the joules in a physics problem and the kWh on a bill.

A short worked check with a falling weight

A 2 kg mass dropped from a height of 10 metres releases about 2 × 9.8 × 10 = 196 J of energy. Dividing by 1,000 gives 0.196 kJ. If a textbook answer shows 196 kJ, something is a thousand times off. Small answers in joules are normal in classroom problems, and kilojoules only become convenient once you pass four digits.

Common mistakes with joules and kJ

• Multiplying when you should divide. Kilojoules are the bigger unit, so the kJ number is smaller than the joule number.
• Mixing up energy and power. Joules measure energy, while watts measure how fast energy is used.
• Confusing kJ with kcal. One kcal is 4.184 kJ, which is 4,184 J.
• Dropping a zero. 4,500 J is 4.5 kJ, not 0.45 kJ, and a decimal place error is a tenfold mistake.