Energy Converter

It starts on kilocalories to kilojoules, the conversion people most often need for food.

1. Type the quantity into Value. For example, enter 250 for a snack or 1 for a unit of electricity.
2. Under From, choose the unit on your label or bill. Read it closely: cal and kcal are listed separately, and they differ by a factor of one thousand.
3. Under To, choose the unit you want. The converted amount shows straight away, followed by a note such as 1 kcal = 4.184 kJ.
4. Press Swap units to flip the two units.
5. Open the In every unit list to see nine versions of the same energy together, from joules up to megajoules, which helps when sources quote different ones.

The calorie on a nutrition label is not the small calorie of a physics lab. Food labels in most countries mean the kilocalorie (kcal), written Calorie with a capital C in the US, while the lower-case cal in this converter is the thermochemical calorie of 4.184 joules. So 1 kcal is 1,000 cal, or 4.184 kJ. Labels in Australia, New Zealand and the EU often show both kJ and kcal, and the figures should match once converted.

• 2,000 kcal = 8,368 kJ
• 250 kcal = 1,046 kJ
• 100 kJ = 23.9 kcal
• 500 kJ = 119.5 kcal

A handy rule: divide kilojoules by about 4.2 to get kilocalories, or multiply kilocalories by 4.2 to get kilojoules. To find how many kilocalories your own day might need, use the calorie calculator, then divide them between protein, carbohydrate and fat with the macro calculator. The conversion is arithmetic only, and says nothing about whether any intake suits you.

Electricity is sold in kilowatt-hours. One kWh is 3.6 million joules (3.6 MJ), or 860.4 kcal, or 3,412 BTU, and one watt-hour is 3,600 J. Take a 60 W bulb left on for one hour. It uses 60 Wh, which is 0.06 kWh or 216 kJ. A kitchen heater drawing 1 kW for an hour uses 1 kWh, the same energy as about 860 kcal of food.

Heating and cooling add a trap. A British thermal unit is 1,055.06 J, but air conditioners and boilers are rated in BTU per hour, which is power, not energy. A 12,000 BTU/h unit delivers 12,000 BTU in each hour, equal to 3.517 kWh of heat per hour of running. This converter handles energy only, so for a power rating, convert the BTU figure to kWh as above, then multiply by running hours. The time converter helps with minutes and decimal hours.

Foot-pounds appear in torque and ballistics, where 1 ft-lb is 1.3558 J. The tool has no watts, horsepower or efficiency figures, and it cannot tell you what a tariff costs, because that depends on your supplier. Pressure work, such as compressed air, sits with the pressure converter.

If you compare appliances, compare them in the same unit. A heater quoted at 5,000 BTU of heat output and another quoted at 1.5 kWh describe the same quantity once converted: 1.5 kWh is about 5,118 BTU. Doing the conversion first removes the guesswork from the label.

Converting between very different units helps build intuition. A 20 Wh power bank stores 72 kJ, or about 17.2 kcal, roughly the energy of a small biscuit. An electric car battery of 75 kWh holds 270 MJ, which is around 64,500 kcal. A single joule, the base unit, is tiny: it is about 0.000239 kcal.

These comparisons are only about the quantity of energy. They say nothing about how fast it is released, how much a battery can deliver in practice, or what share of food energy the body can use.

The calorie was originally defined by heating water, and that link still gives a feel for the numbers. Warming one litre of water from 20 to 100 degrees Celsius takes about 80 kcal, which is roughly 335 kJ or 0.093 kWh, ignoring losses to the kettle and the air.