The word calorie covers three values in this energy converter: 4.184, 4.1868 and 4,186.8 joules. This energy converter lists 54 units, from the electron-volt to the gigaton, and keeps those three calories apart.
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Shaded rows use the updated value of the panel on older tables. A red mark on the left shows units that share one value.
Definition: the Joule, the Calorie, the Foot-Pound and 54 Units
The joule is the SI unit of energy: the amount of energy transferred to an object when a force of one newton acts on it in the direction of its motion over a distance of one meter. The kilojoule is 1,000 joules, and the newton-meter is sometimes used as a unit of work or energy equal to the joule. A calorie, or small calorie, is the amount of energy required to raise the temperature of one gram of water by one degree Celsius. A kilocalorie is 1,000 calories or 4.184 kilojoules, is also called a large calorie or food calorie, and is sometimes written Calorie, which is ambiguous. A foot-pound is the energy transferred when a force of one pound-force is applied over a displacement of one foot.
Around those anchors the converter lists the SI-prefixed joules from the attojoule to the gigajoule, watt-hours from the watt-second to the gigawatt-hour, calories of three kinds, British thermal units and therms, electron-volts, the erg and the dyne centimeter, units built on the gram-force, kilogram-force, pound-force, ounce-force and poundal, horsepower hours, the refrigeration ton-hour, tons of explosives from the ton to the gigaton, fuel oil equivalents, and two atomic units, the hartree and the rydberg. Nine groups of units share one value, such as joule, watt-second and newton meter, or erg and dyne centimeter, which makes 44 distinct values behind 54 names.
History and Origin
The kilojoule, like other SI derived units, uses an SI prefix, here kilo for a multiple of 1,000. The kilocalorie shares its origin with the small calorie, except that it is defined in terms of the kilogram rather than the gram. The term comes from the Latin “calor”, meaning heat, and the calorie was first defined as a unit of heat energy in 1824 by Nicolas Clément. The kilocalorie was not introduced to the American public until 1887, by Wilbur Olin Atwater.
The foot-pound is an English Engineering unit that is part of the British Gravitational system, which is based on the foot-pound-second system, built from the foot for length, the avoirdupois pound for mass or force and the second for time. The use of the newton-meter as a unit of work or energy comes from the fact that the joule is dimensionally equivalent to it.
Current Use, and Three Calories
In countries that have adopted the SI, the kilojoule is widely used as a unit of food energy. Some labels show both kilojoules and kilocalories, although in countries such as the United States only kilocalories are shown, often called Calories. Kilojoules are also used in scientific contexts around the world. The kilocalorie is generally considered obsolete, but it is still widely used alongside the kilojoule for food energy, and it is sometimes seen in chemistry.
The foot-pound is used in countries that have not adopted the SI, and the United States is one of the largest, although even there the joule is preferred in most scientific contexts. The newton-meter is more commonly used as a unit of torque than of energy, and using it for energy is discouraged to avoid confusion about which is meant.
A kj to kcal conversion multiplies by 0.2390057361, so 15 kJ is 3.5850860421 thermochemical kilocalories, and a kcal to kj conversion multiplies by 4.184. A cal to j conversion also multiplies by 4.184 for the thermochemical calorie, while the IT calorie is 4.1868 joules. A ft lb to nm conversion multiplies by 1.3558179483, so 15 foot-pounds is about 20.33726922 newton-meters.
Where Older Tables Are Out of Date
The 2022 CODATA adjustment gives the electron-volt as exactly 1.602176634 × 10-19 joules and the Hartree energy as 4.3597447222060 × 10-18 joules. The rydberg energy is half of the Hartree energy, since the Hartree energy is twice the product of the Planck constant, the speed of light and the Rydberg constant. A kilogram-force is exactly 9.80665 newtons, so a kilogram-force meter is exactly 9.80665 joules, as a power conversion chart shows with 9.80665 watts for the kilogram-force meter per second. A mechanical horsepower is 745.69987158227022 watts, which makes a horsepower hour 2,684,519.5376962 joules.
| Unit | Older value, in J | Value used here, in J |
|---|---|---|
| electron-volt and its multiples | 1.6021766339999 × 10-19 | 1.602176634 × 10-19 |
| kilogram-force meter and kilopond meter | 9.8066499997 | 9.80665 |
| horsepower hour | 2,684,519.5368856 | 2,684,519.5376962 |
| Hartree energy | 4.3597482 × 10-18 | 4.3597447222060 × 10-18 |
| Rydberg constant | 2.1798741 × 10-18 | 2.1798723611030 × 10-18 |
Frequently Asked Questions
How many joules are in a calorie?
4.184 for the thermochemical calorie and 4.1868 for the IT calorie.
How many kilojoules are in a kilocalorie?
4.184 for the thermochemical kilocalorie, and 4.1868 for the IT kilocalorie.
How many joules are in a kilowatt-hour?
3,600,000, since a watt-hour is 3,600 joules.
How many newton-meters are in a foot-pound?
1.3558179483.
Is a newton-meter the same as a joule?
In value, yes, but the newton-meter is more commonly used for torque, and using it for energy is discouraged.
How many joules are in an electron-volt?
Exactly 1.602176634 × 10-19.
Method and sources. Every factor was checked against its exact definition with 50-digit arithmetic. The corrections rest on the 2022 CODATA paper and on secondary pages (an online conversion page and a power chart), not on primary standards pages.
This converter is provided for general reference. For nutrition, engineering, energy billing or scientific work, use the unit and the standard the document specifies. Written by Staff Editorial, NumeraHub.