Energy Unit Converter
Convert an amount of energy between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, electronvolts, ergs, foot-pounds and more. Every factor is the exact NIST definition, the two kinds of calorie and BTU are kept separate, and the converter shows the value in all 18 units at once. Runs entirely in your browser — nothing is uploaded.
A number such as 2000, 0.5 or 1.6e-19. Results are exact, using the NIST definitions of each unit.
2000 kcal (th) = 8368 kJ
8368
Converted value
8.368 × 10³
Scientific notation
1 kcal (th) = 4.184 kJ
Conversion factor
Exact: both units are defined by exact decimal factors, so nothing was rounded.
Your energy in every unit
| Unit | Value |
|---|---|
| joule (J) | 8368000 |
| kilojoule (kJ) | 8368 |
| megajoule (MJ) | 8.368 |
| watt-hour (Wh) | 2324.444444 |
| kilowatt-hour (kWh) | 2.324444444 |
| calorie, thermochemical (cal th) | 2000000 |
| kilocalorie, thermochemical (kcal th) | 2000 |
| calorie, International Table (cal IT) | 1998662.463 |
| kilocalorie, International Table (kcal IT) | 1998.662463 |
| British thermal unit, International Table (BTU IT) | 7931.333663 |
| British thermal unit, thermochemical (BTU th) | 7936.64343 |
| electronvolt (eV) | 5.222894794 × 10²⁵ |
| kiloelectronvolt (keV) | 5.222894794 × 10²² |
| megaelectronvolt (MeV) | 5.222894794 × 10¹⁹ |
| erg | 83680000000000 |
| foot-pound force (ft·lbf) | 6171920.065 |
| litre-atmosphere (L·atm) | 82585.73896 |
| tonne of TNT (t TNT) | 0.002 |
Which calorie? Nutrition tables conventionally use the thermochemical kilocalorie, 4.184 kJ; the International Table calorie is 4.1868 J and is about 0.07% larger. Food-label Calories are kilocalories.
How to Use the Energy Unit Converter
- 1. Enter the amount. Type a number such as 2000, 0.5 or 1.6e-19. A decimal point or comma both work.
- 2. Choose the units. Pick the unit you have and the unit you want. Swap units reverses them.
- 3. Read the result. You get the converted value, the same in scientific notation, the exact conversion factor and a note saying whether the answer is exact or rounded to the number of significant digits you chose.
- 4. Check the table. Below the result is your energy in every unit on the list, which makes it easy to compare a food label in kilojoules with a battery in watt-hours.
How the Units Relate
The SI unit of energy is the joule: the energy transferred by a force of one newton acting over one metre, or one watt for one second. Everything else here is a fixed multiple of it. A kilowatt-hour is 3.6 million joules because an hour has 3600 seconds and a kilowatt is 1000 joules a second. An electronvolt is the energy an electron gains crossing one volt, 1.602176634 × 10⁻¹⁹ J, exactly, since the 2019 redefinition of the SI fixed the elementary charge. An erg, from the old centimetre–gram–second system, is 10⁻⁷ J.
| Unit | Joules | Note |
|---|---|---|
| calorie, thermochemical (cal th) | 4.184 | Used in nutrition tables |
| calorie, International Table (cal IT) | 4.1868 | Used in steam tables and engineering |
| British thermal unit, IT (BTU IT) | 1055.05585262 | Most common in US heating and cooling |
| British thermal unit, thermochemical | 1054.350 | About 0.07% smaller |
| kilowatt-hour (kWh) | 3 600 000 | Electricity billing |
| foot-pound force (ft·lbf) | 1.3558179483314 | From the pound-force and the foot |
| litre-atmosphere (L·atm) | 101.325 | Gas work, from 101 325 Pa |
| tonne of TNT (t TNT) | 4 184 000 000 | A convention, not a measurement |
Which Calorie?
There is more than one calorie, and they differ in the fourth significant figure. The thermochemical calorie is defined as exactly 4.184 J. The International Table calorie, fixed by the Fifth International Conference on the Properties of Steam in 1956, is exactly 4.1868 J, about 0.07% larger. A third, the 15 °C calorie, is roughly 4.1855 J and is not offered here. Nutrition tables conventionally use the thermochemical kilocalorie, so 2000 kcal is 8368 kJ; engineering handbooks that give steam properties use the IT value. If a document does not say which one it means, the thermochemical calorie is the safer reading for food and the IT calorie for engineering, and the difference matters only past the third digit.
The "Calorie" on a food label, sometimes written with a capital C or as Cal, is the kilocalorie: 1000 small calories. In countries that label in kilojoules, the kilocalorie figure is usually given alongside it.
Worked Examples
- 2000 kcal (thermochemical) is exactly 8368 kJ, or 8.368 MJ, or about 2.3244 kWh.
- 1 kWh is 3.6 MJ, or 860.4207 kcal (thermochemical).
- 1 BTU (IT) is 1055.05585262 J, or about 0.2930711 Wh.
- 1 ft·lbf is 1.3558179483314 J, and 1 L·atm is exactly 101.325 J.
- 1 eV is 1.602176634 × 10⁻¹⁹ J, so a 5 MeV particle carries 8.01088317 × 10⁻¹³ J.
Accuracy and Limits
All the factors are definitions, so conversions are exact. The converter works in integer arithmetic rather than floating point and rounds only when it writes the answer, to the number of significant digits you choose. The values are those of the National Institute of Standards and Technology's Guide for the Use of the International System of Units (SP 811, 2008 edition), apart from the electronvolt, which follows the 2019 SI. Energy in joules is not the same as power in watts: for a rate, see the Work and Power Calculator. To combine units with prefixes such as micro or giga, use the Metric Prefix Converter.
Frequently Asked Questions
- How many joules are in a calorie?
- One thermochemical calorie is exactly 4.184 joules, and one International Table calorie is exactly 4.1868 joules. A kilocalorie, the food Calorie, is 1000 of them: 4184 J or 4186.8 J. For nutrition, use the thermochemical value.
- How do I convert kWh to joules?
- Multiply by 3 600 000. One kilowatt-hour is 1000 watts for 3600 seconds, which is 3 600 000 joules or 3.6 megajoules. To go from joules back to kWh, divide by 3 600 000. A 60 W bulb running for an hour uses 0.06 kWh, which is 216 kJ.
- How do I convert electronvolts to joules?
- Multiply by 1.602176634 × 10⁻¹⁹. This is exact: the 2019 redefinition of the SI fixed the elementary charge at that value in coulombs, and one electronvolt is one elementary charge times one volt. To go the other way, divide by the same factor, so 1 J is about 6.241509 × 10¹⁸ eV.
- Why are there two kinds of BTU?
- Like the calorie, the BTU was defined from the heat needed to warm water, and tables built on different assumptions gave slightly different values. The International Table BTU is 1055.05585262 J and the thermochemical BTU is 1054.350 J. Heating and cooling ratings in the US normally mean the IT BTU, and the difference is about 0.07%.
- Is a watt-hour a unit of energy or power?
- Energy. A watt is power, which is energy per second, so a watt-hour is a power multiplied by a time and gives energy: 3600 joules. Battery capacities in Wh and electricity bills in kWh are quantities of energy, while a charger's 20 W rating is power.
- What is a tonne of TNT in joules?
- By convention a tonne of TNT is exactly 4.184 gigajoules, and a kiloton is 4.184 terajoules. The figure is a standard rather than a measurement, because the energy released by real TNT varies; it was chosen so the unit equals a million thermochemical kilocalories.