Measure · 9 units
Energy
Energy, work and heat share one unit, the joule. The calorie comes in two exact versions (4.184 J and 4.1868 J), and the food Calorie is a kilocalorie.
Convert energy
Then and now
Heat was measured as warmth given to water long before it was understood as energy; the calorie and the British thermal unit come from that time.
All energy units
“Exact” means the unit is defined as an exact multiple of the SI unit; the decimal is printed in full when it ends. Rows marked historical or obsolete are kept for reading old documents.
| Unit | Defined as | In SI | Status | Source |
|---|---|---|---|---|
| SI | ||||
| joule J | 1 N · m | 1 J | exact | BIPM SI Brochure |
| kilojoule kJ | 1000 J | 1 000 J | exact | BIPM SI Brochure |
| Electricity | ||||
| kilowatt-hour kWh | 1000 W for 3600 s | 3 600 000 J | exact | BIPM SI Brochure |
| Chemistry | ||||
| calorie (thermochemical) cal | exactly 4.184 J | 4.184 J | exact | NIST SP 811 |
| Engineering | ||||
| calorie (International Table) cal_IT | exactly 4.1868 J | 4.186 8 J | exact | NIST SP 811 |
| British thermal unit (IT) BTU | the IT calorie per gram per kelvin, scaled to the pound and degree Fahrenheit | 1 055.055 852 62 J | exact | NIST SP 811 |
| Nutrition | ||||
| kilocalorie (food Calorie) kcal | 1000 thermochemical calories | 4 184 J | exact Food labels usually use the thermochemical value; some use the International Table value. | NIST SP 811 |
| US engineering | ||||
| foot-pound ft·lbf | 1 pound-force acting through 1 foot | 1.355 817 948 331 400 4 J | exact | NIST SP 811 |
| CGS | ||||
| erg | 1 dyne · cm | 0.000 000 1 J | exact historical | NIST SP 811 |