What Is a Joule?
The complete guide to understanding the joule, the SI unit of energy, work, and heat.
Last updated: 2026-03-15
Definition
A joule (symbol: J) is the SI unit of energy, work, and heat. It is defined as the energy transferred when a force of one newton acts on an object over a distance of one meter in the direction of the force:
1 J = 1 N × 1 m = 1 kg·m²/s²
The joule is also equal to the energy dissipated as heat when one ampere of electric current passes through a resistance of one ohm for one second.
Everyday Examples
| Action | Energy (Joules) |
|---|---|
| Lifting a small apple 1 meter | ~1 J |
| Typing one key on a keyboard | ~0.01 J |
| A heartbeat | ~1-5 J |
| A food Calorie (kcal) | 4,184 J |
| A lightning bolt | ~1,000,000,000 J (1 GJ) |
| 1 kilowatt-hour of electricity | 3,600,000 J |
| Burning 1 liter of gasoline | ~34,000,000 J (34 MJ) |
Relationship to Other Energy Units
| Unit | Joules Equivalent |
|---|---|
| 1 calorie (thermochemical) | 4.184 J |
| 1 food Calorie (kcal) | 4,184 J |
| 1 watt-hour | 3,600 J |
| 1 kilowatt-hour | 3,600,000 J |
| 1 BTU | 1,055.06 J |
| 1 electronvolt | 1.602 × 10-19 J |
History
James Prescott Joule (1818-1889) was an English physicist and brewer who demonstrated that heat and mechanical work are interchangeable forms of energy. His famous paddle-wheel experiment showed that stirring water with a paddle heated it by a measurable amount, directly relating mechanical work to thermal energy. The joule was adopted as the SI unit of energy in 1960.
The Joule in Science and Engineering
Physics
Kinetic energy (KE = ½mv²), potential energy (PE = mgh), and electrical energy all use joules. It unifies mechanical, thermal, and electrical energy under one unit.
Nutrition
Many countries label food energy in kilojoules (kJ) alongside or instead of calories. Australia and much of Europe use kJ as the primary unit on nutrition labels.
Electrical Engineering
Power (watts) is joules per second. A 60W light bulb uses 60 joules of energy every second, or 216,000 joules per hour.
Convert between energy units with our energy converter.