Joules Converter
Convert Joules (J) to any other energy unit. The table below gives the exact figure for each unit, and every conversion is worked from the same engine as the converter.
What joules convert to
One J in every other energy unit.
| Unit | 1 J | Reverse |
|---|---|---|
| Kilojoules | 0.001 kJ | 1,000 J |
| Megajoules | 1e-6 MJ | 1,000,000 J |
| Watt-hours | 2.7778e-4 Wh | 3,600 J |
| Kilowatt-hours | 2.7778e-7 kWh | 3,600,000 J |
| Calories | 0.239006 cal | 4.184 J |
| Kilocalories | 2.3901e-4 kcal | 4,184 J |
| BTU | 9.4782e-4 BTU | 1,055.055853 J |
Multiply a joules figure by the figure in the second column to get the unit in the first, or multiply by the third column to go the other way.
Joules conversion reference
Joules to Kilojoules
One joules is 0.001 kilojoules, and one kilojoules is 1,000 joules.
A kilojoule is 1,000 joules. It is used for food energy in most countries outside the United States, and for small industrial figures.
- 1 J= 0.001 kJ
- 5 J= 0.005 kJ
- 10 J= 0.01 kJ
One kilojoule is exactly 1,000 joules, and the symbol is kJ. It is the joule multiple that food packaging commonly prints, with energy shown in kilojoules beside kilocalories, and chemistry uses it for reaction energies.
Nutrition panels, exercise machines and dietary advice report energy in kilojoules wherever the metric system is used. Materials and fuel data, such as energy content per kilogram, are tabulated in kilojoules as well.
Joules to Megajoules
One joules is 1e-6 megajoules, and one megajoules is 1,000,000 joules.
A megajoule is 1,000 kilojoules, or about 0.278 kilowatt-hours. It is used for the energy content of fuels: a litre of diesel holds roughly 36 megajoules.
- 1 J= 1e-6 MJ
- 5 J= 5e-6 MJ
- 10 J= 1e-5 MJ
One megajoule is 1,000 kilojoules, which is one million joules. Fuels, industrial processes and large energy figures are quoted in megajoules when a count in joules would be unwieldy.
Fuel energy content, blast energy, industrial process heat and large-scale storage are described in megajoules. Engineering calculations for combustion and materials testing also use the unit.
Joules to Watt-hours
One joules is 2.7778e-4 watt-hours, and one watt-hours is 3,600 joules.
A watt-hour is the energy of one watt sustained for one hour — 3,600 joules. It is used for battery and power-bank capacity; a phone battery is typically 10 to 20 watt-hours.
- 1 J= 2.7778e-4 Wh
- 5 J= 0.001389 Wh
- 10 J= 0.002778 Wh
Joules to Kilowatt-hours
One joules is 2.7778e-7 kilowatt-hours, and one kilowatt-hours is 3,600,000 joules.
A kilowatt-hour is 1,000 watt-hours, or 3.6 megajoules. It is the unit on electricity bills, and the 'unit' shown on a prepaid meter.
- 1 J= 2.7778e-7 kWh
- 5 J= 1.3889e-6 kWh
- 10 J= 2.7778e-6 kWh
The kilowatt-hour is the energy of one kilowatt sustained for one hour, and it is exactly 3.6 megajoules. Its symbol is kWh, and it equals 1,000 watt-hours.
Electricity bills, prepaid meter units, solar generation totals and battery or power-bank capacity are all measured in kilowatt-hours. Appliance consumption over time is compared in the same unit.
Joules to Calories
One joules is 0.239006 calories, and one calories is 4.184 joules.
A calorie here is the small calorie of 4.184 joules — the energy needed to raise one gram of water by one degree Celsius. Food labels use the kilocalorie, which is 1,000 of these.
- 1 J= 0.239006 cal
- 5 J= 1.195029 cal
- 10 J= 2.390057 cal
One thermochemical calorie is exactly 4.184 joules, a definition fixed by convention rather than by a water experiment. The food calorie on a nutrition label is a kilocalorie of 4,184 joules.
The small calorie appears in chemistry, physics and heat-capacity problems, where a substance's specific heat is expressed per gram per degree. Nutrition labels use the much larger food calorie instead.
Joules to Kilocalories
One joules is 2.3901e-4 kilocalories, and one kilocalories is 4,184 joules.
A kilocalorie is 1,000 calories, or 4,184 joules. It is the 'calorie' on food labels, so a 2,000-calorie diet means 2,000 kilocalories.
- 1 J= 2.3901e-4 kcal
- 5 J= 0.001195 kcal
- 10 J= 0.00239 kcal
Exactly 1,000 thermochemical calories, or 4,184 joules, make one kilocalorie. A food label that prints calorie, often with a capital letter, means this unit rather than the small calorie of physics.
Food energy on packaging, diet plans, restaurant menus and sports nutrition is stated in kilocalories. Physiology and nutrition research also reports energy expenditure in the same unit.
Joules to BTU
One joules is 9.4782e-4 btu, and one btu is 1,055.055853 joules.
A BTU (British thermal unit) is 1,055.05585262 joules — the energy needed to heat one pound of water by one degree Fahrenheit. It is used for heating and cooling capacity.
- 1 J= 9.4782e-4 BTU
- 5 J= 0.004739 BTU
- 10 J= 0.009478 BTU
What joules are used for
The joule is the SI unit of energy: the work done by one newton over one meter. It is used for small energy amounts in physics and engineering.
Energy in the International System of Units is measured in joules, where one joule is the work done by a force of one newton moving an object one metre. The symbol is J.
Physics and engineering use the joule for mechanical work, heat, radiation and electrical energy, and small appliance labels sometimes state a surge or pulse rating in joules.
Multiples of the joule such as the kilojoule and megajoule are exact powers of one thousand, so no precision is lost. A watt is a rate rather than an amount, so it must be multiplied by time to yield joules.
The usual slip is to quote an electricity meter reading in joules when the meter counts kilowatt-hours. Both name energy, but the joule figure is 3.6 million times the kilowatt-hour figure.
The joule is the base unit, the kilojoule and megajoule are its decimal multiples, and the kilowatt-hour is the accounting unit an electricity meter counts. Joules suit physics and small quantities, while kilowatt-hours suit billing and storage capacity.
All factors are exact definitions, so only the display is rounded.
Energy units are defined against the joule, so every conversion in this dimension is a single exact multiplication by a published number rather than a chain of steps that could accumulate error.
The pair that needs care is the kilowatt-hour, because it looks like a power unit and is not. It is a watt sustained for an hour, which is why an electricity meter counts in kilowatt-hours while a generator is rated in kilowatts: one is an amount, the other is a rate, and only a duration links them.
To check an energy conversion, express both sides in joules or both in kilowatt-hours before comparing. The commonest mistake is treating a food calorie as a calorie when the label means a kilocalorie, which understates the figure a thousandfold.
Tanzania reference points
Real quantities, converted into joules.
One unit of electricity on a Tanzanian prepaid meter
1 kWh = 3,600,000 J
The average domestic rate was about 229.59 TZS per kWh as of December 2025, including taxes and supply. That is an average across domestic customers, not the marginal rate on a bill.
Source: GlobalPetrolPrices — Tanzania electricity prices (compiled from EWURA/TANESCO tariffs) · verified 2026-09-14
Joules conversion tables
Joules to Kilojoules
| Joules (J) | Kilojoules (kJ) |
|---|---|
| 1 | 0.001 |
| 2 | 0.002 |
| 3 | 0.003 |
| 4 | 0.004 |
| 5 | 0.005 |
| 10 | 0.01 |
| 25 | 0.025 |
| 50 | 0.05 |
| 100 | 0.1 |
Joules to Megajoules
| Joules (J) | Megajoules (MJ) |
|---|---|
| 1 | 1e-6 |
| 2 | 2e-6 |
| 3 | 3e-6 |
| 4 | 4e-6 |
| 5 | 5e-6 |
| 10 | 1e-5 |
| 25 | 2.5e-5 |
| 50 | 5e-5 |
| 100 | 1e-4 |
Joules to Watt-hours
| Joules (J) | Watt-hours (Wh) |
|---|---|
| 1 | 2.7778e-4 |
| 2 | 5.5556e-4 |
| 3 | 8.3333e-4 |
| 4 | 0.001111 |
| 5 | 0.001389 |
| 10 | 0.002778 |
| 25 | 0.006944 |
| 50 | 0.013889 |
| 100 | 0.027778 |