⚡ Energy Converter: Joules, Calories, kWh, BTU and More

What is the Energy Converter?

Energy is the capacity to do work, and although it always measures the same underlying quantity, different fields settled on different units for historical and practical reasons. Physics and engineering use the joule (J), the SI unit, defined as one newton of force acting through one metre. Nutrition uses the calorie or, more precisely, the kilocalorie (the food "Calorie" with a capital C is one kilocalorie). Electricity is billed in kilowatt-hours, heating and cooling equipment is rated in British thermal units, atomic and particle physics counts in electronvolts, and older mechanical work is sometimes quoted in foot-pounds. Because they all describe energy, any one can be converted into any other with a fixed multiplier.

The converter works by anchoring everything to the joule. Each unit has a fixed factor: 1 calorie is 4.184 J (the thermochemical calorie), 1 kilocalorie is 4184 J, 1 watt-hour is 3600 J, 1 kilowatt-hour is 3,600,000 J, 1 BTU is 1055.06 J, 1 electronvolt is about 1.602176634 times ten to the minus nineteen joules, and 1 foot-pound is about 1.35582 J. To convert, the tool multiplies your input by its factor to get joules, then divides by the target factor. Converting calories to joules and joules to calories are simply the two directions of that single relationship, which is why the results never drift apart.

A frequent point of confusion is the small calorie versus the large Calorie. The "calorie" in physics is the energy to raise one gram of water by one degree Celsius (4.184 J). The "Calorie" printed on food packaging is a kilocalorie, one thousand of those, which is why a 2000 Calorie daily diet is really 2000 kilocalories, or roughly 8.4 million joules. This converter treats "calorie" as the small calorie and "kilocalorie" as the food Calorie, so choose the kilocalorie row whenever you are working with nutrition figures.

Energy is often confused with power, but they are not the same thing. Power is the rate at which energy is used, measured in watts, while energy is power multiplied by time. A 100 watt bulb draws 100 watts of power, and left on for one hour it consumes 100 watt-hours of energy. This distinction matters on utility bills, where the kilowatt (power) and the kilowatt-hour (energy) look similar but describe different things. This tool converts between energy units only, so if you start with a power rating you must multiply by the running time before converting.

Scale is worth keeping in mind as you read results. A single joule is a small amount, roughly the energy to lift a small apple one metre. A food Calorie is a few thousand joules, a kilowatt-hour is millions of joules, and an electronvolt is a vanishingly tiny fraction of a joule sized for single atoms and particles. When you convert an everyday number into electronvolts you get an enormous figure, and when you convert an electronvolt into joules you get a tiny one. Both are correct, and the tool uses scientific notation where needed to keep very large and very small numbers readable.

Definitions vary slightly between standards, which is why you may see small differences between references. The calorie has thermochemical (4.184 J), international-table (4.1868 J) and mean variants, and the BTU has similar variants. This converter uses the thermochemical calorie and the IT-based BTU of 1055.06 J, the values most commonly cited in general use. The differences appear only in the third or fourth significant figure, so they rarely matter outside precise laboratory or engineering work.

When to use it

• Converting the kilojoules or kilocalories on a food label into the unit your country prints, since some labels show kJ and others show Calories, and many show both.
• Turning a kilowatt-hour reading from your electricity meter into joules or BTU so you can compare the running cost of appliances or heating sources.
• Checking physics and chemistry homework where energy is given in joules but the expected answer is in calories, kilojoules or electronvolts.
• Sizing heating and cooling equipment, where output is quoted in BTU or BTU per hour but you want the figure in kilowatts or kilojoules.
• Comparing the energy content of batteries, fuels or foods that are labelled in different units so you can put them on the same scale.
• Converting mechanical work in foot-pounds from an engineering spec into joules or watt-hours for a report or datasheet.

How to use the Energy Converter

1. Find the unit you already have a value for among the nine input boxes.
2. Type or paste the number into that box.
3. Read the converted value from every other box, all updated at the same time.
4. For nutrition figures, use the kilocalorie box (the food Calorie), not the small calorie box.
5. Clear the box or type a new number to start a fresh conversion.

Formula & method

Worked examples

Energy units expressed in joules (the conversion factors used by this tool)

Handy everyday energy conversions

Rough energy of familiar things, for a sense of scale

Common mistakes to avoid

• Mixing up the calorie and the food Calorie. The "calorie" in science is the small calorie (4.184 J), while the "Calorie" on nutrition labels is a kilocalorie, one thousand times larger. Use the kilocalorie row for diet and food figures, not the calorie row, or your numbers will be off by a factor of 1000.
• Confusing watts with watt-hours. A watt is power (a rate), while a watt-hour is energy (power multiplied by time). A 100 watt bulb running for one hour uses 100 watt-hours. This tool converts energy units, so convert power to energy by multiplying by the run time first.
• Assuming one fixed value for the calorie or BTU. There are slightly different definitions (thermochemical, international table, mean). This tool uses the thermochemical calorie of 4.184 J and the BTU of 1055.06 J, which are the common choices. Other references may differ in the third or fourth significant figure.
• Reading huge electronvolt results as an error. Because one electronvolt is a tiny fraction of a joule, even a small joule value becomes an enormous number of electronvolts. That is expected, not a bug. The electronvolt is sized for single atoms and particles.
• Confusing BTU with BTU per hour. Heating and cooling gear is often rated in BTU per hour, which is a power (a rate), not a total energy. This tool converts BTU as an amount of energy. If your figure is BTU per hour, first decide over how many hours it runs before treating it as energy.
• Dropping the kilo prefix. Kilojoules and joules, or kilocalories and calories, differ by a factor of 1000. Typing a kJ value into the joule box, or a kcal value into the calorie box, produces answers that are 1000 times too small. Match the prefix on your source number to the box you type into.

Glossary

Joule (J)

The SI unit of energy, equal to one newton of force acting over one metre. Every other unit here is defined relative to it.

Kilojoule (kJ)

One thousand joules. Widely used on food labels outside the United States and in chemistry. 1 kJ = 1000 J.

Calorie (cal)

The small calorie, roughly the energy to warm one gram of water by one degree Celsius. This tool uses 4.184 J per calorie.

Kilocalorie (kcal)

One thousand calories, equal to the food "Calorie" with a capital C used on nutrition labels. 1 kcal = 4184 J.

Watt-hour (Wh)

The energy of one watt of power sustained for one hour. Common for batteries and small devices. 1 Wh = 3600 J.

Kilowatt-hour (kWh)

The energy of one kilowatt of power sustained for one hour, the unit electricity is billed in. 1 kWh = 3,600,000 J.

BTU

British thermal unit, the energy to raise one pound of water by one degree Fahrenheit. Common in heating and cooling. 1 BTU = 1055.06 J.

Electronvolt (eV)

The energy gained by an electron moving through a one volt potential difference, used in atomic and particle physics. About 1.602 × 10⁻¹⁹ J.

Foot-pound (ft·lb)

The work done by a force of one pound acting through one foot, used in mechanical engineering. About 1.35582 J.

Frequently asked questions