About the Energy Converter

The energy converter translates between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, British thermal units, foot-pounds, and electronvolts. Whether you are reading a food label in kilocalories, an electricity bill in kilowatt-hours, a heating specification in BTUs, or a physics problem in electronvolts, this tool gives you an instant conversion in either direction.

The joule is the SI derived unit of energy, defined as the energy transferred when a force of one newton acts over a distance of one metre. It is a small unit, so practical measurements often use kilojoules (1000 joules) or megajoules. The calorie, originally defined as the energy to heat one gram of water by one degree Celsius, is now defined exactly as 4.184 joules.

A common source of confusion is the difference between the calorie (4.184 joules) and the kilocalorie (4184 joules), which is the unit used on food labels. When a food label says "200 calories", it means 200 kilocalories, or 200000 small calories. This converter handles both explicitly to avoid mistakes.

How It Works

Each unit has a defined relationship to the joule, the SI unit of energy. The converter uses these factors to translate between any two units:

1 J    = 1 J                    (1 N*m, 1 kg*m^2/s^2)
1 kJ   = 1,000 J
1 cal  = 4.184 J                (thermochemical calorie)
1 kcal = 4,184 J               (food Calorie, 1000 calories)
1 Wh   = 3,600 J               (1 watt for 1 hour)
1 kWh  = 3,600,000 J           (1 kilowatt for 1 hour)
1 BTU  = 1,055.05585262 J      (international table BTU)
1 ft-lb = 1.3558179483314 J    (1 pound-force through 1 foot)
1 eV   = 1.602176634e-19 J     (exact, by 2019 SI redefinition)

Every conversion proceeds in two steps: the input is multiplied by the source unit's factor to give an energy in joules, then divided by the target unit's factor to give the result in the target unit. The factors are exact by international agreement, with the only source of error being floating-point rounding.

The kilowatt-hour is the unit used on electricity bills, defined as the energy delivered by one kilowatt of power sustained for one hour. It equals 3.6 million joules. The British thermal unit is the energy needed to heat one pound of water by one degree Fahrenheit, used in HVAC engineering. The electronvolt is the energy gained by an electron moving across one volt of potential, used in particle physics.

Worked Examples

To convert 1 kilocalorie (food calorie) to joules, the calculation is 1 * 4184 = 4184 joules. To convert the same kilocalorie to kilojoules, the calculation is 4184 / 1000 = 4.184 kJ. This is why food labels in some countries list energy in both kilocalories and kilojoules.

To convert 1 kilowatt-hour to joules, the calculation is 1 * 3600000 = 3,600,000 joules. To convert the same kilowatt-hour to BTUs, the calculation is 3600000 / 1055.05585262 = 3412 BTU. This is why air conditioner capacities are sometimes quoted in both kilowatts and BTUs per hour.

To convert 1 electronvolt to joules, the calculation is 1 * 1.602176634e-19 = 0.0000000000000000001602 joules. This is an extremely small amount of energy, which is why particle physicists use the electronvolt rather than the joule when discussing subatomic processes.

When to Use This Tool

Use the energy converter whenever you need to translate between energy units:

Reading food labels that use kilocalories or kilojoules.
Converting electricity bills between kilowatt-hours and joules.
Interpreting heating or cooling specifications in BTUs.
Working with mechanical energy in foot-pounds or joules.
Converting particle physics energies between electronvolts and joules.
Comparing the energy content of fuels in different units.
Teaching the relationships between thermal, mechanical, and electrical energy units.

Limitations & Disclaimer

This converter handles common energy units used in everyday and engineering contexts. It does not convert thermodynamic units such as the therm, the ton of oil equivalent, or the barrel of oil equivalent. Food calories are handled as kilocalories; the distinction between the small and large calorie must be made by the user. For thermodynamic or HVAC work, verify with domain-specific software. See our disclaimer for full details.

Frequently Asked Questions

What is the difference between a calorie and a kilocalorie?

A calorie (lowercase) is 4.184 joules, the energy to heat one gram of water by one degree Celsius. A kilocalorie is 1000 calories, or 4184 joules. Food labels use the kilocalorie but often call it a Calorie (capital C). When a label says 200 calories, it means 200 kilocalories, or about 837 kilojoules.

Why is electricity measured in kilowatt-hours rather than joules?

Because the kilowatt-hour is a more convenient size for everyday consumption. One kilowatt-hour is 3.6 million joules, so a household using 300 kWh per month is using about 1.1 billion joules. The kilowatt-hour also reflects the way electricity is metered: power (kilowatts) multiplied by time (hours).

What is a British thermal unit and where is it used?

A BTU is the energy needed to heat one pound of water by one degree Fahrenheit, approximately 1055 joules. It is used in HVAC engineering, particularly in the United States, to rate air conditioners, furnaces, and water heaters. One BTU per hour is approximately 0.293 watts.

What is an electronvolt and why is it used in physics?

An electronvolt is the energy gained by an electron moving across one volt of potential, approximately 1.6 times 10 to the minus 19 joules. It is used in particle physics because the energies involved in subatomic processes are tiny when expressed in joules. The 2019 SI redefinition made the electronvolt exact.

Are the conversions exact?

Yes, by international agreement. The thermochemical calorie is exactly 4.184 joules, the BTU is exactly 1055.05585262 joules, the foot-pound is exactly 1.3558179483314 joules, and the electronvolt is exactly 1.602176634 times 10 to the minus 19 joules. The only source of error is floating-point rounding.

What is the relationship between energy and power?

Energy is the capacity to do work; power is the rate at which energy is transferred. One watt of power sustained for one second delivers one joule of energy. One kilowatt sustained for one hour delivers one kilowatt-hour, which is 3.6 million joules. The kilowatt-hour is a unit of energy, not power.

Last updated: July 21, 2026 · Author: HT99 Tools Editorial Team · Reviewed by: HT99 Tools Editorial Team