Energy Converter
Convert energy between joules, calories, kilowatt-hours, BTU, and more.
Results update as you type.
How it works
Energy is a fundamental physical quantity, yet the unit used to describe it depends entirely on context. Physics prefers joules, nutrition labels use kilocalories, electricity bills are calculated in kilowatt-hours, heating and cooling equipment is rated in BTUs, and chemistry often works in calories. An energy converter bridges these worlds so a figure from one domain can be understood in another. A frequent source of confusion is the calorie versus the Calorie. When a food label lists 200 calories, it means 200 kilocalories — a thousand times larger than the physical calorie of 4.184 joules. One kilowatt-hour is the energy a 1,000-watt appliance uses in an hour, equal to 3.6 million joules, and a typical US home consumes roughly 900 kWh each month. One BTU is the heat needed to warm a pound of water by one degree Fahrenheit, which makes it convenient for furnaces and air conditioners. Because every unit has a fixed ratio to the joule, this energy converter normalises the input to joules first and then scales to the target, so pairings from electron volts to kilowatt-hours remain accurate. Physicists, dietitians, HVAC technicians, and utility analysts all depend on an energy converter to compare quantities that would otherwise be impossible to line up.
Convert to joules (SI base unit) then to target unit
Energy conversion is exact because each unit represents a fixed quantity of joules, the SI unit of energy. The converter multiplies the input by its joule ratio to reach the shared base, then divides by the target unit's ratio to produce the answer, so no accuracy is lost across scales. Key ratios: 1 kcal = 4,184 J, 1 kWh = 3,600,000 J, 1 BTU = 1,055.06 J, and 1 cal = 4.184 J.
Source:International System of Units (SI) — NIST
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
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Enter your values
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Adjust the options
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Read your result
Converted Energy updates instantly as you type — no button to press.
Frequently asked questions
What is the difference between a calorie and a Calorie (kcal)?
A 'calorie' (lowercase, also called a small calorie or gram calorie) is the energy needed to raise 1 gram of water by 1°C — equal to 4.184 joules. A 'Calorie' (uppercase, also called kilocalorie or kcal) is 1,000 times larger: the energy to raise 1 kilogram of water by 1°C. Food labels use Calories (kcal) — when a candy bar says '250 Calories,' it means 250 kcal = 250,000 small calories = 1,046,000 joules. The inconsistency of the same word referring to two different amounts causes endless confusion.
How is energy related to power?
Power is the rate of energy use: Power = Energy / Time. A watt is 1 joule per second. A kilowatt-hour (kWh) is the energy used by a 1-kilowatt device for 1 hour: 1 kW × 1 hour = 1 kWh = 3,600,000 joules. Your electricity bill charges per kWh. A 100-watt light bulb running for 10 hours uses 1 kWh. Running it for 100 hours costs about $0.12 at average US electricity rates (~$0.12/kWh).
What is a BTU and when is it used?
A BTU (British Thermal Unit) is the energy needed to heat one pound of water by one degree Fahrenheit — equal to 1,055.06 joules. BTUs are used extensively in HVAC (heating, ventilation, and air conditioning) to rate furnace output and air conditioner capacity. A 12,000 BTU/hour air conditioner (1 ton of cooling) can remove 12,000 BTUs of heat per hour. Home furnaces are rated in thousands of BTUs/hour (e.g., 80,000 BTU/hr). The US also uses BTU for gas appliances and natural gas billing.
How much energy is in a gallon of gasoline?
A gallon of regular gasoline contains about 132,000 BTU or 139,000 kJ (139 megajoules) of chemical energy. However, internal combustion engines convert only about 20–40% of this to motion — the rest becomes heat. So a gallon effectively delivers 26,000–53,000 BTU of mechanical energy. An electric vehicle uses about 25–35 kWh per 100 miles; at $0.12/kWh, that's $3–4.20 vs. a gas car at 30 mpg spending about $3.60 at $3/gallon for the same 100 miles.
What is an electron volt (eV) and when is it used?
An electron volt is the kinetic energy gained by an electron accelerated through 1 volt of electric potential: 1 eV = 1.602 × 10⁻¹⁹ joules. It's an extremely small unit used in atomic and particle physics, where joules are too large to be convenient. Visible light photons have energies of 1.8–3.1 eV. Chemical bond energies are typically 1–10 eV. X-ray photons are thousands of eV (keV). Nuclear reactions involve millions of eV (MeV). The Large Hadron Collider accelerates particles to trillions of eV (TeV).