Calculators

Energy Converter

Swap joules, calories, kilocalories, watt-hours, kilowatt-hours, and electronvolts accurately.

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About this tool

Energy appears in a bewildering variety of units depending on context: physicists use joules, nutritionists use kilocalories, electricians use kilowatt-hours, and particle physicists use electron-volts. The word "calorie" alone means two completely different things — a value of "250 cal" versus "250 kcal" differs by a factor of 1,000. This converter links seven units through a joule baseline: joules (J), kilojoules (kJ), thermochemical calories (cal), food kilocalories (kcal), watt-hours (Wh), kilowatt-hours (kWh) and electronvolts (eV). Enter a value, pick the From and To units, and the result appears instantly to twelve significant digits. Calories use the thermochemical definition of 4.184 J, so 1 kcal = 4.184 kJ, and 1 kWh = 3.6 MJ. Whether reading a nutrition label, checking an electricity bill, or doing a physics problem, the conversion is a single input away.

Why use it

  • Everyday and Scientific Units: Bridges everyday energy units (kcal, Wh, kWh) and scientific ones (J, kJ, eV) in a single interface, so nutrition, electricity and physics values all convert in one place.

  • Food vs Physics Calorie: Explicitly distinguishes the small calorie (cal = 4.184 J) from the food kilocalorie (kcal = 4,184 J), with clear labels so you never mistake one for the other.

  • Live Conversion: The result updates on every keystroke or unit change with no submit button needed.

  • Twelve Significant Digits: Results keep up to twelve significant digits, and tiny values such as electronvolts switch to scientific notation so they stay readable.

  • Privacy First: All calculations happen locally in your browser. No data leaves your device, and there is no account or tracking.

How to use

  1. Type the energy value you want to convert in the Value field
  2. Choose the source unit in the From dropdown
  3. Choose the target unit in the To dropdown (it starts on Kilowatt-hours)
  4. For nutrition values, choose 'kilocalories (food)' — not the small 'calories (thermo)'
  5. Read the converted value and the From → To label, which update as you type

When it helps

  • Converting a food-label kilocalorie value into kilojoules for a European nutrition database.
  • Translating a monthly electricity consumption in kWh into joules for an energy-density comparison.
  • Understanding the energy in an electron-volt for an introductory atomic physics assignment.
  • Cross-checking whether a nutrition app's kJ display matches the kcal on a packaged product.
  • Comparing the energy stored in a battery (Wh) with a food Calorie for a science class demonstration.

Examples

Food calorie to kJ

Input

250 kilocalories (food) → Kilojoules

Expected result

1046 kJ (0.290555555556 kWh)

Electric bill

Input

1 Kilowatt-hours → calories (thermo)

Expected result

860420.650096 cal (3,600,000 J)

Tiny scale

Input

1 Electronvolts → Joules

Expected result

1.602176634e-19 J

Tips

  • On food labels, '1 Calorie' (capital C) always means 1 kcal = 1,000 small calories — the distinction matters for any calculation.
  • 1 kWh = 3.6 MJ is a fundamental conversion worth memorising for energy-system and electricity-bill math.
  • For BTU-rated appliances, convert to joules here and divide by about 1,055 to get BTU.
  • Power (W) × time (s) = energy (J) — never add watts and joules as if they were the same unit.
  • Electron-volts are convenient for atomic-scale energies because the joule values would require scientific notation with large negative exponents.

Frequently Asked Questions

What is the difference between calorie and Calorie?
A lowercase calorie (cal) is the gram calorie equal to 4.184 joules — a small amount of energy used in physics and chemistry. An uppercase Calorie (Cal or kcal) is the kilocalorie equal to 4,184 joules and is the unit used on food nutrition labels.
How many joules are in 1 kWh?
Exactly 3,600,000 joules (3.6 MJ). One watt equals one joule per second; one kilowatt-hour is 1,000 watts running for 3,600 seconds, giving 3.6 million joules.
Why do food labels use kcal but say 'Calories'?
Food 'Calories' (capital C) have always meant kilocalories, a convention dating to 19th-century nutritional science. The European Union now mandates both kcal and kJ on labels to reduce this ambiguity.
Can I convert BTU or megajoules?
Not directly — the tool offers J, kJ, cal, kcal, Wh, kWh and eV. For megajoules, convert to kJ and divide by 1,000. For BTU, convert to joules and divide by about 1,055.06.
How is energy different from power?
Energy is the capacity to do work, measured in joules or kWh. Power is the rate of energy transfer, measured in watts (joules per second) or kilowatts. Energy = Power × Time. A 1 kW heater running for 1 hour uses 1 kWh of energy.
What's an electron-volt used for?
An electron-volt (eV) is the kinetic energy gained by one electron accelerating through a 1-volt potential difference, equal to 1.602176634 × 10⁻¹⁹ joules. It is the standard unit in atomic, nuclear, and particle physics because joules produce unwieldy small numbers at those scales.
Which calorie definition does the tool use?
The thermochemical calorie, 4.184 J. Some engineering tables use the International Table calorie of 4.1868 J, which gives results about 0.07% different — for example 1 kWh is 860,420.65 thermochemical calories but about 859,845 IT calories.
What's the SI unit of energy?
The joule (J) is the SI unit of energy, defined as one kilogram times metres squared per second squared (kg·m²/s²). It is also equal to one watt-second and one newton-metre.

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Glossary

Joule (J, SI Energy)
The SI derived unit of energy equal to one Newton-metre or one watt-second (kg·m²/s²). All other energy units are defined as exact multiples or fractions of the joule, making it the universal reference for energy conversion.
Calorie (cal)
The gram calorie, equal to exactly 4.184 joules. It represents the energy needed to raise 1 gram of water by 1 °C at standard pressure. Not to be confused with the food Calorie (kilocalorie).
Kilocalorie (kcal / 'food calorie')
Equal to 1,000 gram calories or 4,184 joules. Nutrition labels list energy in kcal, often written as 'Calories' (capital C). The European Union additionally requires kJ on packaged food labels.
Kilowatt-hour (kWh)
A practical energy unit equal to exactly 3,600,000 joules (3.6 MJ). Used universally for electricity billing and battery capacity. One kWh is the energy consumed by a 1,000-watt device running for one hour.
British Thermal Unit (BTU)
A legacy energy unit approximately equal to 1,055.06 joules, defined as the heat needed to raise one pound of water by one degree Fahrenheit. Widely used in US HVAC ratings, boiler capacities, and natural-gas billing.
Electron-volt (eV)
An energy unit equal to the kinetic energy acquired by a single electron accelerating through a 1-volt potential difference (~1.602 × 10⁻¹⁹ joules). The standard unit in atomic, nuclear, and particle physics where joule values are impractically small.
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