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Energy Converter

Convert joules, calories, kcal, kWh, BTU, eV and more

Privacy: your files never leave your device. All processing happens locally in your browser.

How to use

  1. 1.Type the amount you want to convert into the energy value field.
  2. 2.Choose the unit you are converting from and the unit you want the answer in (for example kcal to kJ, or calories to joules).
  3. 3.Read the converted result instantly, or tick 'Show all units at once' to see joules, calories, kWh, BTU, eV and therms together.

About Energy Converter

An energy converter turns any amount of energy between joules (J), kilojoules (kJ), calories (cal), kilocalories (kcal), watt hours (Wh), kilowatt hours (kWh), British thermal units (BTU), foot pounds-force (ft·lbf), electronvolts (eV) and therms — instantly and entirely in your browser, with nothing uploaded. Every unit is converted through one common base, the joule (J), the SI unit of energy: your value is first turned into joules, then divided into the target unit, so every pairing stays consistent.

The most common conversion people need is calories to joules. This tool uses the thermochemical calorie, which is defined exactly as 1 cal = 4.184 J, so 1 kcal = 4184 J = 4.184 kJ. Be careful here, because 'calorie' is ambiguous. There are two definitions: the thermochemical calorie (4.184 J, used in chemistry and physics, and what this converter uses) and the International Table calorie (4.1868 J, used in some steam and engineering tables). They differ by only about 0.067%, but for precise work you should know which one a source intends. This converter states its choice up front: thermochemical.

The bigger trap is the food Calorie. On nutrition labels, the capital-C 'Calorie' is actually a kilocalorie (kcal) — one thousand small calories. So a snack listed as '200 Calories' really holds 200 kcal = 200,000 cal = 836,800 J = about 0.233 kWh of chemical energy. Mixing up the small calorie and the food Calorie is one of the most frequent unit mistakes, which is why this tool labels kcal as the 'food Calorie' and keeps cal and kcal as separate units.

Electrical energy is where kilowatt hours matter: 1 Wh = 3600 J because one watt sustained for one hour (3600 seconds) delivers 3600 joules, and 1 kWh = 3,600,000 J. That is why 1 kWh ≈ 860 kcal — a single unit of electricity carries a surprising amount of energy. Heating and gas bills often use the BTU (International Table, 1 BTU = 1055.05585262 J) and the US therm (1 therm = 105,480,400 J ≈ 100,000 BTU). Mechanical work sometimes appears as the foot pound-force (1 ft·lbf = 1.3558179483314 J), and atomic and particle physics use the electronvolt (1 eV = 1.602176634 × 10⁻¹⁹ J, fixed exactly by the 2019 SI redefinition of the elementary charge).

Because these units span an enormous range — from the sub-attojoule electronvolt to the multi-megajoule kilowatt hour, roughly 25 orders of magnitude — the results switch to scientific (exponential) notation at the extremes so no significant figures are lost. Enter any number, choose your source and target units, and read the answer immediately; tick 'Show all units at once' to see joules through therms side by side. Everything runs client-side, so your values never leave your device.

Frequently asked questions

How many joules are in a calorie?
This tool uses the thermochemical calorie, so 1 cal = 4.184 J exactly, and 1 kcal = 4184 J = 4.184 kJ. To convert calories to joules, multiply by 4.184; to convert kcal to kJ, multiply by 4.184 as well since both scale by 1000.
What is the difference between the thermochemical and International Table calorie?
The thermochemical calorie is defined as exactly 4.184 J and is used in chemistry and physics. The International Table (IT) calorie is 4.1868 J and appears in some steam and engineering tables. They differ by about 0.067%. This converter uses the thermochemical calorie.
Is a food Calorie the same as a calorie?
No. The capital-C 'Calorie' on nutrition labels is a kilocalorie (kcal) — 1000 small calories. So a '200 Calorie' food actually contains 200 kcal = 200,000 cal = 836,800 J. In this tool, use the kcal (food Calorie) unit for nutrition values.
How many joules are in a kWh?
1 kilowatt hour equals 3,600,000 joules (3.6 × 10⁶ J), because one kilowatt sustained for one hour of 3600 seconds delivers that much energy. That also works out to about 860 kcal or 3412 BTU, showing how much energy a single unit of electricity carries.
What is an electronvolt (eV) in joules?
1 eV = 1.602176634 × 10⁻¹⁹ J, a value fixed exactly by the 2019 SI redefinition of the elementary charge. Because it is roughly 25 orders of magnitude smaller than a kWh, the converter uses scientific notation for eV so no significant figures are lost.

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