Calories or kilocalories?

What appears as "Calories" (Cal, capital C) on food labels is actually kilocalories (kcal, 1000 gram-calories). One kcal = 4184 J = 4.184 kJ. One gram-calorie (cal, lower c) = 4.184 J, used only in scientific contexts. Official nutrition labels in the EU must show both kcal and kJ.

What is one kWh in "everyday" terms?

One kWh = 3.6 MJ = 860 kcal (food calories). Enough to: boil about 10 litres of water from 20°C to 100°C, drive an EV about 6 km, run a 10 W LED for 100 hours, or run a heat pump (COP 3) for an hour and get 3 kWh of heat.

British Thermal Unit, the energy to heat one pound of water by 1°F. One BTU = 1055 J ≈ 0.293 Wh. Mainly used in the US and UK oil industry. Air conditioners are marketed in "BTU/hour", typically 5000–24,000 BTU/h. 12,000 BTU/h ≈ 3.5 kW, a "ton" of cooling.

What is an electronvolt?

One electronvolt (eV) is the energy an electron gains when accelerated through 1 V of potential difference. 1 eV = 1.602·10⁻¹⁹ J. Used in atomic and particle physics where joules are "too big". Visible light has about 1.65–3.26 eV per photon, X-rays 100 eV–100 keV, gamma rays MeV to TeV. The LHC produces particles at 13 TeV = 13·10¹² eV.

Energy Converter - Joules, kWh, calories

Unit	Joule (J)	Kilojoule (kJ)	kWh	kcal
1 J	1	0.001	2.77778e-7	0.000239
1 kJ	1000	1	0.000278	0.239006
1 kWh	3 600 000	3600	1	860.421
1 cal	4.184	0.004184	1.16222e-6	0.001
1 kcal	4184	4.184	0.001162	1

Unit

Joule (J)

Kilojoule (kJ)

kWh

kcal

1 J

0.001

2.77778e-7

0.000239

1 kJ

1000

0.000278

0.239006

1 kWh

3 600 000

3600

860.421

1 cal

4.184

0.004184

1.16222e-6

0.001

1 kcal

4184

4.184

0.001162

How energy conversion is calculated

Energy is the capacity to do work. The official SI unit of energy is the joule (J), which equals the work done by a force of one newton acting through a distance of one meter. Other units like kilowatt-hours are commonly used for electricity, while calories are used to measure energy content in food.

1 kilojoule (kJ) = 1,000 joules (J)
1 kilowatt-hour (kWh) = exactly 3,600,000 joules (J) = 3.6 megajoules (MJ)
1 calorie (cal, thermochemical) = exactly 4.184 joules (J)
1 kilocalorie (kcal / food calorie) = 1,000 calories = 4,184 joules (J)

About this tool

The energy converter maps between joules (J), kilojoules (kJ), kilocalories (kcal), watt-hours (Wh), kilowatt-hours (kWh), megajoules (MJ), electronvolts (eV), BTU and foot-pounds. Useful for diet and calories (kcal ↔ kJ), electricity consumption (kWh), heating (MJ), physics work (J and eV), and US HVAC (BTU). All updates in real time, no server needed.

How to use it

Type a value in any field.
See all other units update in real time.
Switch tabs for diet units (kcal) or physics (eV).
Copy the result with one click.

Examples

Chocolate energy

Input540 kcal per 100 g

Output2260 kJ per 100 g

Monthly electricity use

Input450 kWh

Output1620 MJ = 1.62 GJ

One kWh is the energy a 1000 W device uses in one hour. A Norwegian average household uses 15,000–20,000 kWh per year (including heating).

Physics: photon energy

Input2.5 eV (green light)

Output4.005 · 10⁻¹⁹ J

Common use cases

Diet: convert food energy labels (kcal ↔ kJ).
Electric bills: turning kWh into joules or heat equivalents.
Heating: sizing heat pumps and radiators in MJ or BTU.
Physics work: from macro scale (J) to atomic scale (eV).
US appliances: BTU to watts for HVAC and stoves.

Frequently asked questions

Calories or kilocalories?: What appears as "Calories" (Cal, capital C) on food labels is actually kilocalories (kcal, 1000 gram-calories). One kcal = 4184 J = 4.184 kJ. One gram-calorie (cal, lower c) = 4.184 J, used only in scientific contexts. Official nutrition labels in the EU must show both kcal and kJ.
What is one kWh in "everyday" terms?: One kWh = 3.6 MJ = 860 kcal (food calories). Enough to: boil about 10 litres of water from 20°C to 100°C, drive an EV about 6 km, run a 10 W LED for 100 hours, or run a heat pump (COP 3) for an hour and get 3 kWh of heat.
What is BTU?: British Thermal Unit, the energy to heat one pound of water by 1°F. One BTU = 1055 J ≈ 0.293 Wh. Mainly used in the US and UK oil industry. Air conditioners are marketed in "BTU/hour", typically 5000–24,000 BTU/h. 12,000 BTU/h ≈ 3.5 kW, a "ton" of cooling.
What is an electronvolt?: One electronvolt (eV) is the energy an electron gains when accelerated through 1 V of potential difference. 1 eV = 1.602·10⁻¹⁹ J. Used in atomic and particle physics where joules are "too big". Visible light has about 1.65–3.26 eV per photon, X-rays 100 eV–100 keV, gamma rays MeV to TeV. The LHC produces particles at 13 TeV = 13·10¹² eV.

Technical background

All energy values are converted via joule (J) as the SI base unit. The joule is now (since the 2019 SI revision) anchored via Planck’s constant h = 6.62607015·10⁻³⁴ J·s (exact). Constants: 1 kJ = 1000 J, 1 MJ = 10⁶ J, 1 GJ = 10⁹ J. 1 Wh = 3600 J = 3.6 kJ, 1 kWh = 3.6 MJ, 1 MWh = 3.6 GJ. 1 kcal (thermochemical) = 4184 J (exact), 1 kcal (IT) = 4186.8 J; the difference is negligible for practical use. 1 BTU (IT) = 1055.05585 J (exact), 1 BTU (thermochemical) = 1054.35 J, 1 therm = 100,000 BTU. 1 eV = 1.602176634·10⁻¹⁹ J (exact since 2019). 1 ft·lb = 1.35582 J. Conversions are direct multiplications, with IEEE 754 double precision well beyond even high-energy physics needs. The tool does not automatically map between energy and mass (E = mc²) or energy and power (P = E/t): those are separate operations.

Energy Reference Table