Energy Converter

Convert between joules, calories, kilowatt-hours, BTU, and more.

Value

From

Result

4.184

Kilojoules (kJ)

Common conversions

Conversion	Result
1 kcal → kJ	4.184
1 kWh → kJ	3600
1 BTU → cal	252.16539197
1 kJ → BTU	0.94781339
1 Wh → J	3600
1 kcal → Wh	1.16222222

Energy units explained

Energy is measured in many units depending on the discipline. Scientists use joules (SI base unit) and kilojoules. Nutritionists use kilocalories (kcal), often labelled "Calories" on food packaging. Electricians use watt-hours and kilowatt-hours (kWh) for consumption billing. Engineers in the US use BTU (British Thermal Units) for HVAC and heating calculations, while mechanical engineers use foot-pounds. This converter routes every value through joules, so all units stay consistent. All conversions run entirely in your browser — no data leaves your device.

Built and maintained by Meet Shah · Last updated 19 July 2026

What this tool is used for

Comparing an appliance's kilowatt-hour figure against a joule-based specification.
Converting a food energy figure between calories and kilojoules.
Reading a heating output in BTU against a metric requirement.
Converting a battery capacity figure into consistent units.
Comparing two energy figures quoted in different systems.

Frequently Asked Questions

Which calorie does it use?: The thermochemical calorie, 4.184 J exactly. That matters because at least three definitions exist and they differ in the third decimal — the 15 °C calorie is 4.1855 J and the International Steam Table calorie is 4.1868 J. Nutrition labelling uses the thermochemical one.
Is a food Calorie the same as a calorie?: No — the one on a food label is a kilocalorie, 1000 of the physics calories, and a capital C is the only thing distinguishing them in print. A 500 Calorie meal is 500 kcal, about 2.1 megajoules, which is why European labels print kJ alongside it.
How does a kilowatt-hour relate to a joule?: 1 kWh is exactly 3,600,000 J, because a watt is a joule per second and an hour is 3600 seconds. It is a unit of ENERGY despite sounding like a rate — the hour cancels the per-second inside the watt, which is the single most useful thing to understand on an electricity bill.
Why is the BTU 1055.06 J and not a round number?: Because it is defined by a physical effect — the heat to raise one pound of water by one degree Fahrenheit — which depends on the starting temperature. 1055.06 J is the ISO value; the thermochemical and 39 °F variants differ by about 0.5%, real money on an HVAC quote.
What is an electronvolt for?: Atomic and particle physics, where a joule is absurdly large: 1 eV is 1.602 × 10⁻¹⁹ J, the energy an electron gains crossing one volt. A visible photon is roughly 2–3 eV and chemical bonds are a few eV, so the unit exists to avoid writing nineteen leading zeros.

Common errors and gotchas

Confusing the calorie with the kilocalorie, a factor of a thousand that food labelling routinely blurs.
Mixing energy with power, where a watt is a rate and a joule is a quantity.
Assuming a kilowatt-hour is a unit of power. It is energy — a kilowatt sustained for an hour.
Using the wrong BTU definition, since several slightly different ones are in use.
Comparing a fuel's energy content against delivered energy without accounting for efficiency.