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Energy Converter

Joules, calories, kWh, BTU

By Guga — BrowserTools maintainer, Tbilisi, Georgia Updated 2026-09-06
Free Private No uploads
What is Energy Converter?

Energy units differ less in size than in origin, which is exactly where the mistakes happen. A kilocalorie is a thousand small calories, and a food label that says 250 kcal is talking about a dietary Calorie with a capital C — the same thing. One kilocalorie is therefore 4184 joules, so that label figure is 1 046 000 J. The calorie used here is the thermochemical one at exactly 4.184 J, and the BTU is the international table BTU at 1055.05585262 J; other countries' BTU definitions differ slightly and are not carried here. An electronvolt is 1.602176634 × 10⁻¹⁹ J, exact by SI definition but small enough that the display switches to scientific notation. Eight units in total, live in both directions, with everything shown at once below.

Convert energy between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours and BTU. Great for physics homework, nutrition labels and electricity bills.

Private Energy Converter In Your Browser

Energy Converter is processed locally with client-side JavaScript. Your files never touch a server, and there is no queue to wait in.

Step-by-Step

How to use Energy Converter

1

Enter the quantity in the Value box.

2

Set From unit to the unit the figure is currently in.

3

Set To unit to whichever scale the result should be shown in, or press swap to invert the pair.

4

Scan the All energy units grid, which shows the same quantity in all eight units at once.

Key Capabilities
  • Eight units from the joule to the electronvolt
  • Thermochemical calorie at 4.184 J, so 1 kcal is exactly 4184 J
  • BTU held at the international table value of 1055.05585262 J
  • All eight energy units shown live beneath the converter
Common Use Cases
  • A dietitian converting 1800 kJ from a nutrition label into kilocalories for a meal plan.
  • A physics student working through a joule-to-calorie problem in a unit on thermal energy.
  • An engineer comparing a boiler's annual output in kWh against a BTU figure from an older specification.
Clear Answers

Frequently Asked Questions

What is the difference between a calorie and a kilocalorie?
A factor of a thousand, and the capital letter matters. A small calorie is 4.184 J; the Calorie printed on food packaging is really a kilocalorie, so 250 kcal is 1 046 000 J. A day's 2000 kcal is two million joules, which is why calorie counts look implausibly small next to physics figures.
Why does the electronvolt show in scientific notation?
Because it is small. One electronvolt is 1.602176634 × 10⁻¹⁹ J, and anything under a millionth switches the display to notation with seven significant figures — 1.602177e-19. Converting a joule into electronvolts gives 6.241509e+18 for the same reason. Both are exact values; only the printed precision is reduced.
Is one BTU the same everywhere?
Not quite. This table uses the international table BTU at exactly 1055.05585262 J. The thermochemical and US thermochemical definitions give 1054.35 and 1054.68 J, a spread of about 0.07%. If a document specifies which BTU it means, that specification wins.
Why is a kilowatt-hour listed as an energy unit?
Because a watt-hour is genuinely an amount of energy rather than a rate: one kilowatt-hour is exactly 3 600 000 joules, while a kilowatt is power in joules per second. Household bills are metered in kilowatt-hours, which is why the unit appears on utility documents and physics exams alike.