How this calculator works
This converter covers the energy units you meet in nutrition labels, utility bills and heating specifications: joules and their multiples, calories and kilocalories, watt-hours and kilowatt-hours, and British thermal units. Every unit is defined in joules, so any pair converts through one exact ratio.
The calorie used here is the thermochemical calorie (4.184 J), matching the 'Calories' printed on food labels (a food Calorie is one kilocalorie). The BTU used here is the International Table BTU (1,055.05585262 J), the modern engineering standard published by NIST; a slightly different 'thermochemical BTU' also exists but is rarely used outside historical references.
How it converts
result = value × (joules per from-unit) ÷ (joules per to-unit)Exact anchors: 1 cal = 4.184 J · 1 kWh = 3.6 MJ · 1 BTU (IT) = 1,055.05585262 JWorked example: 2,000 kcal to kilojoules
- A 2,000 kilocalorie daily food energy target uses the thermochemical calorie: 1 kcal = 4.184 kJ.
- 2,000 kcal × 4.184 kJ/kcal = 8,368 kJ.
- As a sanity check, converting back: 8,368 kJ ÷ 4.184 kJ/kcal = 2,000 kcal.
Frequently asked questions
Is a food 'Calorie' the same as a chemistry 'calorie'?
A food Calorie (capital C) is one kilocalorie, or 1,000 small calories. Nutrition labels list Calories, so '200 Calories' means 200 kcal, which this converter treats as the kcal unit.
Why does this converter use 4.184 J per calorie instead of 4.1868 J?
4.184 J is the thermochemical calorie, the convention used for food energy and most modern references. 4.1868 J is the International Table calorie, used in some older steam-engineering tables. The difference is under 0.07% and rarely matters outside historical unit reconciliation.
How does a kilowatt-hour relate to my electricity bill?
Utilities bill electricity in kilowatt-hours: the energy delivered by one kilowatt of power sustained for one hour, equal to 3.6 megajoules. A 100 W bulb left on for 10 hours uses 1 kWh.
What BTU value does this calculator use?
The International Table BTU, defined as exactly 1,055.05585262 joules. This is the value used by NIST and most modern HVAC and appliance specifications.
How do I convert calories to joules by hand?
Multiply the calorie value by 4.184, the exact thermochemical conversion factor this page uses (1 cal = 4.184 J exactly). A 2,000 kcal daily food-energy target, for example, converts to 2,000 × 4.184 = 8,368 kJ.