Energy Converter

About the Energy Converter

This converts an amount of energy between units — joules, calories, kilowatt-hours and BTU. It's useful for nutrition (food Calories to joules), electricity bills and appliance ratings (kWh), heating and cooling figures (BTU), and physics or engineering values in joules. Choose the units and enter a value for an instant result.

Converting other physical units? See the Temperature Converter or the Pressure Converter.

How it works

Each unit is defined relative to the joule (J), the SI unit; your value converts to joules and then to the target unit. Key definitions:

• 1 kilojoule (kJ) = 1000 J.
• 1 calorie (cal) = 4.184 J; a food Calorie (kcal) = 1000 calories = 4184 J.
• 1 watt-hour (Wh) = 3600 J, so 1 kWh = 3,600,000 J = 3.6 MJ.
• 1 BTU ≈ 1055.06 J (British thermal unit, common in heating/cooling).

So a 500 kcal meal is about 2092 kJ, and 1 kWh is about 860 kcal.

Assumptions and behaviour

• The food "Calorie" (capital C) on nutrition labels is a kilocalorie (kcal) — 1000 small calories. This tool uses that convention for the kcal unit.
• Uses the thermochemical calorie (4.184 J), the value most commonly used for nutrition.
• Results show about 7 significant figures.
• The Swap button reverses the two units.

Limitations

• Watch calorie vs Calorie. The 'calories' on food labels are really kilocalories (kcal). Converting a food value, use the kcal unit, not the small cal — mixing them up is a factor-of-1000 error.
• There are a few slightly different definitions of the calorie and BTU; this tool uses the common thermochemical calorie and the international-table-style BTU, which is fine for everyday use but may differ in the last digits from a specialist definition.
• Displayed values are rounded, so hand-chaining rounded figures can drift slightly.

Privacy

The conversion runs entirely in your browser. Nothing is uploaded or stored.