Energy Converter
Convert energy between joules, kilojoules, calories, kilocalories, watt-hours, kWh, BTU and foot-pounds, with a reference table.
About the Energy Converter
This energy converter moves between the units used in physics, nutrition and utility bills: joules, kilojoules, calories, food Calories (kcal), watt-hours, kilowatt-hours, British thermal units, foot-pounds and electronvolts. Type a value, pick your units, and see the result instantly.
Energy takes many guises — the joule is the SI base, food labels use kilocalories (1 kcal = 4184 J), electricity is billed in kilowatt-hours (1 kWh = 3.6 million J), and heating and cooling systems often quote BTU. Having them all in one converter makes cross-domain comparisons easy.
All conversions use exact factors through a joule base unit and run locally in your browser for fast, accurate results.
How to Use the Energy Converter
- 1Enter the energy value to convert.
- 2Select the source and target units.
- 3Read the result and reverse conversion instantly.
- 4Use the reference table for common amounts.
Frequently Asked Questions
How many joules are in a food Calorie?
A food Calorie (with a capital C) is a kilocalorie, equal to 1000 small calories or 4184 joules. So a 250-Calorie snack contains about 1,046,000 joules. Nutrition labels outside the US often list energy in kilojoules directly — 250 kcal is about 1046 kJ.
How do I convert kWh to joules?
One kilowatt-hour is 3,600,000 joules (3.6 megajoules), because it's 1000 watts sustained for 3600 seconds. So a 5 kWh daily usage equals 18 million joules. This converter handles the factor automatically in both directions.
What is a BTU and how does it relate to joules?
A British thermal unit is the energy to raise one pound of water by 1 °F, about 1055 joules. It's widely used for heating, air conditioning and gas appliances. Air-conditioner capacity is often given in BTU per hour, which is a power (energy per time) rather than energy itself.
What's the difference between a calorie and a Calorie?
A lowercase calorie is the energy to warm one gram of water by 1 °C — 4.184 joules. A capital Calorie, used on food labels, is 1000 of those (a kilocalorie). The 1000× difference matters: your daily 2000 'Calories' is 2000 kcal, or about 8.4 million joules.
Why include electronvolts?
The electronvolt (1.602×10⁻¹⁹ J) is the natural energy unit in atomic and particle physics, where joules are impractically large. It's included for students and researchers who need to bridge between everyday energy units and the sub-atomic scale.