Energy Converter
Convert between joules, calories, watt-hours, and BTU.
Quick reference
- 1 food Calorie (kcal) = 4,184 joules
- 1 kilowatt-hour = 3,600,000 joules
- 1 BTU ≈ 1,055 joules
- 1 watt-hour = 3,600 joules
About this converter
Energy shows up in a strangely wide range of units depending on the field: joules in physics, calories on a nutrition label, kilowatt-hours on an electric bill, BTU on an air conditioner box. This converts between all of them.
The food-label "Calorie" is a good example of how these units get muddled. A food Calorie (capital C) is actually 1,000 of the small calories that a chemistry class uses, a quirk that goes back to 19th-century American food science and never got cleaned up. This converter's "cal" option is the small calorie; for a nutrition label, use kcal instead. And if you've ever seen HVAC capacity listed in "tons," that's not a weight at all, it's short for a ton of refrigeration, defined by how much heat it takes to melt one ton of ice in a day, which works out to 12,000 BTU per hour.
Frequently asked questions
- Is a food "Calorie" the same as a science "calorie"?
- No. A food Calorie (capital C, also written kcal) is 1,000 small calories. This converter's "cal" unit is the small calorie; use the kcal option for anything off a nutrition label.
- Why does my AC's capacity get listed in "tons"?
- It's a leftover unit from before mechanical refrigeration, a ton of refrigeration is the cooling effect of melting one ton of ice over 24 hours, which comes out to 12,000 BTU/hr. Nothing to do with weight.
- How does a kilowatt-hour relate to a joule?
- A kilowatt-hour is just power multiplied by time: 1,000 watts running for one hour, which comes out to exactly 3,600,000 joules. It's used for electric bills because the numbers stay reasonably sized; nobody wants a bill in joules.