Energy Converter
Convert energy between joules, calories, kWh, BTU and more.
Results update as you type.
About this calculator
An energy converter changes a quantity of energy between joules and kilojoules, calories and kilocalories (the food Calorie), watt-hours and kilowatt-hours, the BTU, the foot-pound and the electronvolt. Every value passes through the joule as a common base.
The conversions people reach for most are 1 food Calorie (kcal) = 4,184 joules, 1 kilowatt-hour = 3,600,000 joules and 1 BTU ≈ 1,055 joules. Because a food Calorie is really a kilocalorie, a "200 Calorie" label means 200 kcal, or about 837 kJ.
Worked example: to convert 200 kilocalories to kilojoules, multiply by 4.184, giving about 837 kJ. To convert 1 kWh to megajoules, note it is 1,000 W sustained for 3,600 s, so 3.6 MJ. A results table shows the same energy in every unit at once, useful for nutrition labels, utility bills and physics homework.
Frequently asked questions
How many joules are in a food Calorie?
A food Calorie is a kilocalorie, which equals 4,184 joules (4.184 kJ). So a 200 Calorie snack holds about 837 kJ of energy.
How many joules in a kilowatt-hour?
One kilowatt-hour is 3.6 million joules (3.6 MJ), because it is 1,000 watts sustained for 3,600 seconds. It is the unit your electricity bill is measured in.
What is a BTU?
A British Thermal Unit is the energy to raise one pound of water by 1 °F, about 1,055 joules. It is common in heating and air-conditioning ratings.
How many calories are in a kilojoule?
One kilojoule is about 0.239 food Calories (kcal). So a 2,000 kcal daily intake is roughly 8,368 kJ, the figure used on European nutrition labels.
What is the difference between a calorie and a Calorie?
A small calorie (cal) is 4.184 joules, while a food Calorie with a capital C is a kilocalorie — 1,000 small calories, or 4,184 joules. Nutrition labels always mean the large one.
How do I convert kWh to joules with this energy converter?
This energy converter multiplies kilowatt-hours by 3,600,000 to get joules, because one kWh is 1,000 watts for 3,600 seconds. So 2 kWh is 7.2 million joules.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Anything you omit uses the same default this page is pre-filled with; an unknown parameter is a 400, never a silent zero. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/energy/curl
curl "https://calculator.free/api/v1/energy/?amount=1&from=kcal&to=kj"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/energy/?" + new URLSearchParams({
"amount": "1",
"from": "kcal",
"to": "kj"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.