Energy Converter
Convert between any two joule units in your browser — instant, accurate, and private.
Example
1 J = 0.001 kJ. Switch the unit menus to convert between any pair.
How it works
Every value is converted through a single joule base unit using internationally defined conversion factors, so any from/to pair stays consistent.
Good to know
Energy shows up under a different name in almost every field, which is exactly why a converter that pivots through the joule is so handy. A nutrition label lists food energy in kilocalories (often just called "Calories" with a capital C) or kilojoules, your electricity bill counts kilowatt-hours, a furnace or air conditioner is rated in BTU, mechanical work problems use foot-pounds, and a physicist measuring a single particle reaches for the electronvolt. They all describe the same quantity, so converting between them is the only way to compare a 2,000 kcal daily diet, a 0.5 kWh appliance cycle, and a 10,000 BTU heater on equal footing.
The units split cleanly by origin. The joule, kilojoule, calorie and kilocalorie are metric/SI, with the kilowatt-hour layered on top (1 kWh = 3.6 million joules, because it is one kilowatt sustained for 3,600 seconds). The BTU and foot-pound are imperial heat-and-work units, while the electronvolt is a physics convenience unit equal to about 1.602 x 10⁻¹⁹ J — the energy one electron gains across a one-volt difference. A dietary Calorie is a kilocalorie, never the tiny "small" calorie.
A few rounding shortcuts make mental math easy:
- kilojoules to dietary Calories: divide by about 4.184 (or quick-and-dirty, divide by 4.2), so 2,000 kJ ≈ 478 kcal.
- kilowatt-hours to BTU: multiply by about 3,412, so a 1 kWh space heater puts out roughly 3,400 BTU of heat.
- kilowatt-hours to megajoules: multiply by 3.6.
The classic mistake is the calorie/Calorie mix-up: confusing the small calorie (4.184 J) with the kilocalorie (4,184 J) throws every food calculation off by a factor of 1,000. A second caveat is the BTU itself — several slightly different definitions exist (thermochemical, ISO, 39°F, 59°F), and they disagree by a fraction of a percent, so treat BTU results as accurate to about three or four significant figures rather than as an exact integer.
Frequently asked questions
What does the energy converter do?
It converts between common joule units instantly. Pick a unit to convert from and a unit to convert to, type a value, and the result updates live.
Which units does this converter support?
It includes 9 units: Joule, Kilojoule, Calorie, Kilocalorie, Watt-hour, Kilowatt-hour, BTU, Foot-pound, Electronvolt.
Is this converter free and private?
Yes. It runs entirely in your browser, so your inputs never leave your device, there is no sign-up, and it works offline once loaded.
Are the conversions exact?
Conversions use internationally defined factors and are exact where the definitions are exact (for example, 1 inch = 2.54 cm). Displayed results are rounded for readability.
People also ask
How many joules are in a kilowatt-hour?
One kilowatt-hour equals exactly 3,600,000 joules (3.6 megajoules), because it is one kilowatt (1,000 watts) sustained for 3,600 seconds.
How many BTU are in a kWh?
One kilowatt-hour is about 3,412 BTU. So a 1 kW electric heater running for an hour delivers roughly 3,412 BTU of heat.
What is the difference between a calorie and a Calorie?
A small calorie equals 4.184 joules, while a food Calorie (capital C) is actually a kilocalorie, equal to 1,000 small calories or 4,184 joules. Nutrition labels always mean the kilocalorie.
How do you convert kilojoules to calories on a food label?
Divide kilojoules by 4.184 to get dietary Calories (kilocalories). For example, 1,000 kJ is about 239 kcal.
How many joules is one electronvolt?
One electronvolt equals about 1.602 x 10⁻¹⁹ joules. It is a tiny unit used in atomic and particle physics for the energy of single particles.
How many joules are in one BTU?
One BTU is about 1,055 joules (using the international-table definition). Different BTU definitions vary by under half a percent, so the exact figure ranges from roughly 1,054 to 1,060 J.
How much energy is a foot-pound in joules?
One foot-pound equals about 1.356 joules. It is an imperial unit of work, the energy needed to lift one pound a distance of one foot against gravity.
How many kilojoules are in a kilowatt-hour?
One kilowatt-hour equals 3,600 kilojoules (3.6 megajoules), the same as 3,600,000 joules.
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