Energy converter: joules, calories, kilowatt-hours, and BTU

Convert between joules, kilojoules, calories, kilocalories, kilowatt-hours, watt-hours, and BTU at ToolHQ's energy converter, all common energy units in one tool. Free, no account required.

Energy is measured in different units depending on the context. Your electricity bill uses kilowatt-hours. Your air conditioner's capacity is rated in BTU. Food is labeled in Calories (which are actually kilocalories). Physics and chemistry use joules. Converting between these units requires knowing which unit system applies, and the formulas.

ToolHQ's energy converter handles all common energy units and returns equivalents across all of them simultaneously.

Key Takeaways

• 1 kWh = 3,600,000 joules = 860 kcal = 3,412 BTU
• Food "Calories" on nutrition labels are kilocalories, not small calories
• BTU is used for HVAC and heating equipment ratings in the US
• kWh is the standard unit for electricity consumption worldwide
• No data is stored or transmitted, all calculations run locally in your browser

---

Energy unit overview: which unit for which context

Different fields use different energy units, not because they measure different things, but because different scales and historical conventions took root in different industries.

Unit	Abbreviation	Used in
Joule	J	Physics, chemistry, scientific work (SI base unit)
Kilojoule	kJ	Science, food energy (used on nutrition labels outside US)
Calorie (small)	cal	Chemistry (heat to raise 1g water by 1°C)
Kilocalorie / Calorie (large)	kcal or Cal	Food nutrition (US labels say "Calories")
Watt-hour	Wh	Small electrical energy amounts
Kilowatt-hour	kWh	Electricity consumption, billing
British Thermal Unit	BTU	HVAC, heating, air conditioning (US)
Therm	therm	Natural gas billing (1 therm = 100,000 BTU)

All calculations run locally in your browser, no data is stored or transmitted.

---

The calorie confusion: cal vs. Calorie vs. kcal

The single biggest source of confusion in energy units is the calorie:

• 1 small calorie (cal): The energy required to raise 1 gram of water by 1°C. Used in chemistry and physics.
• 1 large Calorie (Cal) = 1 kilocalorie (kcal): The energy required to raise 1 kilogram of water by 1°C. This is what nutrition labels mean by "Calories."
• Food labels always use kilocalories. When a food package says 200 Calories, it means 200 kilocalories (kcal), which equals 200,000 small calories (cal). The capital "C" is supposed to indicate kilocalorie, but this distinction is widely misunderstood.

In practice: When you talk about food energy in everyday life, "calories" means kilocalories. When doing chemistry or scientific calculations, "calorie" means the small calorie (4.184 joules). ToolHQ's energy converter keeps these distinct and lets you convert between both.

---

Energy conversion reference table

Starting unit	Joules (J)	Kilocalories (kcal)	kWh	BTU
1 Joule	1 J	0.000239 kcal	0.000000278 kWh	0.000948 BTU
1 Kilocalorie (kcal)	4,184 J	1 kcal	0.00116 kWh	3.97 BTU
1 Kilowatt-hour (kWh)	3,600,000 J	860 kcal	1 kWh	3,412 BTU
1 BTU	1,055 J	0.252 kcal	0.000293 kWh	1 BTU
1 Kilojoule (kJ)	1,000 J	0.239 kcal	0.000278 kWh	0.948 BTU

Key reference values:

• 1 kWh = 3,600,000 J = 3,600 kJ = 860 kcal = 3,412 BTU
• 1 kcal = 4.184 kJ = 4,184 J = 0.00116 kWh = 3.97 BTU
• 1 therm = 100,000 BTU = 29.3 kWh = 105.5 MJ

Mini-story 1: Sophie was a nutrition student trying to understand how much physical energy a 2,000-calorie daily diet represented. She knew 2,000 food Calories meant 2,000 kcal, but wanted to know what that was in kilowatt-hours, just to have a sense of scale. She entered 2,000 kcal into ToolHQ's energy converter and saw the result: 2.326 kWh. That was roughly what a standard electric kettle uses in about two hours of boiling water. The comparison made the energy scale feel concrete: an entire day's food energy is roughly equivalent to running a kettle for two hours.

Convert energy units free at ToolHQ

---

How to use the ToolHQ energy converter

1. Go to the tool. Navigate to ToolHQ's energy converter. No account or sign-up required.
2. Enter your value. Type any number in the input field.
3. Select your source unit. Choose from joules, kilojoules, calories (small), kilocalories, watt-hours, kilowatt-hours, or BTU.
4. Read all results. The converter returns the equivalent in all supported units simultaneously.

---

When energy unit conversions come up in practice

Electricity billing: Your utility bill charges per kWh. If you want to know how many BTU a device uses, or how many joules your monthly usage represents, you convert from kWh.

HVAC and heating: Air conditioners, heaters, and heat pumps are rated in BTU per hour in the US. A 12,000 BTU/hr air conditioner is called a "1-ton" unit. To compare with a device rated in kW, you convert BTU to watts (1 BTU/hr = 0.293 watts).

Food and nutrition: Nutrition labels in the US list Calories (= kcal). Labels in Europe, Australia, and most of the world list kilojoules (kJ) alongside kilocalories. To convert from kJ to kcal, divide by 4.184.

Physics and chemistry: Joules are the SI unit of energy. Laboratory calculations, thermodynamics, and chemistry use joules or kilojoules. Converting from food energy or electrical energy to joules lets you work within a single unit system.

For power (energy per unit time), ToolHQ's power converter converts between watts, kilowatts, horsepower, and BTU/hour.

Mini-story 2: Daniel was comparing two gas-fired boilers for his home. One listed its output in kW (24 kW) and the other in BTU/hour (82,000 BTU/hr). He needed to know if they were equivalent. He entered 82,000 BTU into ToolHQ's energy converter and then divided by 3,412 (the BTU per kWh) to get approximately 24 kWh per hour, or 24 kW. The two boilers had nearly identical output. He also converted 24 kWh to megajoules (86.4 MJ) to check against the manufacturer's European spec sheet, which listed output in MJ/hr, and confirmed they matched.

---

Frequently asked questions

What is the difference between a calorie and a kilocalorie?

A small calorie (cal) is the energy to raise 1 gram of water by 1°C, equal to 4.184 joules. A kilocalorie (kcal) equals 1,000 small calories. Nutrition labels call kilocalories "Calories" (with a capital C). When people say "I ate 500 calories," they mean 500 kcal (kilocalories).

Why does my electricity bill use kWh instead of joules?

Kilowatt-hours are a more practical scale for electricity consumption. One joule is a very small amount of energy, a 100-watt bulb running for one second uses 100 joules. A kilowatt-hour (3,600,000 joules) is a more useful billing unit. One kWh is roughly the energy used by a 100-watt bulb for 10 hours.

What does BTU stand for and when is it used?

BTU stands for British Thermal Unit, the energy required to raise one pound of water by 1°F. Despite the name, BTU is primarily used in the United States for HVAC equipment, heating systems, and natural gas. The rest of the world uses watts and kilowatts for the same purposes.

How do I convert kJ on a food label to Calories?

Divide kilojoules by 4.184 to get kilocalories. Example: 840 kJ / 4.184 = 200 kcal = 200 food Calories.

Is the energy converter free?

Yes. ToolHQ's energy converter is completely free, with no account, no sign-up, and no usage limits.

---

The short version

Energy units reflect the fields where they developed: joules for science, kWh for electricity, BTU for HVAC, and kcal for food. The most important conversion to know is 1 kWh = 3,600,000 J = 860 kcal = 3,412 BTU. The most important distinction to understand is cal vs. Calorie: food labels always mean kilocalories (kcal). ToolHQ's energy converter handles all common units and returns simultaneous results across all of them.

For related tools, ToolHQ's power converter converts between watts, kilowatts, horsepower, and BTU/hour, and the electricity cost calculator estimates energy bills from wattage and usage time. Explore more converter tools at ToolHQ.

Convert energy units free at ToolHQ

---