Energy Converter
Enter a value to instantly convert between energy units.
Energy conversions span an unusually wide range of everyday and technical contexts: nutrition labels list food energy in calories or kilojoules, electricity bills measure consumption in kilowatt-hours, heating systems are often specified in BTUs, and scientific work uses joules as the base unit. Because energy is a fundamental physical quantity that appears in mechanics, electricity, heat, and chemistry, the units in this category come from several different traditions — which is why a single converter covering calories, joules, BTU, and kilowatt-hours together is so useful.
1 Kilocalorie = 4.184 Kilojoule
Key Formulas
Kilocalorie → Kilojoule
kJ = kcal × 4.184Kilojoule → Kilocalorie
kcal = kJ × 0.239006Kilowatt-Hour → Joule
J = kWh × 3600000Joule → Kilowatt-Hour
kWh = J × 2.77778e-7Popular Conversions
All Energy Conversions
About Energy Conversions
History & Background
The joule, the SI unit of energy, is named after James Prescott Joule, whose 19th-century experiments established the equivalence between mechanical work and heat — a foundational result in thermodynamics. The calorie originated specifically as a unit of heat (the energy needed to raise the temperature of 1 gram of water by 1°C), and survives today primarily in nutrition labeling, where it's almost always actually a kilocalorie (1,000 calories) despite being labeled simply 'Calories.' The BTU (British Thermal Unit), still standard for HVAC and heating equipment ratings in the US, similarly originates from heating water (the energy to raise 1 pound of water by 1°F). The kilowatt-hour, used for electricity billing worldwide, is a unit of energy defined from power (kilowatts) and time (hours) — a practical unit for metering rather than a scientific base unit.
How to Use This Converter
Select your starting and target energy units and enter a value for an instant conversion. Note that nutrition labels' 'Calories' are kilocalories (1 Cal = 1,000 cal = 4,184 J) — if you're converting food energy values, make sure you're using the kilocalorie/Calorie conversion, not the small calorie, to avoid a 1,000x error.
Frequently Asked Questions
Why does a food label say 'Calories' but scientists use 'calories' (lowercase) differently?
In nutrition labeling, 'Calorie' (often capitalized, sometimes written as 'kcal') refers to a kilocalorie — 1,000 small calories. In strict scientific usage, the lowercase 'calorie' is the smaller unit (the energy to raise 1 gram of water by 1°C). This naming overlap is a long-standing source of confusion; when converting nutrition information, always treat the food 'Calorie' as a kilocalorie (4,184 joules), not 4.184 joules.
How is a kilowatt-hour related to a joule?
A kilowatt-hour is the energy equivalent of using 1,000 watts of power for 1 hour. Since a watt is 1 joule per second, and an hour is 3,600 seconds, 1 kWh = 1,000 × 3,600 = 3,600,000 joules (3.6 megajoules). This is why your electricity bill's 'kWh' figure is a measure of total energy consumed, not power — power (kW) is the rate, and kWh is the total over time.
What is a BTU used for, and how big is it?
A BTU (British Thermal Unit) is approximately 1,055 joules — the energy needed to raise the temperature of 1 pound of water by 1°F. It's still the standard unit for rating heating, ventilation, and air conditioning (HVAC) equipment in the US, where an air conditioner's cooling capacity is often given in BTUs per hour (sometimes shortened to just 'BTU').
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