Convert Energy Units
Energy Converter
Energy Conversion Chart
Key relationships between energy units. The SI unit is the joule (J), defined as 1 newton·meter or 1 watt·second.
SI / Metric
| 1 kJ | = | 1,000 J | exact |
| 1 MJ | = | 1,000,000 J | exact |
| 1 Wh | = | 3,600 J | exact |
| 1 kWh | = | 3,600,000 J | exact |
| 1 kWh | = | 3.6 MJ | exact |
Calories (two definitions)
| 1 cal (small) | = | 4.184 J | exact |
| 1 kcal | = | 4,184 J | exact |
| 1 Cal (food) | = | 1 kcal | exact |
| 1 Cal (food) | = | 1,000 cal | exact |
"Calorie" on food labels (Cal, capital C) = 1 kilocalorie (kcal) = 1,000 small calories (cal).
Thermal & Imperial
| 1 BTU (IT) | = | 1,055.06 J | |
| 1 therm | = | 100,000 BTU | exact |
| 1 therm | = | 105.506 MJ | |
| 1 ft·lbf | = | 1.35582 J |
1 thermochemical calorie = 4.184 J exactly (IUPAC). The kilowatt-hour (kWh) is defined as exactly 3.6 MJ — it measures energy consumed, while the kilowatt (kW) measures power (rate of energy use). Do not confuse kWh (energy) with kW (power).
About Energy Units
Food & Nutrition
Food energy is labeled in Calories (Cal) in the US and kilojoules (kJ) in Europe/Australia. 1 food Calorie = 1 kcal = 4.184 kJ. A typical adult needs 2,000–2,500 Cal/day (8,400–10,500 kJ/day). The capital-C "Calorie" on labels is actually a kilocalorie.
Electricity & Home
Electricity bills use kilowatt-hours (kWh). 1 kWh = running a 1,000 W appliance for 1 hour = 3.6 MJ. An average US household uses about 30 kWh/day. Do not confuse kWh (total energy) with kW (instantaneous power).
Heating & HVAC
Natural gas is billed in therms (US) or kWh (Europe). 1 therm = 100,000 BTU ≈ 105.5 MJ. Heating capacity is rated in BTU or BTU/h. The British Thermal Unit is the energy needed to raise 1 pound of water by 1°F.
Science & Engineering
The joule (J) is the SI unit: 1 J = 1 N·m = 1 W·s. Chemistry uses kJ/mol for reaction energies. Nuclear physics uses electron-volts (1 eV = 1.602 × 10⁻¹⁹ J). Mechanical work uses foot-pounds (ft·lbf).
The most common confusion in energy units is calorie vs Calorie: the small calorie (cal) is a physics unit; the large Calorie (Cal) on food labels equals 1,000 small calories = 1 kcal. When a food label says "200 Calories", it means 200 kilocalories = 200,000 calories = 836.8 kJ.
Popular Energy Conversions
Joules & Calories
Electrical (kWh)
Thermal & Mechanical
Conversion Charts & Tables
Printable charts and reference tables for energy conversions:
Frequently Asked Questions
Why are joules used in scientific calculations?
Joules are the standard unit of energy in the International System of Units (SI), making them universally recognized in scientific research worldwide. They provide a consistent base for all energy measurements, from molecular interactions to mechanical work, ensuring accurate scientific communication and calculations across all fields of physics and engineering.
When should I use BTU or BTU/h instead of watts?
A single BTU is a unit of energy (approximately 1,055 joules), while a watt is a unit of power (1 joule per second) — they measure different physical quantities. In HVAC, the relevant unit is BTU/h (BTU per hour), which IS comparable to watts: 1 BTU/h ≈ 0.293 W. Use BTU for energy content in fuels; use BTU/h for heating and cooling capacity of furnaces, air conditioners, and heat pumps — especially in the United States.
What's the difference between calories and kilocalories?
A calorie (cal) is the energy needed to raise 1 gram of water by 1 degree Celsius, while a kilocalorie (kcal) equals 1,000 calories. Food labels showing "calories" actually refer to kilocalories - so when you see "100 calories" on food packaging, it is really 100,000 scientific calories or 100 kcal. This is why food "calories" are sometimes called "large calories".
How do energy units relate to electricity bills (kWh)?
Kilowatt-hours (kWh) measure the total energy consumed over time. If you run a 1,000-watt (1 kW) appliance for 1 hour, you consume 1 kWh of energy. Your electricity bill is calculated by multiplying the kWh used by the rate per kWh. For example, 500 kWh multiplied by $0.12/kWh equals $60 on your monthly bill.
Related Converter Categories
- Power Converter — Power = energy ÷ time (kW vs kWh)
- Temperature Converter — Heat energy: BTU, calorie
- Weight Converter — Energy content of fuels per unit mass
