How to Use This Calculator
Select your conversion mode using the tabs above. Single Conversion converts between two specific units. Convert to All Units shows your value in every supported energy unit. Energy Reference provides lookup tables for common energy values.
To convert energy: enter your value, pick the starting unit from the "From" dropdown, choose the target unit in "To", and click Convert. Use the swap button (⇄) to quickly reverse your conversion direction. Quick conversion buttons set common unit pairs, while preset buttons populate typical energy values.
Understanding Energy Units
Energy measures the capacity to do work or transfer heat. Physicists use joules, nutritionists use kilocalories, HVAC technicians use BTU, and electric bills show kilowatt-hours—same quantity, different numbers.
The Joule (J) — The SI unit of energy. One joule equals the work done when a force of one newton moves an object one meter, or one watt-second (1 J = 1 W·s). The unit is named after physicist James Prescott Joule. A joule is small—lifting an apple one meter takes about one joule.
The Calorie (cal and kcal) — Originally defined as the energy needed to raise one gram of water by one degree Celsius. The "calorie" on food labels is actually a kilocalorie (kcal), also written as Calorie with a capital C. This trips people up constantly: 1 food Calorie = 1 kcal = 1,000 cal = 4,184 J. When nutritionists say a banana has 105 Calories, they mean 105 kcal or about 439 kJ.
The British Thermal Unit (BTU) is the standard unit for HVAC and heating in the United States. One BTU raises one pound of water by one degree Fahrenheit—about 1,055 joules. Air conditioner ratings, furnace specs, and natural gas bills typically use BTU.
Kilowatt-hour (kWh) — Your electric bill uses this unit. One kWh equals the energy of 1,000 watts running for one hour—3.6 megajoules. A 100-watt bulb running 10 hours uses 1 kWh. At $0.12/kWh, that's about 1.2 cents per hour for an LED bulb versus 7 cents for an incandescent.
In atomic and nuclear physics, the electronvolt (eV) is the preferred unit. One eV is the energy gained by an electron accelerating through one volt—about 1.6 × 10⁻¹⁹ joules. At atomic scales, joule values get unwieldy, so physicists use eV instead. Chemical bonds: 1-10 eV. Nuclear reactions: MeV range.
The Joule-Calorie Relationship
One thermochemical calorie equals 4.184 joules. This value comes from James Joule's experiments relating heat to mechanical work.
Quick Mental Math
- kcal to kJ: multiply by 4 (exact is 4.184)
- kJ to kcal: divide by 4 (exact is ÷4.184)
- BTU to kJ: roughly 1:1 (exact is 1.055)
- kWh to MJ: multiply by 3.6 (exact)
- 1 food Calorie = 1 kcal = 4.184 kJ
Energy in Everyday Life
A 2,000 kcal diet provides about 8.4 MJ per day. Running a 1,500W space heater for 8 hours uses 12 kWh costing roughly $1.44 at average US rates. A gallon of gasoline contains about 132 MJ—equivalent to roughly 36 kWh of electricity.
Energy = 3,500W × 8h = 28,000 Wh = 28 kWh
Cost = 28 kWh × $0.12 = $3.36
Power, Energy, and Fuels
Energy changes form but the total stays constant. Burning gasoline turns chemical energy into heat and motion. Car engines convert roughly 25% of fuel energy into forward movement; the rest escapes as heat. Power plants typically achieve 33-60% efficiency.
Power measures how fast energy flows. Energy equals power multiplied by time. A 100-watt bulb uses 100 joules every second. Run it for an hour and you've used 360,000 joules = 0.1 kWh. Your electricity bill charges for energy (kWh), not power (kW). A high-power appliance used briefly can use less energy than a low-power device left on all day.
Gasoline holds about 34 MJ/L. Hydrogen has 120 MJ/kg but only 10 MJ/L when compressed—high energy per kilogram, low energy per liter. Batteries hold far less energy per kilogram than liquid fuels.