Energy Converter
Convert an energy value between joules, calories, kilowatt-hours, BTUs, and other units. Enter a value, then choose the From unit and To unit.
Convert Energy Units Across Systems
Energy Formula
Where:
- \(E_{\mathrm{from}}\) — the energy value entered in the From unit
- \(E_{\mathrm{to}}\) — the converted energy value in the To unit
- \(f_{\mathrm{from}}\) — joules per From unit
- \(f_{\mathrm{to}}\) — joules per To unit
The converter first expresses the entered value in joules, then divides by the number of joules in one target unit. This changes the unit without changing the amount of energy.
Energy – Calculation Example
Example: With the default settings, convert 1000 joules to kilocalories. One kilocalorie equals 4184 joules.
\[ 1000\ \mathrm{J}\times\frac{1\ \mathrm{J/J}}{4184\ \mathrm{J/kcal}}=0.2390057361\ \mathrm{kcal} \]
Result: 1000 J is approximately 0.2390057361 kcal.
How to use the calculator
- Enter a nonnegative number in Energy value. You can use a decimal value.
- Choose the unit of that number in From unit.
- Choose the unit you need in To unit.
- Read Converted Energy Value. To reverse a conversion, exchange the From unit and To unit and enter the value you want to convert.
The default fields are set to 1000, Joule (J), and Kilocalorie (kcal), matching the worked example above. Selecting the same unit in both menus leaves the numerical value unchanged.
What energy units mean
Energy is a quantity used to describe work, heat, and other physical processes. The joule (J) is the SI unit of energy, but other units are convenient at different scales. Kilojoules and megajoules express larger quantities without long strings of digits; watt-hours and kilowatt-hours are common for electrical energy; and electronvolts are useful for energies at the atomic scale.
Calories and kilocalories are often used for heat and food energy. In this converter, one calorie (cal) equals 4.184 J, and one kilocalorie (kcal) equals 1000 cal, or 4184 J. A food-label “Calorie” with a capital C generally means a kilocalorie, not the smaller calorie. Choosing cal instead of kcal changes the displayed number by a factor of 1000.
A conversion tells you how the same energy quantity is written in another unit; it does not tell you how efficiently energy is transferred or used. For example, converting kWh to J does not establish how much useful heat an appliance delivers. That depends on the process or equipment involved. A foot-pound here means a foot-pound of energy; the same unit name can also appear in discussions of torque, which is a different physical quantity.
Conversion factors used
The calculator uses the following number of joules per unit. For any pair of units, multiply the entered value by the From-unit factor and divide by the To-unit factor.
| Unit | Joules per unit |
|---|---|
| Joule (J) | 1 |
| Kilojoule (kJ) | 1000 |
| Megajoule (MJ) | 1,000,000 |
| Calorie (cal) | 4.184 |
| Kilocalorie (kcal) | 4184 |
| Watt-hour (Wh) | 3600 |
| Kilowatt-hour (kWh) | 3,600,000 |
| Electronvolt (eV) | \(1.602176634\times10^{-19}\) |
| BTU (British Thermal Unit) | 1055.05585 |
| Erg | \(1\times10^{-7}\) |
| Foot-pound (ft·lb) | 1.3558179483 |
Assumptions and practical tips
The converter uses fixed factors, including 1055.05585 J per BTU. BTU definitions can vary slightly by convention, so a source using a different definition may produce a slightly different result. For calculations requiring a specified BTU convention, check which factor that source uses.
The result is rounded for display to 10 decimal places. Very small results may therefore appear as zero, especially when converting from electronvolts or ergs to a much larger unit. Try a smaller target unit to see a more useful number. The number of displayed digits should not be taken as a guarantee that the original measurement was equally precise.
Use this converter when you already know an energy quantity and only need to change its unit. If you first need to calculate energy transferred as heat, use the heat transfer calculator. For a material’s heat capacity per unit mass, see the specific heat calculator.
Frequently Asked Questions
How do I convert kilowatt-hours to BTU by hand?
Multiply kilowatt-hours by approximately 3,412.14 to get BTU. For example, \(2\ \mathrm{kWh}\times 3{,}412.14\ \mathrm{BTU/kWh}\approx 6{,}824.28\ \mathrm{BTU}\). For a more precise calculation, divide 3,600,000 joules per kilowatt-hour by the BTU factor used by your source; BTU definitions can differ slightly.
Can I convert joules or kilowatt-hours directly to watts?
No. Joules and kilowatt-hours measure energy, while watts measure power—the rate of energy transfer. You also need a time interval. Divide energy in joules by time in seconds to get watts, or divide energy in kilowatt-hours by time in hours to get kilowatts. For example, using 2 kWh over 2 hours corresponds to an average power of 1 kW.
What is a normal or good energy value?
There is no universally good energy value; it depends on what you are measuring. A food portion, an electrical appliance, and a heating system operate on very different scales. To judge whether a value is reasonable, compare it with a measurement for the same activity, equipment, and time period. Converting units alone cannot tell you whether energy use is efficient.
How do I use an energy conversion to estimate electricity cost?
Convert the energy used to kilowatt-hours, then multiply by your electricity price per kilowatt-hour. For example, 500 Wh is 0.5 kWh; at $0.20 per kWh, its energy charge would be $0.10. Your actual bill may also include fixed charges, taxes, and prices that vary by time or usage, so check your tariff before estimating a total.
Can energy be negative, and why does this converter require a nonnegative value?
An amount of energy is normally entered as zero or positive, which is why this converter accepts nonnegative values. A negative sign can be meaningful for an energy change: it may indicate that a chosen system lost energy, depending on the sign convention. To convert such a change, convert its positive magnitude and keep track of the sign separately.
Can I convert energy in joules to a temperature change?
Not from energy alone. For heating or cooling a substance, you also need its mass and specific heat capacity. If these are known and the specific heat is approximately constant, use \(\Delta T=Q/(mc)\), where \(Q\) is heat transferred. Phase changes, heat losses, and changing heat capacity can make a simple estimate inaccurate.