What this calculator does
Turn an amount of energy from one unit into another, such as food calories into kilojoules. Scientists measure energy in joules, but daily life uses many other units. Food labels list kilocalories or kilojoules. Electricity bills count kilowatt-hours. Furnaces and air conditioners are rated in BTU (British thermal units). Physics at the scale of atoms uses electronvolts. Each of these is a fixed number of joules.
Enter an amount of energy and choose the units. The converter shows the result and the working, which goes through joules. It also gives a table of the same energy in all nine units. Very large and very small values are shown in scientific notation (a number times a power of 10).
Where it's used
Why this math matters outside the classroom:
- Food energy. A 250 kcal snack contains about 1,046 kJ, the figure many food labels outside the US print.
- Electric bills. A 1,000 W heater run for 3 hours uses 3 kWh, which is 10.8 MJ.
- Heating fuels. When comparing heating options, 1 kWh of electricity equals about 3,412 BTU.
- Physics problems. Lifting a 10 kg box 2 m stores about 196 J, roughly 145 ft·lbf of energy.
- Particle physics. Physicists express tiny energies in electronvolts, where 1 eV is about 1.602 × 10⁻¹⁹ J.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter the amount of energy. Zero and decimals are fine; negative values aren't.
- Choose the unit you're converting from and the unit you want.
- Read the result or scan the table. The working shows how many joules are in each unit.
What the inputs mean
- Energy
- The amount of energy to convert, in the “From” unit.
- From
- The unit your number is in. The food “Calorie” with a capital C is the kilocalorie.
- To
- The unit you want the answer in.
Formula and method
result = value × (from-unit in J) ÷ (to-unit in J) 1 cal = 4.184 J (thermochemical); 1 kcal = 4,184 J 1 Wh = 3,600 J; 1 kWh = 3,600,000 J 1 BTU = 1,055.05585262 J (International Table) 1 eV = 1.602176634 × 10⁻¹⁹ J; 1 ft·lbf ≈ 1.35582 J
J stands for joules. The converter stores each unit as the number of joules it holds. To convert, it multiplies by the size of the unit you start with, then divides by the size of the unit you want. In the list, cal is calorie, Wh is watt-hour, eV is electronvolt and ft·lbf is foot-pound.
Several units come in more than one version. This converter uses the thermochemical calorie (4.184 J). That's also the basis of food energy labels in many countries. It uses the International Table BTU too. A watt-hour is the energy one watt delivers in one hour, and an hour has 3,600 seconds. That's why it equals 3,600 joules. The electronvolt has had an exact value since the metric system (SI) was redefined in 2019.
Worked example
A snack provides 500 kcal. How many kilojoules is that?
- 500 kcal × 4,184 J = 2,092,000 J.
- 2,092,000 J ÷ 1,000 = 2,092 kJ.
Result: 500 kcal is exactly 2,092 kJ.
Understanding the result
Handy reference points: 1 kWh is about 3,412 BTU and about 860 kcal. And 1 kcal is 4.184 kJ.
Converting energy units doesn't account for efficiency. A kilowatt-hour of electricity and a kilowatt-hour of heat from burning gas are the same amount of energy. But devices turn them into useful work with different losses.
Assumptions and limitations
- The calorie is the thermochemical calorie of exactly 4.184 J.
- The BTU is the International Table BTU of 1,055.05585262 J.
- The International Table calorie (4.1868 J) and other BTU versions differ slightly and aren't offered.
- Therms, tons of TNT and barrels of oil equivalent aren't included.
- It converts energy, not power. Watts and BTU per hour are rates, so they need a length of time to become energy.
Frequently asked questions
Is a food Calorie the same as a calorie?
No. The Calorie on food labels, sometimes written with a capital C, is a kilocalorie. That's 1,000 small calories, or 4.184 kJ. Choose kcal here when you convert food energy.
How many BTU are in a kilowatt-hour?
About 3,412.14. One kWh is 3,600,000 J, and one International Table BTU is 1,055.05585262 J.
What is the difference between kW and kWh?
A kilowatt is how fast energy is used, while a kilowatt-hour is an amount of energy. Running a 2 kW heater for 3 hours uses 6 kWh. This converter works with amounts such as kWh, not rates such as kW.
Why is the electronvolt so small?
It's the energy one electron gains as it moves across one volt, about 1.6 × 10⁻¹⁹ J. That tiny scale suits atoms and particles, which is where the unit is used.
How this calculator is checked
Method: Thermochemical calorie = 4.184 J; Wh = 3,600 J; IT BTU = 1,055.05585262 J; eV = 1.602176634 × 10⁻¹⁹ J (SI 2019); ft·lbf from the international foot and pound with g = 9.80665 m/s². The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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