About the Energy Converter
This energy converter changes a quantity of energy from one unit to any other: joules and kilojoules (the SI units), food calories (kcal), watt-hours and kilowatt-hours (electricity bills), British thermal units and therms (heating and gas), foot-pounds (mechanics) and electronvolts (physics). Choose a "from" and "to" unit for the headline answer, and the table lists the same amount in every unit.
Students use it for physics and chemistry problems, homeowners use it to compare a gas bill in therms with an electricity bill in kWh, athletes translate kJ on European food labels into Calories, and HVAC buyers relate BTU ratings to kilowatts of energy. Because everything passes through the joule, any pair of units works.
The calorie used here is the thermochemical calorie (4.184 J), which is the one food labels are based on; 1 kcal = 1 food Calorie. The BTU is the International Table BTU (1,055.056 J) and a therm is 100,000 BTU. The electronvolt uses the exact 2019 SI value of the elementary charge.
With the default inputs, the converted amount is 3,412.141633. Change any value above to recalculate instantly.
How to use the energy converter
- 1Enter the amount of energy.
- 2Choose the unit you are converting from.
- 3Choose the unit you want the answer in.
- 4Read the headline result, or scan the table for every other unit.
Formula and method
Every unit is defined as a fixed number of joules, so the amount is multiplied by the joules in the source unit and divided by the joules in the target unit. Key factors: 1 kWh = 3,600,000 J exactly, 1 kcal = 4,184 J, 1 BTU = 1,055.056 J, 1 therm = 100,000 BTU and 1 eV = 1.602176634 × 10⁻¹⁹ J.
Different "calorie" and "BTU" definitions exist (International Table, thermochemical, 59 °F); they differ by less than 0.1%, which only matters for laboratory work.
- J
- Joule, the SI unit of energy (1 W for 1 s)
- kWh
- Kilowatt-hour, 3.6 MJ — the unit on electricity bills
- kcal
- Kilocalorie = 1 food Calorie = 4.184 kJ
- BTU
- Energy to heat 1 lb of water by 1 °F ≈ 1,055 J
Worked examples
1 kWh to BTU
1 kWh is 3,600,000 J. Dividing by 1,055.056 J per BTU gives 3,412.14 BTU, and dividing by 4,184 J per kcal gives about 860.4 kcal.
A 2,000-Calorie daily diet in kJ and kWh
2,000 kcal × 4.184 = 8,368 kJ — the figure you would see on an Australian or EU label. That is 8,368,000 J ÷ 3.6 million ≈ 2.32 kWh of energy.
One therm of gas in kWh
A therm is 100,000 BTU = 105.5 MJ. Dividing by 3.6 MJ per kWh gives about 29.31 kWh, useful for comparing gas and electric heating costs.
Frequently asked questions
How many joules are in a calorie?+
One small calorie (cal) is 4.184 joules. A food Calorie is a kilocalorie, so 1 kcal = 4,184 J = 4.184 kJ.
How many BTU are in a kWh?+
One kilowatt-hour equals about 3,412 BTU. Going the other way, 1 BTU is roughly 0.000293 kWh, so a 100,000 BTU furnace output is about 29.3 kWh.
What is the difference between kW and kWh?+
kW measures power — how fast energy is used. kWh measures energy — power multiplied by time. A 2 kW heater running for 3 hours uses 6 kWh.
How do I convert kJ on a food label to Calories?+
Divide kilojoules by 4.184. A snack labelled 840 kJ contains about 200 kcal, which is the same as 200 food Calories.
What is an electronvolt used for?+
The electronvolt (eV) is the energy an electron gains crossing one volt. It is used in atomic, particle and semiconductor physics, where joules are inconveniently large: 1 eV = 1.602 × 10⁻¹⁹ J.