Energy Converter
Convert between joules, calories, kilowatt-hours, BTU, and electronvolts. Useful for physics, engineering, and nutrition.
Energy Converter
Enter an energy value and select its unit β all other units update instantly.
Quick References
How to Convert Between Joules, Calories, kWh, BTU, and Electronvolts
Your electricity bill shows you used 450 kWh last month. You want to compare that to the energy content of natural gas to see which heating option is cheaper. Converting 450 kWh to BTU gives 1,535,460 BTU, while the same amount in joules is 1.62 billion J. Enter the number, pick kWh, and every other energy unit appears instantly. The copy button lets you paste the BTU figure into your cost comparison spreadsheet.
The converter covers 11 units from electronvolts to therms. A nutritionist comparing a 2,500 kcal daily target with a European label showing 10,460 kJ can confirm they match. A physicist converting a photon energy of 2.5 eV to joules gets 4.005 x 10^-19 J. The quick-reference strip at the bottom shows everyday energy values: an AA battery at 10,800 J, a smartphone charge at 36,000 J, a gallon of petrol at 120 million J.
Nutrition labels where kcal and kJ cause confusion
A British cereal box lists 380 kJ per 30 g serving. An American equivalent shows 90 kcal. Converting 380 kJ to kcal: divide by 4.184 to get 90.8 kcal, essentially the same. But a shopper who confuses kJ with kcal might think the British cereal has 380 Calories per serving, which is actually a full meal worth of energy. The converter makes these cross-label comparisons instant and error-free.
A homeowner wants to compare a 1500W electric heater running for 10 hours (15 kWh) against a gas furnace rated at 40,000 BTU/hr. The electric heater uses 15 kWh = 51,182 BTU total. At $0.12/kWh, that costs $1.80. The gas furnace producing the same 51,182 BTU uses about 0.51 therms. At $1.00/therm, that costs $0.51, gas is roughly 3.5 times cheaper per BTU. The converter bridges the unit gap between the two energy sources.
A solar panel engineer needs to compare photon energies. Visible light photons have energies of 1.8 to 3.1 eV. A 2.0 eV photon corresponds to 3.204 x 10^-19 J, or a wavelength of about 620 nm (orange light). The converter lets the engineer quickly translate between eV (used in semiconductor physics) and joules (used in thermal calculations) without memorising the conversion factor 1.602 x 10^-19.
Related tools
How the Energy Converter Handles Joules, Calories, kWh, and Electronvolts
Every unit in the converter is stored as a multiple of one joule. When you enter 1 kilowatt-hour, the tool multiplies by 3,600,000 to get 3.6 million J, then divides by each target unit's factor. The conversion to BTU gives 3,412.14 BTU (using the IT BTU factor of 1055.06 J). Because all conversions pass through joules in one step, there is no rounding drift from chained multiplications.
Why the calorie has three different definitions
The thermochemical calorie is defined as exactly 4.184 J. The international steam table calorie is 4.1868 J. The 15 degrees C calorie is 4.1855 J. These differ because early experimenters measured the energy needed to heat water at different starting temperatures, and the results varied slightly. The thermochemical definition (4.184 J) is the most widely used in chemistry and nutrition. This converter uses the thermochemical value. The difference between the definitions is less than 0.1%, which matters only in high-precision calorimetry.
The electronvolt is defined by fixing the elementary charge at exactly 1.602176634 x 10^-19 C and multiplying by 1 volt. This makes it exact by definition. The BTU (ISO) is defined as 1055.06 J, which is approximate, the original definition (energy to heat 1 lb of water by 1 degree F) depends on the starting temperature because water's specific heat capacity varies with temperature. The therm (100,000 BTU) is used by natural gas utilities and is exact relative to the BTU definition.
Why food calories are capitalised
The "Calorie" (capital C) on food labels is actually a kilocalorie (kcal) = 1,000 small calories. This convention was introduced in 1940 to reduce confusion, but it backfired, most people still do not know the difference. A 200 Calorie snack contains 200,000 small calories, or 836,800 joules. The energy is enough to heat 200 grams of water from room temperature to boiling. This converter shows both kcal and kJ, which is useful because European food labels list energy in kJ while US labels list kcal.
The formatting thresholds adapt to the enormous range of the converter. Electronvolts (10^-19 J) display in scientific notation. Joules and calories use 6-8 decimal places. Kilowatt-hours and therms use locale-formatted numbers with 2-4 decimal places. This keeps the output readable whether you are converting atomic-scale energies or household electricity consumption.
Frequently Asked Questions
How many joules are in 500 kcal?
500 kcal equals 2,092,000 joules (500 x 4.184 = 2,092). This is roughly the energy in a large fast-food meal. For comparison, running at 8 km/h for one hour burns about 500 kcal, the same energy as the meal. The converter shows both values instantly: enter 500 kcal, see 2,092 kJ, 581.1 Wh, or 1,984 BTU.
Why does my electricity bill use kWh instead of joules?
A kWh is a more practical unit for household electricity. One kWh = 3.6 million joules. If your bill listed joules, you would see numbers in the billions, 900 kWh/month becomes 3.24 billion J. The kWh keeps the numbers manageable (900 instead of 3.24 billion) and directly relates to appliance wattage: a 100W bulb running 10 hours uses exactly 1 kWh.
What is the energy of a photon of visible light?
A photon of visible light has an energy of about 1.8 to 3.1 electronvolts (eV). Red light (700 nm) is about 1.8 eV; blue light (450 nm) is about 2.8 eV. Converting 2.5 eV to joules: 2.5 x 1.602 x 10^-19 = 4.005 x 10^-19 J. This tiny energy per photon is why you need billions of photons to see anything, the human eye can detect as few as 5-10 photons at the retina.
How do I compare gas and electricity costs?
Convert both to the same unit (BTU or kWh) and compare the price per unit. Gas is typically sold in therms (100,000 BTU) at $1.00/therm. Electricity is sold in kWh at $0.12/kWh. Converting: 1 therm = 29.3 kWh. So $1.00 for gas buys 29.3 kWh worth of energy, while $1.00 for electricity buys only 8.33 kWh. Gas is roughly 3.5 times cheaper per BTU.
What is a food Calorie in terms I can visualise?
One food Calorie (1 kcal) has enough energy to heat 1 kg of water by 1 degree C. A 200 Calorie banana could theoretically heat 200 kg of water (a large bathtub) by 1 degree C. A 2,000 Calorie daily diet provides enough energy to heat 2,000 kg of water by 1 degree C, or one bathtub by 2 degrees C. This helps visualise why food energy matters for body heat production in cold weather.