Why this matters
Energy shows up in wildly different contexts with wildly different units. A nutrition label lists Calories, an electricity bill charges in kilowatt-hours, a physicist talks in electronvolts, and an HVAC engineer specifies cooling capacity in BTUs. These are all measuring the same physical quantity, but the numbers differ by orders of magnitude — one food Calorie equals 4,184 joules, and one BTU equals roughly 1,055 joules. Converting between them by hand means remembering or looking up conversion factors every single time, which is error-prone when the factors have five or more significant digits.
This converter covers nine units — joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, foot-pounds, and electronvolts — and uses exact conversion factors throughout. The thermochemical calorie is fixed at exactly 4.184 joules, the kilowatt-hour is exactly 3.6 million joules, and the electronvolt is defined by the 2019 SI redefinition at exactly 1.602176634 times 10 to the negative 19 joules. You enter one value, pick the source unit, and every equivalent appears with a copy button so you can grab whichever number you need.
Unit reference table
| Unit | Symbol | Exact factor to Joules | Primary domain |
|---|---|---|---|
| Joule | J | 1 | SI base unit |
| Kilojoule | kJ | 1,000 | Food labeling (non-US) |
| Calorie | cal | 4.184 | Chemistry, physics |
| Kilocalorie | kcal | 4,184 | US food labels (as Calorie) |
| Watt-hour | Wh | 3,600 | Battery capacity |
| Kilowatt-hour | kWh | 3,600,000 | Electricity billing |
| BTU | BTU | 1,055.06 | HVAC, heating |
| Foot-pound | ft-lb | 1.35582 | US mechanical engineering |
| Electronvolt | eV | 1.602e-19 | Particle physics, semiconductors |
How to use it
Type your energy value into the input field.
Select the source unit from the dropdown (joules, calories, kWh, BTU, etc.).
View all nine equivalent values simultaneously, each with its own copy button.
Testing your result
Verify accuracy with a known conversion: enter 1 kilocalorie and confirm the joule result reads 4,184 exactly. Enter 1 kWh and check that kilocalories show 860.42 (3,600,000 divided by 4,184). Try the electronvolt range by entering 1 and confirming the joule value is on the order of 10 to the negative 19 — if it shows anything larger, the unit selection is wrong. Cross-reference a BTU value against your air conditioner's specifications: a 12,000 BTU window unit should convert to approximately 3.516 kWh. All math runs locally, so you can test with any value without network latency.
Common mistakes
Confusing calorie (small c, 4.184 J) with Calorie (large C, 4,184 J) — the capitalization on food labels matters enormously.
Assuming BTU is an SI unit; it is an imperial unit and its conversion to joules involves a non-round factor.
Using watt-hours when you mean watt-hours per hour (which is just watts) — a common source of confusion in electricity calculations.
Edge cases and limitations
Electronvolt conversions produce extremely small or extremely large numbers in scientific notation, which some browsers may truncate in display. The converter uses exact factors as defined by SI, so precision is not an issue, but readability can be at extreme scales. Foot-pounds are a US customary unit primarily used in mechanical engineering and torque calculations — if you work exclusively in metric, you can safely ignore that row. The tool does not handle compound units like megajoules or gigawatt-hours directly; enter the value in the base unit and convert from there.
Real-world use cases
Comparing the energy content of food (kcal) against electrical energy expenditure (kWh) during exercise on a stationary bike.
Converting an air conditioner's BTU rating to kilowatts to calculate expected electricity costs.
Translating a physics problem's energy from electronvolts to joules for calculation with SI formulas.
Frequently asked questions
Q: What is the difference between calorie and Calorie?
A: 1 calorie (small c) = 4.184 J. 1 Calorie (large C, used on food labels) = 1 kilocalorie = 4,184 J. The capitalization matters.
Q: What is a BTU?
A: British Thermal Unit — energy to heat 1 lb of water by 1 degree F. 1 BTU is approximately 1,055 J. Used for HVAC, heaters, and air conditioners.
Q: What is an electronvolt?
A: Energy gained by an electron moving across 1 volt potential = 1.602 times 10 to the negative 19 J. Used in particle physics, semiconductor physics, and quantum mechanics.
Q: Are conversions exact?
A: Yes — 1 cal = 4.184 J (thermochemical), 1 kWh = 3.6 MJ exactly, 1 eV = 1.602176634 times 10 to the negative 19 J exactly (2019 SI redefinition).
Q: Is my data uploaded?
A: No — all conversions happen in your browser.
Start using it now
Convert between nine energy units with exact SI factors using the Energy Converter. For related unit conversions, try the Temperature Converter or the Pressure Converter. Calculate the cost of your energy usage with the Electricity Cost Calculator.
