Energy crops up in nutrition (Calories), utility bills (kWh), heating (BTU), mechanics (foot-pounds), and physics (electronvolts) — all measuring the same quantity in very different sizes. This reference converts any energy value into every supported unit at once, using exact factors.
Why so many energy units?
Each field developed its own unit before standardisation. Physics settled on the joule as the SI unit, but nutrition inherited the calorie from 19th-century calorimetry, electrical engineering adopted the watt-hour for practical billing, US engineering uses the BTU because it relates to familiar quantities (pounds of water, Fahrenheit degrees), and particle physics uses the electronvolt because it is naturally scaled to atomic energies. A single converter saves the translation between all of them.
| Unit | Exact joules | Where used |
|---|---|---|
| Joule (J) | 1 | SI standard |
| Kilocalorie (kcal) | 4,184 | Food labels (“Calories”) |
| Watt-hour (Wh) | 3,600 | Electricity |
| Kilowatt-hour (kWh) | 3,600,000 | Utility bills |
| BTU (IT) | 1,055.056 | Heating, air conditioning (US) |
| Foot pound-force (ft·lbf) | 1.35582 | US mechanical engineering |
| Electronvolt (eV) | 1.602×10⁻¹⁹ | Atomic and particle physics |
How it works
Each unit stores an exact number of joules per unit, with the joule as the SI base. Conversion is two steps:
joules = value × joulesPerUnit[from]
result = joules ÷ joulesPerUnit[to]The useful anchors are 1 kWh = 3.6 × 10⁶ J, the thermochemical 1 cal = 4.184 J (so the food Calorie = kcal = 4184 J), the IT 1 BTU = 1055.05585262 J, and the physics 1 eV = 1.602176634 × 10⁻¹⁹ J. The mechanical foot pound-force is ≈ 1.3558 J.
The Calorie confusion
The single most common energy unit error: food labels in most countries use “Calorie” (capital C) or “kcal” to mean the kilocalorie — 4,184 joules. The small calorie (lower-case c) used in chemistry and physics is 4.184 joules, a factor of 1,000 smaller. So a 300-Calorie meal has 300,000 small calories, or about 1,255,200 joules. The tool correctly treats the food Calorie as a kilocalorie.
Worked examples
- A 200-Calorie snack is
200 kcal = 200 × 4,184 = 836,800 J ≈ 0.232 kWhof food energy. - Your electricity bill:
1 kWh = 3.6 MJ, so a 3 kWh appliance run uses10.8 MJof energy. - An air conditioner rated at 10,000 BTU/h provides energy at the rate of
10,000 × 1,055 ≈ 10.55 MJ per hour, which is about2.93 kWh/h.
Extremely large or small results — such as electronvolts or joule equivalents of BTUs — appear in scientific notation so they remain readable. All conversions run locally in your browser.