A diet, a battery, and an air conditioner walk into a bar
You ate 2000 Calories today. Your phone battery holds about 40,000 J. The window AC is rated 10,000 BTU. Three numbers, three unit systems, one physical quantity — energy. Convert them to a common unit and you can compare a sandwich to a power bill.
The Energy Unit Converter handles 11 units across five domains, all funnelled through the joule. Conversion is the easy part — multiply by the source's joule factor, divide by the target's. The interesting part is the four places a sensible number is quietly wrong: the calorie that isn't the food Calorie, the calorie that isn't the calorie, the BTU that isn't the BTU, and the electron-volt that used to be approximate and is now exact.
The joule is the hub
Every unit has a stored toJoules factor. To convert A → B, the tool multiplies by A's factor to get joules, then divides by B's. No unit talks to another directly — they all route through J — so a two-step conversion (kWh → BTU) is exactly as accurate as a one-step (J → kJ).
| Unit | Symbol | Joules | Domain |
|---|---|---|---|
| Joule | J | 1 | SI |
| Kilojoule | kJ | 1,000 | SI |
| Megajoule | MJ | 1,000,000 | SI |
| Calorie | cal | 4.184 | Thermal |
| Kilocalorie | kcal | 4,184 | Thermal |
| BTU | BTU | 1,055.06 | Thermal |
| Therm | thm | 1.055×10⁸ | Thermal |
| Watt-hour | Wh | 3,600 | Electrical |
| Kilowatt-hour | kWh | 3.6×10⁶ | Electrical |
| Foot-pound | ft-lbf | 1.35582 | Mechanical |
| Electron-volt | eV | 1.602176634×10⁻¹⁹ | Atomic |
Two of these are exact, not measured: 1 kWh = 3.6 MJ exactly (1 W = 1 J/s, so 1 Wh = 3600 J by definition), and 1 eV = 1.602176634×10⁻¹⁹ J exactly since the 2019 SI redefinition pinned the elementary charge. Before 2019 eV was a measured constant; now it's defined — eV↔J is one of the few exact conversions in physics.
The calorie trap (read this if you only read one section)
There are two traps with the word "calorie," and they bite different people.
Trap one — the food Calorie is a kilocalorie. The "2000 Calorie" diet on a nutrition label is 2000 kcal, not 2000 cal. Capital-C Calorie = kcal = 4184 J. Type 2000 into the tool's kcal field, not cal, or you're off by 1000×. The most common energy-conversion error, built into the English language, not the math.
Trap two — there is no single "calorie." The tool uses the thermochemical calorie, 1 cal = 4.184 J, the one most chemistry textbooks use. At least three others exist:
- International Table calorie = 4.1868 J
- 15 °C calorie = 4.1855 J
- Mean calorie = 4.190 J
The spread is ~0.07% — irrelevant for a recipe, enough to miss a published answer on a thermochem problem set. If a source says "calories" with no subscript, it almost always means this 4.184 value, but check before you publish.
The BTU trap is the same shape
The tool's BTU = 1055.06 J, the International Table BTU — the one HVAC and US heating equipment use, the right default for "a 10,000 BTU air conditioner." The thermochemical BTU is 1054.35 J, the 59 °F BTU is 1054.8 J. Same 0.07% nuisance. Match the BTU to the domain: AC specs → 1055.06 (this tool), a lab calorimetry paper → maybe not.
The therm is 100,000 BTU (US, IT), so 1 therm = 1.05506×10⁸ J — the tool rounds to 1.055×10⁸. UK and some US gas bills use therms; others use CCF (hundred cubic feet), roughly but not exactly a therm, since gas energy content varies. Don't assume CCF = therm.
The diet that costs thirty cents
Convert the 2000-kcal diet through the hub: 2000 × 4184 = 8,368,000 J = 8.37 MJ = 2.32 kWh. At ~$0.15/kWh, that's $0.35 of grid energy. Your entire day of food is worth roughly thirty cents as electricity — the body runs at ~100 W, comparable to a bright incandescent bulb.
More cross-domain anchors, all from the joule hub:
- 1 kWh ≈ 3412 BTU ≈ 860 kcal ≈ 3.6 MJ (the HVAC↔electricity bridge)
- 1 BTU ≈ 0.252 kcal ≈ 1055 J (roughly a kilojoule)
Gotchas
- Pick kcal, not cal, for food. The capital-C Calorie is a kilocalorie. The 1000× error is the classic one.
- eV needs scientific notation. 1 eV in J is 1.6×10⁻¹⁹ — the tool switches to exponential below 1×10⁻⁶, so you'll see
1.602176e-19. Don't paste that into a system expecting fixed-point; keep the exponent. - The calorie and BTU are domain-defined. This tool uses the thermochemical calorie (4.184) and the IT BTU (1055.06). Both are common defaults; both differ from their thermochemistry variants by ~0.07%.
- ft-lbf is energy, lbf-ft is torque. Same dimensions, different context. The tool's "Foot-pound" is the work form (1.35582 J). A bolt-tightening spec is torque — don't convert it here.
Summary
- All 11 units convert through the joule hub: multiply by the source factor, divide by the target. Two factors are exact — 1 kWh = 3.6 MJ and 1 eV = 1.602176634×10⁻¹⁹ J (defined in the 2019 SI).
- The food Calorie (capital C) = kcal = 4184 J. Type food energy into kcal, never cal, or you're off by 1000×.
- The tool's calorie is the thermochemical 4.184 J and its BTU is the International Table 1055.06 J — both common defaults, both within ~0.07% of their thermochemistry variants.
- A 2000-kcal diet ≈ 8.37 MJ ≈ 2.32 kWh ≈ $0.35 of electricity — the body runs on pocket change.
- Convert in the Energy Unit Converter. For other physical quantities: temperature, length, weight, and data storage.