Convert between twelve energy units in one tool, routed through joules as the canonical base. Source and target dropdowns cover metric SI units (J, kJ, MJ), commercial electricity units (Wh, kWh), calorie variants (small cal, food kcal, IT cal), imperial energy units (BTU, ft·lbf, therm), and the CGS erg. Quick-pick buttons jump to common conversion pairs.
The formula
Every conversion routes through joules:
Every factor is exact or effectively exact to floating-point precision.
Supported units
Metric SI:
- Joule (J) — SI base unit. 1 J = 1 N·m = 1 W·s.
- Kilojoule (kJ) — 1000 J.
- Megajoule (MJ) — 1,000,000 J.
Commercial electricity (derived from the watt):
- Watt-hour (Wh) — 3600 J (= 1 W × 3600 s).
- Kilowatt-hour (kWh) — 3,600,000 J = 3.6 MJ.
Calorie variants (be careful):
- Small calorie (cal) — thermochemical, 4.184 J exactly. Physics and chemistry use this.
- Food calorie / kilocalorie (kcal, Cal) — 4184 J exactly (= 1000 small cal). Nutrition labels always mean this.
- IT calorie (cal_IT) — International Steam Table calorie, 4.1868 J. Legacy engineering unit.
Imperial:
- BTU — British thermal unit (IT), 1055.05585262 J.
- Foot-pound (ft·lbf) — 1.3558179483… J (= 0.3048 × 0.45359237 × 9.80665).
- Therm — 100,000 BTU = 105,505,585.262 J. US natural gas billing.
CGS:
- Erg — 1e-7 J = 100 nJ. Physics legacy unit, still seen in some astronomy papers.
The calorie mess, explained
The one non-trivial thing about energy conversion is the three different “calories”:
| Unit | Joules | Where it’s used |
|---|---|---|
| small cal (thermochemical) | 4.184 | physics, chemistry |
| food kcal (= Cal) | 4184 | nutrition labels worldwide |
| IT cal (legacy) | 4.1868 | older engineering references, steam tables |
The nutrition convention of using “calorie” to mean “kilocalorie” is entrenched — you won’t change it, and pretending it doesn’t exist leads to 1000× errors. This converter has explicit rows for “small cal” and “kcal” so you can’t accidentally mix them. When someone says “a 300-calorie snack”, they mean 300 kcal = 1.2552 MJ = 0.349 kWh.
Example: food energy to electric equivalent
A 2000 kcal daily diet is:
- 2000 kcal = 8.368 MJ = 2.324 kWh
That’s about what an electric kettle uses to boil 25 litres of water, or what a 100 W bulb burns in 23 hours. Remarkably efficient — the human body runs on less daily energy than most household appliances, and converts a large fraction of it into useful muscular work.
Example: home gas bill
A typical US home uses around 80 therms of natural gas per month during winter:
- 80 therms = 80 × 100,000 BTU = 8,000,000 BTU = 8.4 GJ = 2,344 kWh
At $1.20 per therm that’s $96. At the equivalent electricity rate of about $0.20 per kWh, the same energy in electric form would cost $469 — nearly 5× more. That ratio is why natural gas heating dominates in areas where it’s available, and why electric heat pumps (which deliver 3–4× more heat than the electricity they consume) are the only electric heating strategy that competes.
Example: mechanical engineering
Torque wrench and some automotive specs use ft·lbf:
- 100 ft·lbf = 135.58 J
- 1 kWh = 2,655,223 ft·lbf (a useful check — the number is big because ft·lbf is a small unit)
- 1 J = 0.7376 ft·lbf
Metric equivalents (N·m, used in the rest of the world for torque and mechanical energy) are basically the same size as the joule — 1 N·m = 1 J exactly, by definition.
Example: astronomy
Energy in astronomy is often quoted in ergs because the relevant values are tiny on human scales:
- Human metabolism: ~100 W × 86,400 s × 1 J × 1e7 erg/J ≈ 8.6 × 10¹³ erg/day
- Lightning bolt: ~1 GJ = 10¹⁶ erg
- Solar energy per second: ~4 × 10³³ erg/s
The large powers of 10 are why physicists kept the erg around — multiplying-and-dividing cosmic numbers in joules produces weirdly small prefixes, but in ergs the exponent lands in a comfortable range.
What this tool does not do
It doesn’t convert to eV (electron-volts) or keV, MeV, GeV — those are particle-physics units and would require a separate converter.
It doesn’t handle tons of TNT or kilotons — nuclear yield units that map onto energy but usually appear in their own specialist references.
It doesn’t compute power (energy per time) — power is a different physical quantity and has its own conversion system (W, kW, MW, horsepower, etc.). For the “per time” side of the math, the time unit converter turns hours into seconds and vice versa.