Energy Unit Converter

Convert between joules (J, mJ, kJ, MJ, GJ), watt-hours and kilowatt-hours (W·h, kW·h, MW·h, GW·h — the electrical-engineering and utility-billing standard), the thermochemical and International-Table calories and kilocalories (cal, kcal — the food-energy convention), the BTU in both flavours (th and IT), electron-volts and their keV / MeV / GeV multiples, the CGS erg, the mechanical ft·lb, and tons of TNT equivalent (t, kt, Mt — the nuclear-yield and explosive-energy convention). 24 units with NIST SP 811 / ISO 31-4 / CODATA 2018 factors, round-trip stable. Type a value, pick a source unit, see every other unit at the same instant. Nothing leaves your browser.

About the units
  • J, mJ, kJ, MJ, GJ — SI prefixes of the joule. The joule is the SI unit of energy, one newton-metre.
  • W·h, kW·h, MW·h, GW·h — watt-hours. 1 W·h = 3600 J exactly. kW·h is the utility-billing standard; a typical European household uses ~10 kW·h per day.
  • cal (th) — the thermochemical calorie, exactly 4.184 J. This is the older chemistry convention.
  • cal (IT) — the International-Table calorie, exactly 4.1868 J (ISO 31-4). Differs from the thermochemical calorie by 0.0028 J, or about 0.067 %. Modern food-energy labels use kcal (IT).
  • kcal (th), kcal (IT) — food "Calorie" = 1 kcal. A banana is about 100 kcal (IT); an adult uses ~2000 kcal (IT) per day.
  • BTU (IT) — the IT British thermal unit, exactly 1055.05585262 J. The North American HVAC standard.
  • BTU (th) — the thermochemical BTU, 1054.3502644888888 J. Differs from BTU (IT) by ~0.07 %.
  • eV, keV, MeV, GeV — electron-volt, the energy gained by one electron crossing one volt. Defined by CODATA / SI as exactly 1.602176634e-19 J. keV dominates atomic and X-ray physics; MeV and GeV dominate nuclear and particle physics.
  • erg — the CGS unit, exactly 1e-7 J. Common in older physics literature and in astrophysics.
  • ft·lb — foot-pound-force, 1.3558179483314004 J exactly. The mechanical-engineering and ballistic-energy convention.
  • t TNT, kt TNT, Mt TNT — ton of TNT equivalent, exactly 4.184e9 J per ton. The nuclear-yield and explosive-energy convention. The Hiroshima bomb was ~15 kt TNT (6.3e13 J); the largest thermonuclear weapon tested was the Tsar Bomba at ~50 Mt TNT (2.1e17 J).

Not shipped: "large calorie" / "small calorie" ambiguity (the food "Calorie" is kilocalorie, capital C; this lib uses kcal throughout). Conversion is round-trip stable to within 1 ULP of IEEE-754 precision. The ~0.067 % gap between thermochemical and IT calories / BTUs is the IUPAC / ISO specification, not a numerical artefact.