How this calculator works
Energy is the capacity to do work, and the joule is the SI unit: one newton-metre. Calories, watt-hours, Btu, electronvolts, ergs and tons of TNT are all alternative units for the same quantity, each with an exact or conventional definition. The readout converts everything through the joule, so the order of the units does not matter. The three extra lines use the mass-energy relation, the photon energy relation and the thermal energy relation, which are equivalences rather than energy conversions in the usual sense.
Formula
joules = entered value x F(unit). 1 cal = 4.184 J (thermochemical calorie). 1 Btu = 1,055.05585262 J (International Table Btu). 1 Wh = 3,600 J. 1 eV = 1.602176634 x 10^-19 J. 1 erg = 10^-7 J. 1 therm = 100,000 Btu. 1 ton of TNT = 4.184 x 10^9 J. Equivalent mass = E / c²; photon wavenumber = E / (h c); temperature = E / k, with c = 299,792,458 m/s, h = 6.62607015 x 10^-34 J s and k = 1.380649 x 10^-23 J/K.
Worked example
1 kWh is 3,600,000 J, 860.421 kcal, 3,412.14 Btu and 2.247 x 10^25 eV. 1 eV is 1.602176634 x 10^-19 J, the energy gained by one electron across one volt. 1 kg of mass is equivalent to 8.987551787 x 10^16 J.
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Frequently asked questions
Is the calorie here the food calorie?
No. The small calorie (cal) is 4.184 J. The food calorie is the large calorie or kilocalorie (kcal), which is 4,184 J, and it is the value on nutrition labels.
Which Btu is used?
The International Table Btu of 1,055.05585262 J. Other Btu definitions differ by a few parts per million, which matters only in precise engineering work.
Is the mass readout something I can use for fuel?
It is the mass that would be converted completely to energy. Ordinary burning releases only a tiny fraction of a material's rest energy, so the number is not the fuel mass needed.