Energy Converter

Enter an amount of energy in joules, kilojoules, megajoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, foot-pounds or electronvolts, and every other unit updates at once. Everything routes through joules, so no conversion compounds on another.

The calorie is the unit most likely to bite, because the word means two different things a thousand apart. A calorie in physics is the energy to warm one gram of water by one degree; the Calorie on a nutrition label is a thousand of those, and the same number appears on packaging in some countries as a kilocalorie. A 250-Calorie snack is 250 kcal, 250,000 small calories, or about 1,050 kilojoules — which is why European labels print both kJ and kcal and why the two figures on a packet differ by roughly a factor of four rather than being a mistake.

The calorie has a second problem: there is more than one of it. The thermochemical calorie is defined as exactly 4.184 joules, the international steam-table calorie as 4.1868, and a handful of others sit between them, because warming water takes slightly different energy depending on the starting temperature. They disagree by about 0.07 percent, invisible in nutrition and occasionally relevant in engineering. The BTU has the same family problem for the same reason.

The rest of the table is exact by definition and spans an enormous range. A kilowatt-hour is exactly 3.6 megajoules, a definition rather than a measurement, which is what makes electricity billing straightforward. An electronvolt is 1.602176634 × 10⁻¹⁹ joules exactly, fixed when the SI redefined the elementary charge in 2019 — so this table reaches from the energy of a single photon to the daily output of a power station in the same row of boxes. A foot-pound is the odd one out culturally rather than physically: it is a unit of energy with the same dimensions as the torque unit that shares its name, which is why engine specifications carefully write one as lb·ft and the other as ft·lb.