Speed of light squared (mass-energy conversion factor)
| Value | 8.987551787368176e16 J/kg |
| Status | Exact by definition — no uncertainty |
| Source | Derived from the exact SI value of c (SI Brochure, 9th edition, 2019) |
| Categories | Universal & Atomicphysicsrelativity |
| joule per kilogram | 8.9875518e+16 J/kg |
| kilojoule per kilogram | 89,875,518,000,000 kJ/kg |
| calorie per gram | 21,480,764,000,000 cal/g |
| BTU per pound | 38,639,518,000,000 BTU/lb |
Learning zone
Squaring an exact constant leaves it exact: c² = 89 875 517 873 681 764 J/kg, with no uncertainty whatsoever. It is the slope of the mass-energy line, and its size is the whole reason nuclear processes dwarf chemical ones — burning a kilogram of coal releases about 3 × 10⁷ J, while annihilating a kilogram of anything releases 9 × 10¹⁶ J, three billion times more.
Nothing engineered converts mass completely. Uranium fission liberates roughly 0.09 % of the fuel's rest mass, hydrogen fusion in the Sun about 0.7 % — enough for the Sun to shed 4.3 × 10⁹ kg of mass every second as light. The same factor works in reverse and in miniature: the mass defect of a helium nucleus, a mere 0.5 % of its constituents, is what a binding-energy calculation is really weighing.