Megaton

A megaton (abbreviation MT or Mt) is a unit of energy approximately equivalent to the energy released in the detonation of 1,000,000 tons of TNT. Used to specify the energy output, and hence destructive power, of nuclear weapons.

A kiloton is one-thousandth of a megaton.

The Little Boy weapon dropped on Hiroshima had a yield of approximately 13 kilotons. The largest nuclear weapon ever detonated was the Tsar Bomba, which had a yield of 50 megatons.

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From The Effects of Nuclear Weapons, 3rd ed., compiled and edited by Samuel Glasstone and Philip J. Dolan, prepared and published by the U.S. Department of Defense and the U.S. Department of Energy, Washington, D. C., 1977. (original at http://www.wws.princeton.edu/cgi-bin/byteserv.prl/~ota/disk3/1979/7906/790613.PDF)

Megaton Energy: Defined strictly as 10¹⁵ calories = 4.2 x 10¹⁵ joule = 4.2 PJ (petajoule). This is approximately the amount of energy that would be released by the explosion of 1,000 kt (1 million tons) of TNT.