What is the maximum yield of any existing nuclear weapon?

Answer 1

Strategic, rocket launched, nukes carry yields ranging from 250KT up through 1 MT (or thereabouts...it's been a while since I was in the business). As I recall, the largest nuke device, not a weapon, was about 10 MT. The trend over the years has been to have smaller yields but pack in more warheads (reentry vehicles).

Strategic nukes have considerably more yield than tactical nukes for several reasons. Strategic targets are typically larger and/or tougher to destroy; so a bigger bang is needed. A bigger bang is also needed because strategic targets are typically much farther away than tactical targets. The bigger bang compensates in part for the increased chance of missing the strategic target.

Answer 2

The yield-to-weight ratio is the amount of weapon yield compared to the mass of the weapon. The theoretical maximum yield-to-weight ratio for fusion weapons is 6 Megatons per metric ton (6 Mt/t). [2] The practical achievable limit is somewhat lower. For current US weapons 600 kt/t (2.5 TJ/kg) to 2.2 Mt/t (9.2 TJ/kg). By comparison, for the Davy Crockett it was 0.4 - 40 kt/t (0.002 - 0.167 TJ/kg), for Little Boy 4 kt/t, and for the Tsar Bomba 2 Mt/t (8 TJ/kg) (deliberately reduced from the possible maximum which was twice as much), and for the Mk-41 5.2 Mt/t.

The largest pure-fission bomb ever constructed had a 500 kt yield, which is probably in the range of the upper limit on such designs. Fusion boosting could likely raise the efficiency of such a weapon significantly, but eventually all fission-based weapons have an upper yield due to the difficulties of dealing with large critical masses. However there is no known upper yield limit for a fusion (e.g, hydrogen) bomb. In principle a fusion bomb could be many thousand megatons. Because of the maximum theoretical yield-to-weight ratio is about 6Mt/t, and the maximum achievable ratio about 5.2 MT/t, there is a practical limit on air delivery of the weapon.