The energy release by the bomb is given by the total mass ie tamper plus fission material divided by 2 and multiplied by the velocity of expansion squared. This velocity is the expansion to second criticality divided by the characteristic expansion time. For example the in the Nagasaki bomb the Plutonium was 6.2 Kg and the tamper 108 Kg. The plutonium was compressed to about 2.2 times it's alpha density making it critical. The charateristic time was estimated as 33 ns. the plutonium expanded from 3.27 cm to 4.14 ie 0.87 cm in 33ns . This yielded 8.6 x 10^20 ergs and at say 4 x 10^20 ergs per 10Ktons of tnt is 21.6 Ktons. 1055 gms underwent fission and an efficiency of 17%. Total life time of the explosion was about 580ns. Mass converted to energy, just under1 gm.

The energy release by the bomb is given by the total mass, ie., tamper plus fission material divided by 2 and multiplied by the velocity of expansion squared. This velocity is the expansion to second criticality divided by the characteristic expansion time. 

For example in the Nagasaki bomb, the Plutonium was %6.2% kg and the tamper $108$ kg. The plutonium was compressed to about 2.2 times it's alpha density making it critical. The characteristic time was estimated as $33$ ns. The plutonium expanded from $3.27$ to $4.14$ cm, i.e., $0.87$ cm in $33$ ns.

This yields $8.6\times10^{20}$ ergs and at, say $4\times 10^{20}$ ergs per $10$ kilotons of TNT is $21.6$ kilotons.

$1055$ g underwent fission and an efficiency of $17\%$. Total life time of the explosion was about $580$ ns. Mass converted to energy, just under $1$ g.