For the meat and potatoes see this post from arms control wonk.

The dietary supplement is this. Multiply kg on hand (that isn’t converted to fuel plates which have to be reprocessed) by the square of the enrichment for “effective kilograms.” If this number is less than 28 there is not enough for a bomb.

Iran has refined 232.8 kg of uranium to the 20% level. They have converted 95.5 to fuel plates leaving 137.3 kg available. 20% squared is .04. That gives us 4.492 effective kg, or a bit less than 1/6 a significant mass (approximate amount required to go critical.)

Iran will not have the bomb (from its own production) in six months.