How strongly does working memory predict IQ?

Working memory capacity correlates with fluid intelligence at roughly r = 0.5 to 0.7 when measured with several tasks, one of the strongest links in cognitive psychology. The two are not the same thing: a 2005 meta-analysis put the correlation closer to 0.5 and argued against treating them as identical. Working memory also predicts school achievement independently of IQ.

Source: Kane, Hambrick & Conway (2005), Psychological Bulletin; Ackerman, Beier & Boyle (2005), Psychological Bulletin