The following are approximate equivalents; however, as with all substitutions, you should check to make sure you're obtaining gauge.
2 strands fingering = one strand sportweight
2 strands sport = one strand worsted weight
2 strands worsted = one strand chunky to super bulky weight*
*2 strands of a lighter worsted yarn held together may approximate the thickness of a chunky yarn, while 2 strands of a heavier worsted weight yarn held together may approximate the thickness of a super bulky yarn. Because of this range, again, it is recommended you should make a gauge swatch to test.
(From the Lion Brand Website)