Fly rod weights correspond to the fly line they are rated to throw, similar to spinning rods lure weight rating. Fly lines should weigh close to the attached chart, but just like spinning rod power ratings (UL, L, ML, etc.) there is no true industry standard. A heavier fly line can throw bigger, more wind resistant flies, but also requires a heavier fly rod with more power.
Really rude and crude method, but for a general idea you can use the rule of 16. Divide the fly rod weight by 16 and that is the max lure weight rating for a similar spinning rod.
3 wt = 3/16 oz; Ultralight power spinning rod
5 wt = 5/16 oz; Light power spinning rod
6 wt = 6/16 oz (3/8 oz); Medium-Light power spinning rod
8 wt = 8/16 oz (1/2 oz); Medium power spinning rod
The conversion gets less accurate as the weights get higher.
Attachments:

fly-line.jpg