Peckish gave us a resource for superior threads but I don't know how to find it so I'll give you what I copied out.
Thread Weights:
A=20 miles weighs 1 lb.
B=40 miles weighs 1 lb. (labelled 40 wt. and finer)
C=60 miles weighs 1 lb. (labelled 60 wt. and finer yet) hopes this helps