Every fiber weighs differently, so the count numbers don't correlate between different types of yarn (darn!). That means an 8/2 wool is a VERY different size than an 8/2 cotton. Following is a chart for the (a) standard number of yards per pound in a size-1 strand:

Cotton, Rayon: 840

Linen, Hemp: 300

Woolen spun wool: 256

Worsted spun wool: 560

You can use this chart and the yarn's count to determine the number of yards per pound of any particular yarn. You simply multiply this 'magic number' by the yarn's actual count number to get the number of yards per pound. For example, a pound of size-1 cotton would give you 840 yards of yarn (840 x 1); and a pound of 8/1 cotton would be 6,720 yards of yarn (840 x 8). What about plys, you ask? Easy! Simply multiply by the count, then divide by the number of plys. That means one pound of 8/2 cotton would have half as many yards as a pound of 8/1 cotton.

The same process works for all other fibers, you just have to substitute the correct number for the type of fiber that the yarn is made of. Which brings up the next complication... Fiber mixes. What if you have a yarn that is part cotton and part linen? (or any other mix) Unfortunately, the correct number to use will differ by manufacturer. Some will use the count number for one of the fibers in the mix, others will use a new count number that falls somewhere between the two fibers. It is helpful to ask (or look at the yarn in person) if you are unsure of the actual size of a mixed yarn. The Swedish cottolin (cotton-linen blend) that we carry from Bockens is labeled 22/2, but it is actually very close in size to an 8/2 cotton and can be substituted for it in any draft.

Next complication! Some countries will swap the count and the ply order, or just swap it for some types of fiber (in the US we tend to swap the order on wool yarns). In Canada, an 8/2 cotton would be called  2/8 instead. Pay attention to the context and the correct meaning is usually clear (an 8-ply, size-2 yarn would be rope, and not good for dishtowels).