Section 1: Mass
To find out the information here, I picked the top 10 most popular yarn of each weight on Ravelry. I found out how many meters were in a skein and how many grams each skein weighed. I then found out the meters per gram (m/g) of each, and then averaged them (added all 10 together then divided by 10). I also found the grams per meter (g/m) of each as well. This is what I found:
|weight||meters per gram||grams per meter|
I left out some of the less common ones, because you can guess what weight it is if it's in between. Of course, your best bet would be to check what kind of yarn it is to begin with. If you have mystery yarn, you can use Math for Knitters: Part 1 (The Math) to find out more information about the yarn you have.
Not all yarns will fit exactly in with these numbers. These are guidelines only, and can vary greatly depending on the material used. If I have a pound of feathers and a pound of bricks, one will take up more space. If I spun the brick and the feathers into yarn that knits up at the same gauge (so I had 1 lb of brick worsted and 1 lb of feather worsted), their meterages would both be very different. Your best bet is always to go with the gauge information as prescribed by the CYC (below).
Section 2: Gauge
If you want gauge information, I suggest checking out Craft Yarn Council Standard Yarn Weight System. I won't republish their information, because it's super easy to find (following the link I just gave you).
I hope you find this post useful! Again, any feedback you have, please let me know!