How do you figure out the weight of recycled yarn?

I’m thinking of making a felted bag with some recycled yarn I dyed but I don’t know what weight it is, I know I’ll need to double it (it isn’t very thick) but I want to make sure to use the right needle size to get a good felted result. Any suggestions? Or maybe someone could tell me how loose the stitches are supposed to look?

The best thing to do is the wraps per inch (WPI) method. There are a few good places online to check this out. This one has a simple way of explaining it along with a simple chart to work out your needle sizes.


Edit: don’t forget to felt a test swatch first, you don’t want to do all that work and find that your lovely recycled yarn won’t felt.