Yarn Held Double - Needle Sizing

Hey everyone!
This is my first post, and I’m hoping someone can easily answer my question! I’ve tried searching but can’t find what I’m looking for.

I’m knitting a project that I hold the yarn double… now according to the information on the yarn it recommends size 10 (6mm) needles.

My question is, if I am holding the yarn double, do I still use size 10 needles? or what would be best? what is the rule of thumb here?
How about if I was to hold it triple?

I hope someone can help me!

The rule of thumb for doubling yarn (and I have to say I’ve never tried this) is to add the recommended needle sizes in mm together. So for size 10 (6mm) that would be 12mm or size 17. I don’t know if you can scale up for triple the yarn (~size 35). That’s really getting up there!

If you double the yarn you have to make the needles larger or it’s going to be too thick and stiff. A size 17 or 19 would be good.