How do I use different yarn than what the pattern calls for?

Ok, stupid question but I already found out that I get answers in this joint!!

I want to make a tank top that calls for a cotton blend yarn out of a ribbon yarn…is there a formula for converting different yarns to fit the pattern guage or do I have to hurt my brain trying to figure it out?
Help!

Thanks so much!!

Dain

p.s. I finally finished my first sweater and couldn’t have done it without the help of the good folks here! (will post pic. soon)

Try this.
It is a bunch of different knitting calculators. One of them should be able to help!

Now see, that’s what I’m talking about! I had barely hit the “send” button on my question and a reply already came in! You rock!

Wow, that is an awesome site-I hope it’s posted for others to use-it was exactly what I needed!!! Thanks Holly!

Dain-still trying to figure this kitting thing out!