Hi, and welcome to KH.
This isn't a technically correct answer, but I would start by looking at the gauge given in the pattern and see what I have or is available for purchase that would give me the same number of stitches over the inches or centimeter specified. Worsted weight yarn is pretty thick and worked as a single strand gives 4-5 stitches per inch. Here's a Yarn Comparison Chart
that might be helpful.