Handy Hint: Wraps per Inch

If you substitute or spin a yarn for a project, you can compare the weight of the yarn to the project yarn by comparing wraps per inch. To do this, wrap your yarn around a ruler for on inch and count the number of wraps. If you have more wraps per inch, your yarn is too thin; fewer wraps per inch, your yarn is too thick.

Taken from: http://www.knittingdaily.com/knittinghelp/techniques.html