Yarn weights and gauges are given in Knitspeak on Pages 202 and 203,
but there is a measure that was not included in the book that is used
by spinners and weavers, and more
lately, on the Ravelry.com site to measure yarn thickness so yarns can
more easily be compared. That measurement is Wraps Per Inch, or WPI.
As far as I know, there is no standard (yet) for WPI equivalents to the
yarn weights as defined by the Craft Yarn Council of America, so Iíve
included information from a few different sources so you can decide which
will meet your needs.
To get wraps per inch, wind yarn around a pencil, dowel, or ruler so
that the wraps lay next to each other -- but not so tightly that the
yarn is stretched. Count the number of wraps around the pencil in one
inch (count the number of wraps in two inches and divide by two for a
more accurate reading). Mark off inches (or cm) on the edge of your copy
of the pattern so that you don’t need to haul a ruler or tape measure
around all over the place.