Yarn Weights and what is WPI?

Most crocheters have heard the terms worsted weight, and many in the US understand (4) or (5) on a package of yarn.  But what does yarn weight mean?

Yarn weight is not how many ounces it weighs but how many "Wraps Per Inch" (WPI) it has.   You can determine a yarn's weight using a ruler and wrapping the yarn around the ruler and determining how many wraps it takes to make an inch.  The width of the ruler is irrelevant, because you are not measuring length, but measuring the width of the yarn.  So, if you have yarn that you have no idea what it is or what weight it is, grab your ruler!

The WPI charts vary around the world; which can make things even more confusing.  And even more frustrating is commercial yarns are not universal either.   Some yarns state worsted weight, when they are clearly lighter weight than other worsted weight yarns.  What adds even more confusion is the "light worsted weight" and "heavy worsted weight" designations some yarns state.

The Yarn Wench has a great chart http://www.yarnwench.com/php/WPITable.php that is helpful.  I also like the information at Knit Picks, because it matches commercially available yarns (most of the time).  http://www.knitpicks.com/tutorials/Yarn_Weights__D64.html

So, what do you do when you want to substitute yarns, and the "yarn weights" are not universal.  One really easy trick is to match the suggested hook or needle size.  So if it recommends a 4.5 mm (#7) then you would find another yarn that recommends the same size needle or hook.  This can be beneficial when wanting to substitute in a pattern.  If the pattern calls for Homespun yarn, you know that it recommends a K hook.  So you search for yarns from your stash that use a K hook.

It is really important to not substitute a yarn that matches the recommended hook size in the pattern!  Many patterns are designed to be worked really loose by using a much larger hook than the yarn would normally use, or very tight, by using a slightly smaller hook.  So be careful not to match the hook size recommended in the pattern, but the size recommended by the yarn itself.   Lets look at an example:

Pattern calls for Knit Pick's Shine Sport Cotton blend yarn.  The pattern uses a J hook.  You don't have any Shine Worsted Cotton and you need to find a substitution.  You go to Knit Picks website,  http://www.knitpicks.com/yarns/Shine_Sport_Yarn__D5420122.html, and you notice that the yarn recommends E hooks.  So, you then look at your yarn stash for a yarn that recommends an E hook.  If you tried to match with a yarn that uses a J hook, your work would not match at all.

Again, we have to use the tools we are given, and not all of our tools are specifically helpful.  However, we can get better at yarn substitutions by thinking a little outside of the box.

One website that does a GREAT job at yarn substitutions is LionBrand.  They have a great substitution chart that shows want yarns will substitute for others.  Mind you it is only their yarns they discuss, but it may be a helpful tool for you!  Lion Brand Yarn Substitutions

Below are some further helpful links that I have used as resources.

http://www.ravelry.com/help/yarn/weights
http://www.craftyarncouncil.com/standards.html
http://www.craftyarncouncil.com/weight.html