Loom FAQS: What is WPI and Yarn Weights?

The one thing all yarn arts have in common is the yarn itself.  And what a variety of yarn there is!  All the different weights or thicknesses of yarn can be overwhelming.  It can be confusing as to what can be made with certain weight yarns if you have never used it before.  Questions always abound when it comes to yarn weights.  What is yarn weight?  Which weight yarn do I use?  How many strands do I use to equal a heavier weight?  Is 4ply and worsted the same?  What in the world is WPI???

Let’s start at the beginning.

What is yarn weight?

Well first of all, yarn weight has nothing to do with the net weight of the hank, skein, or ball of yarn.  It has nothing to do with the yardage either.  When yarn weight is discussed, it is referring to the thickness or diameter of the yarn.

Here in the USA, yarn is labeled by a number system to differentiate between the different thicknesses of yarn although there are other terms or common names associated with those weights as well.  The yarn weight can be found on the label of the yarn.  It is a symbol of a yarn skein or ball with a number on the label like this one.

Not all yarns will have this since it depends on where the company is actually located.

Yarn weight is actually determined by the wraps per inch or WPI instead of the actual diameter of the yarn.

What is WPI?

WPI stands for Wraps Per Inch.  You can easily determine the weight of “mystery” yarn you have in your stash that has lost its label, handspun yarn you either spun yourself or bought, or mill ends by counting how many wraps are in an inch.  There are different ways you can do this.  You can buy a WPI tool or just use a pencil and use a ruler to measure.  Or you can just use the ruler to wrap the yarn and measure at the same time.

The WPI tool is a very neat tool that has the inches marked on the round stem with a notch at the end to hold the yarn so you can wrap the stem.  It usually comes with a card that has the instructions on one side and the yarn weights with WPI on the other.  Very handy but not necessary.

You can use a pencil or pen to do the same thing.  Then measure and count the wraps in an inch by using a ruler.  In the picture below, I wrapped more than an inch and started counting from the second wrap until I reached the 1″ mark.  There are 9 wraps in an inch.

You can also just use a ruler to wrap the yarn around.  The problem with this method is the possibility of twisting the yarn while wrapping which will stretch it so care is needed when using just a ruler.  I would recommend starting at the 1″ mark and wrapping to the 2″ mark on the ruler instead of starting at the end of the ruler.  If you start at the end, it is harder to keep the end wraps from falling off the ruler.  As you can see below, it is harder to read the marks on the ruler when the yarn is wrapped on it instead of a pencil.  There are 6 wraps in an inch.

When you wrap the pencil or ruler, you need to make sure the yarn is not pulled tight or pushed together.  It needs to be relaxed.  You just roll the pencil or turn the ruler to wind the yarn on whichever you are using.  Rolling instead of just wrapping will keep the yarn from being twisted which will cause the yarn to pull tighter and be thinner than it actually is.  Do not pull on the yarn at all.  Tension will stretch the yarn and cause it to be thinner than it is.  Let each wrap rest next to the previous wrap without being pushed together.  If the yarn has a halo, like mohair, you will need to give the yarn more room between wraps for the “fuzzy” hairs to expand.  This is why mohair yarn always has a heavier weight than it would appear to be.  It is not measured by just the diameter of the yarn but also how far the halo extends as well.

Count the number of times the yarn is wrapped around for 1 inch.  Some instructions will say to wrap 2 inches, count the wraps, and divide by 2.  This is not necessary unless you are measuring yarn with “character” like a thick and thin yarn.  Then you would need to wrap 3 inches and divide by 3 to get a good count.

The number counted in 1 inch is the WPI.  Then compare that number to the chart below to find your yarn weight.

What are the different weights of yarn available?

Since the new weight classification has been added, there are now 8 different categories in the USA yarn weight system.  Some of the common names overlap depending on location and how it was taught.  Please note that some people will include aran as a bulky weight yarn.  While it is thicker than worsted, aran is still included in the medium weight category due to it’s WPI.  Also, Caron Simply Soft and Red Heart Boutique Unforgettable are considered medium weight as well even though both are thinner which can make the entire medium weight category confusing for some people.

Weight #                          Common Name                                       WPI

0 – lace                           Cobweb/Thread/Lace/Sock                  23 and greater

1 – super fine                  Lace/Sock/Fingering                              19 – 22

2 – fine                            Baby/Sport/Lace                                    15 – 18

3 – light                           Sport/DK/Baby                                        12 – 14

4 – medium.                    Worsted/Aran                                           9 – 11

5 – bulky                          Bulky                                                        7 – 8

6 – super bulky.               Super Bulky                                              5 – 6

7 – jumbo                         Jumbo                                                      4 and less

Why are the weights different other places?

Not all countries use the same names for yarn weights.  It can sometimes get confusing since the internet makes the world smaller.  There are pattern writers all over the world that use the yarn classifications of their country.  I have bought yarn from the UK on several occasions.  You will need to know what the names of the yarn weights are so you can buy the correct yarn.  For example, 4 ply is NOT 4 weight yarn.  It is a lot thinner.  Yarn weight is not determined by the number of plies.  And that leads us to our next question…

What are the yarn weight equivalents between the USA and UK?

USA Weight                  UK Term

0 – lace                            1 – 3 ply

1 – super fine                  4 ply

2 – fine                             5 ply

3 -light                             DK/8 ply

4 – medium                     Aran/10 ply

5 – bulky                          Chunky/12 ply

6 – super bulky              Super Chunky

7 – jumbo                        unknown

What weight yarn do I use with certain looms?

A lot of times, the weight of yarn you use with certain looms will depend on the stitch pattern or the way you want the finished project to look.  Each person has their own idea of what is too tight or too loose.  Tension is a factor as well since each person’s tension is different.  It is sometimes hard to say what weight yarn is best for each gauge loom, but it can be helpful to have a starting place until you learn which is best for you.  If you are unsure what gauge loom you have, you can learn more about gauge here.

Extra fine gauge – lace/super fine

Fine gauge – super fine/fine/light

Small gauge – light/medium

Regular gauge – light/medium/bulky

Large gauge – bulky/super bulky/jumbo

Extra large gauge – super bulky/jumbo

How many strands will equal a heavier weight?

Needing a heavier weight yarn than you have in your stash?  All you need to do is use more than 1 strand.  But how many strands will equal what you need?

If you use 2 strands of yarn, it will be equivalent to the next heavier weight.  So if you have a 4 weight yarn and need a 5 weight yarn, use 2 strands together as 1.

Here is an easy way to see what you need:

2 strands of 1 weight = 1 strand of 2 weight

2 strands of 2 weight = 1 strand of 3 weight

2 strands of 3 weight = 1 strand of 4 weight

2 strands of 4 weight = 1 strand of 5 weight

2 strands of 5 weight = 1 strand of 6 weight

2 strands of 6 weight = 1 strand of 7 weight

But what if I only have 4 weight and need a 6 weight?  Since 2 strands of 4 weight equals 1 strand of 5 weight and 2 strands of 5 weight equals 1 strand of 6 weight, then 4 strands of 4 weight will equal 1 strand of 6.  3 strands of 4 weight will be a heavier 5 or a lighter 6 weight yarn.

Hopefully this will help in trying to decide which yarn weights are best for which looms and for finding out what weight that mystery yarn is that keeps getting pushed aside.

Keep asking questions!  Questions lead to answers, and answers lead to knowledge.