How to Choose the Right Yarn for a Sweater

Owning a yarn store allows me to have a lot of conversations about knitting sweaters (and vests) in particular. Often I’m asked about which yarn to pick. I used to have simple answers, but as time goes by, my advice has become more nuanced and it involves a lot more questions for the maker.

Field Slipover designed by Camilla Vad
West Yorkshire Spinners Fleece Bluefaced Leicester in the color Fellside

 

These questions are more than yarn weight and color. It starts with what do you want from a sweater. How do you want it to function? How long do you want it to last? How well do you want it to wear? How warm should it be? Do you want it to be machine washable? Not all yarns will answer these questions and no yarn can be all things. There is always a compromise. But, in answering these questions, you can make better, more informed decisions about the yarn and construction methods you choose. I will leave aside talk about how to pick a pattern, this is just about how yarn choice will affect the nature of the resulting sweater or any garment for that matter. 

What do you want from the sweater?
Is it a utilitarian garment (which does not mean unattractive or unfashionable) meant for regular use or meant to last for a long time? Is it a decorative garment? Under what conditions will you wear this garment: warm weather, cool weather, layering? Maybe you want a cardigan that can button up tight at the neck so it can provide some warmth when open or more warmth when fully buttoned up. Maybe you want a cute little top that can be worn in spring and fall or even in summer.

How warm do you want it to be?
Several factors come into play here. If you want warmth consider one or more of these options: a heavier weight yarn; a woolen spun or blown yarn, a non-superwash wool, blends with alpaca, yak, and cashmere, or a wool yarn with a mohair or suri lace yarn carried with it. Woolen spun and blown yarns trap more air than a worsted spun yarn (see our yarn glossary for definitions). 

If you want a cooler sweater consider these options: a lighter weight yarn, a worsted spun yarn, a superwash wool yarn, wool blends with plant based fibers, or yarns made entirely from plant based fibers or silk. 

How well do you want it to wear?
This is more than taking care of our sweaters or how often it will be worn. Will it be prone to pilling? Will it be saggy and lose its shape? These are questions about durability. There are a number of factors that go into making a sweater more durable. The main factors are a durable yarn worked at a fairly tight gauge.

So what makes a durable yarn? Here the news is tough for the folks who love soft yarns. Merino is a very popular and common wool fiber because it is soft. The softness comes from how fine the fiber is. We measure fiber diameter in microns (one thousandth of a millimeter) and merino fibers are usually 19-22 microns in most commercial yarns. Many people find wools with micron count in the mid to high 20s to be too coarse for skin contact. For people who are less sensitive this starts in the low 30s. As you can see it is a fairly small range. For comparison, human hair averages 70-120 microns.

Aside from softness, merino is not always a good choice for sweaters. Merino has a short staple length for wool (staple refers to the length of each individual fiber); merino is usually 2.6-3.9 inches long (6.5-10cm). Shorter fibers are more prone to pilling.

There are other soft wools that perform better than Merino due to a longer staple length, a larger micron count, or both. These wools aren’t as common, but demand is growing. See our article on Sheep Breeds for more information on wool and specific wool breeds.

The wool is just the first part. How the fiber is processed and how the yarn is spun and plied makes a difference too. Most yarns are made with wool that is either carded or combed. Combing wool removes short bits of fiber and aligns all the fibers so they are parallel. The yarns made from combed fibers are denser, more abrasion resistant, less likely to pill, and have a smoother appearance. Carded wool is blended in a disorganized way. This traps more air in the yarn making it warmer. It also makes a yarn that weighs less, has a more rustic texture, and greater yardage due to its lighter weight.

How a yarn is spun changes its qualities. A soft fiber that is overspun or spun firmly will lose some of its softness. This has led some yarn manufacturers to underspin their yarn to increase the perceived softness. The downside of underspinning is that this causes greater pilling and garments that wear out faster because of lower abrasion resistance due to being underspun. If the yarn is more tightly spun it will feel less soft, but wear better and pill less.

Then you need to factor in how you use the yarn. Garments wear longer when they are knit at a tighter gauge. If you take a fingering weight yarn that knits at 7-8 stitches per inch and knit it at a DK gauge of 5.5 stitches per inch, you are making a less durable sweater. It is going to pill faster, snag more easily, lose its shape more readily, and have a shorter lifespan. If you take a slightly less soft yarn that is more firmly spun and knit it at its intended gauge, you will get a 15 year sweater. If you use a coarser, firmly spun yarn knit at tight gauge you will get a 30+ year sweater. Think of traditional fisherman’s sweaters worn every day on fishing boats. 

Ranunculus designed by Midori Hirose
Speckle & Friends Wisp in Tequila Sunrise colorway held with
Black Squirrel Yarns Undyed 4-ply Fingering

Ranunculus close-up: you can see that the gauge is loose. This makes for a light, ethereal, and drapey sweater. It is perfect as a light layer or warm weather top. 

Ærø Icelandic Sweater designed by Hanne Rimmen
Main Color: Full Belly Organic Worsted in natural white
Contrast Color: Queensland Brisbane in the color Triggerfish

Ærø close-up: the gauge is tighter; you can't see through the stitches. This yarn is more rugged and knit as a denser gauge making a longer lasting and more durable sweater. It is ideal for outer wear.

 

Do you want it to be machine washable? 
Sometimes the answer to this is obvious. If it is for a baby, we often recommend washable wool or cotton. Baby garments get dirty often and washed regularly. Expecting new parents to handwash and lay flat to dry is a big ask.

What is superwash (a.k.a. treated) wool and why does it matter?
First, let’s understand the structure of wool. Each strand of wool is covered in small scales. These scales have benefits. They trap air making wool garments warmer, they cause the wool to have a high surface area making wool capable of holding more water while remaining warm, and the scales hold onto each other giving body, memory, and elasticity to garments. Untreated wool is also dirt repellent, water repellent, anti-bacterial, and antimicrobial.

The scales also present problems. Under the right conditions, a combination of heat, agitation, and soap, these scales open and interlock shrinking the garment. This process is called felting and it cannot be reversed. Once a wool item is felted it won't return to its original shape.

CSIRO, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Common

The superwash process either chemically removes the scales, uses a plastic resin to clog the scales, or uses a combination of these two processes. Typically both processes are used. Chlorine compounds are usually used to strip or dull the scales. The second step is to coat the wool fibers in a petroleum based resin. This process is not sustainable or eco friendly. Not only does it create toxic waste not accepted by wastewater treatment plants in many countries, but the process is energy intensive. Because of this, most yarn is made superwash in China and other parts of the world with less stringent environmental laws.

Superwash wool behaves differently. The benefits are a smoother surface that takes colors very well and is machine washable while having great drape and a softer and smoother feel. It will show off lace details beautifully. The downsides of superwash wool is that it greatly diminishes many of wool’s natural qualities like elasticity, its natural warmth (both dry and wet), and its ability to hold shape. When working with superwash wool, it needs to be worked at a tighter gauge to keep its shape, it is less crisp for cables and texture patterns, and will have more gaps in stranded colorwork. In addition to the environmental impacts, the added processing and shipping makes it more expensive, and it will shed microplastics.

 

What about non wool options?
Wool isn’t the only game in town. There are plant fibers like cotton, linen, and hemp that are well suited to warm weather garments. They are usually machine washable, but tend to sag and are cold when wet. Bamboo, Tencel, and other similar plant fibers are not natural fiber. These are rayons manufactured from cellulose from bamboo, wood pulp, or other plant matter. These rayons tend to be very shiny and have tremendous drape, but will continue to grow in length over time.

There are also non-wool animal fibers like alpaca, llama, silk, mohair, camel, yak, and cashmere. Camel, yak, and cashmere are expensive and often used as a smaller percentage in a yarn. Their properties won’t govern, but will add softness and warmth. Alpaca, llama, and silk will add softness, warmth, and drape. Garments made from these fibers will not hold their shape, must be hand washed, and will grow in length. Brushed mohair is not just about texture, it adds warmth and softness while providing some structure. It can be a great addition to superwash wool or other fibers which have less memory. We have also noticed that garments made with a fluffy yarn tend to pill less. Perhaps because the fluffiness protects the main yarn from abrasion and therefore from pilling. This is a recent observation. Have you noticed this? It is something we are watching.

Flax Worsted designed by Tin Can Knits
Juniper Moon Farm Santa Cruz in the color Fandango

 

Picking the right yarn
Clearly there isn’t a simple or single answer. It is about weighing the options while considering the needs of the pattern and the wearer of the sweater. The answer can be different for every sweater. Knitting for a baby, college student, or someone who likely will not handwash? Pick a superwash wool, Easy Care wool, or cotton. Want a warm cozy sweater? Consider woolen spun or blown yarns, adding a fluffy yarn, or a blend with warm fibers like alpaca or yak. Want a structured sweater? Use a worsted spun, untreated wool, worked at a fairly tight gauge.

 

What sweater do you want to make? Is there a condition that isn’t addressed here? Have questions about something I've said? Leave your comments below and I will answer them.

Note: if there is a word or term you don’t know, check our yarn glossary. You can find the glossary, and more, in the footer of every page of our website under Useful Information.