You've picked the perfect pattern and now you just need to find the perfect yarn. Maybe you feel guilty for buying new yarn when your carefully curated stash is calling your name or perhaps the yarn the designer suggests is our of stock or out of your budget. There are many reasons you may need to pick a different yarn from the one used in the pattern's sample, all valid and always a tricky situation with no guarantees. These steps will help you make that yarn substitution with confidence and ease.
Find the Characteristics of the Suggested Yarn
Before you can search for a suitable substitution, you need to know the characteristics of the yarn called for in the pattern. What is the weight? The fiber type? Will it matter if you use a variegated color? All of this information should be found in the pattern.
I like to put the yarn information right at the top of my information page and I leave it generic so it's easier for knitters to find a good substitution. Every designer is different though so look for the basic information you would find on a ball band; the weight, yards/meters, and fiber content.
Start with the yarn weight
Yarn weights are somewhat standardized within the industry, though the weights may have different names in different countries and regions. Here is a good table to help you understand how the numbers and names like fingering, sport, and DK relate to each other. For the most part, you should get similar gauge results is you substitute a yarn of the same weight.
Pay Attention to the Fiber Content
Each fiber has its own unique quirks. Animal fibers behave differently than plant fibers and even if you are look at two wool yarns, but from different breeds of sheep, they may behave differently once knit and blocked. If you want to stay true to the designers sample, then match your yarn in fiber type as closely as possible.
Woolen vs. Worsted Yarn Preparation
Once you have found the same weight and fiber content for your substitution, take a look at the yarn preparation. In the spinning world you will see this described as woolen spun or worsted spun. In commercial yarn you are more likely to see descriptions like "hand-spun" or "rustic" when describing the light and airy woolen spun yarn. Worsted spun yarn (not to be confused with worsted weight yarn) is more dense and smooth.
In the photo to the right, the bottom yarn with loose plies and a fuzzier appearance is woolen spun. The smooth, tightly plied strand on top is worsted spun.
To get the results the designer has in their sample, you really need to knit a swatch. Including washing and blocking it before you measure. It really is a necessary evil if you want to avoid surprising results after all of your hard work.
There is good news in this though; if you swatch, you can throw all of the rules I mention above out the window! That's right, go crazy and use any yarn and needle combination that makes you happy, as long as you make a swatch and adjust your measurements accordingly.