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Yarn Weight Substitution

As part of my podcast, I have a Questions for Karen thread where viewers post questions that I then answer on the next episode. The following question was asked recently:

"When you substitute a heavier weight yarn in a pattern, do the proportions remain the same? For example, a pattern calls for 400 yards of fingering weight yarn. If I substitute worsted weight yarn and increase the hook size, how much yarn would I need?"

I gave a very basic answer to this question on the podcast (Episode 68), which is essentially that most people say that if you use a different weight of yarn, the yardage needed may vary. But I couldn't really find any specific answers. (They are probably out there, but I didn't come across them in my searches.) But I did say that I wanted to do an experiment to find out.

I didn't do it before that podcast episode, but I've done it now, and I wanted to share the results with you.

The Setup

I picked out two yarns, both superwash merino. One was a fingering weight and the other a worsted weight. I picked needle sizes that corresponded to what I usually use for those yarns: US 4 for fingering weight and US 8 for worsted weight.

I knit a garter stitch swatch for each yarn by casting on 20 stitches using a long tail cast on and then knitting 20 rows. I bound off using a traditional bind off, and I cut the yarn tails at the beginning and end of the work so I'd only be measuring the actual amount of yarn used for the stitches.

Here are the swatches. The pink/purple/grey at the top is the fingering weight yarn and the blue on the bottom is the worsted weight yarn.

Yarn Weight Substitution

The Results

The sizes of the swatches obviously differ due to gauge for the weight of yarn, but they are as follows:
Fingering weight swatch: 3 inches by just under 1.5 inches
Worsted weight swatch: Just under 4 inches by just under 2.5 inches

Then I unraveled each swatch and measured the yarn using a yard stick to see how much yarn I used for each swatch.
Fingering weight yarn: 7 yards and 2 inches (254 inches)
Worsted weight yarn: 9 yards and 24 inches (348 inches)

Conclusions

I think most of us understand that if we use a heavier weight yarn and adjust the needle size to that yarn, we're going to end up with a bigger finished object. Sometimes that's the desired goal: there's a shawl designed for laceweight yarn but the finished shawl is smaller than you'd like so you want to use a heavier weight yarn to automatically make the finished shawl bigger.

But then comes the question of just how much yarn you need. Can you just go by the yardage listed in the pattern and get that much yardage in the heavier weight yarn? (Which is essentially what the original question asked.)

According to this experiment, the answer to that is no. You would need more yardage of a worsted weight yarn to complete a project originally designed for fingering weight yarn. And if you look at the example, that's a pretty big difference of 94 inches (just over 2.5 yards) just in this tiny swatch. It's a percentage increase of 37%!

Now, results will vary based upon jumping just one weight of yarn up (for example, lace to fingering, which probably won't be as large of a percentage increase) or based upon the type of stitches in the project you are doing. But this experiment does seem to suggest that you'll want to make sure you have more yarn in the heavier weight than that called for in the original pattern.

Applying this to a project

How can you figure out how much more yarn you might need? Do this experiment for your pattern. Knit a swatch (based upon the stitch pattern used in the project you are making) in both weights of yarn (the one in the pattern and the one you want to use). Then unravel and determine the length of yarn used for each one. Find the percentage of increase (you can find calculators online that will figure that out for you) and then use that percentage on the yardage from the pattern to figure out the new yardage you'll need for the heavier weight yarn.

So, let's say this project was a garter stitch scarf that called for 400 yards of fingering weight yarn. I figured out through my swatch experiment that the worsted weight swatch uses 37% more yarn. So I multiply 400 by .37 to get the additional yardage needed for the entire project, which in this case is 148. Add the original yardage amount (400) to get a total of 548 yards of worsted weight yarn for this project. (You can also multiply 400 by 1.37 to do the calculations in one step.)

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