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The "M" Word: Using Different Yarn Weights with the Help of a Little Math

In my last post, I told you a bit about substituting yarns. I mentioned that you're best off substituting a yarn that will give you the same gauge and fabric as the original called for in the pattern, and I stand by that. However, in the real world, you can't always find a yarn that you like that matches the original. Or maybe you're dying to use one of your stash yarns for a particular pattern, but it's just not quite the right gauge. The good news is, very often you can still substitute. The bad news is, you will have to do math. And you really have to do it, not just fudge it. Use a calculator to do the actual math, but write everything down with a pencil and paper.

So if you can't match gauge, how do you figure out how many stitches to cast on or chain? Since gauge is what determines how wide your sweater is, this is pretty important, especially with some of the beautiful new fitted patterns out there. But if you plug in a few numbers, you can often make a size larger or smaller to meet your yarn's gauge, or rework the pattern to meet your numbers. Let's look at an example.

My pattern calls for gauge to be 4 stitches per inch, and has options for knitting a 34, 38, 43, or 46 inch size sweater (the sizes refer to the chest measurement of the finished garment). I need to make the 43 inch sweater for it to fit me properly, but my yarn is giving me 4.5 stitches to the inch. The important thing to remember is that the higher the number of stitches in your gauge, the smaller your stitches are. So if you get more stitches per inch than the pattern calls for, you'll need to make a larger size. This is counter-intuitive, I know, but trust me. Here's how you can see this mathematically:

For a 43 inch sweater knit at 4 stitches to the inch, you would knit 172 stitches total for the front and back. That's desired inches x number of stitches per inch. This should match the number of stitches the pattern is telling you to cast on for the front and back. Now, to find out what your circumference would be if you cast on the same number of stitches but knit more stitches per inch, you divide: number of stitches cast on ÷ number of stitches per inch, or in this case, 172 ÷ 4.5, which gives us a chest circumference of 38.2. It’s amazing what a half stitch per inch can do, isn’t it? That’s basically going down a whole size from what we’re aiming for. But if we use our first formula above to figure out how many stitches we have to cast on with our 4.5 stitch per inch yarn (43 x 4.5 = 193.5 – you can't knit a half stitch so you'll have to round up or down to the next whole number), we can look at the other sizes to see whether they have a similar number of cast on stitches, and just knit following the instructions for that size. This is the easiest and quickest way to up or down size a pattern.

If you are doing this sort of substitution, it is even more important than usual to make a proper gauge swatch. I would also advise making a gauge swatch in any pattern stitch, even if the pattern itself only requires you to make one in stockinette. (I learned this the hard way after making a lace sweater in a yarn that frankly, looked awful. The numbers worked to substitute the gauge into one of the smaller sizes, but it was so heavy that the lace just looked completely wonky. Learn from my mistakes.)

Hopefully -- with a little math and a little swatching -- you'll now be able to make your pattern and your yarns work together!

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