Now that you have commited to converting a pattern from one weight of yarn to another (and if you have not done that yet, read here to make sure you are ready), here are some math tips on how to match your gauge swatch to the pattern’s gauge.
You have not made a gauge swatch yet? Stop what you are doing, and knit a gauge swatch on the yarn you want to use. Launder it how you would launder the final piece, block it, and then measure it. Without this step, you are going to make a garment or item that will not fit.
To convert a pattern, you may need to use algebra if the conversion isn’t incredibly simple. In order to know how many stitches you will need to make your piece the same size as the gauge in the pattern, you divide the number of stitches in your four-inch gauge swatch by the number of stitches in the pattern’s four-inch gauge swatch. Use this number to multiply throughout the pattern, like this:
Pattern gauge: 16 stitches and 18 rows in a four-inch square.
Your gauge: 20 inches and 23 rows in a four-inch square.
Divide 20 by 16, and you get 1.25. Multiply every stitch count by 1.25 throughout the pattern. If you end up with an uneven number and the end of it is .5 or more (say, 16.65), then round up to the next number of stitches (17 in this case). If your number ends in .01 to .49 (like 16.27), then just use the whole number (in this case, 16).
If you want to make sure you are doing your math correctly (or, like most of the world, make sure your smart-phone or mobile device is doing the math correctly), keep in mind that if you are using a thinner yarn than the original pattern, you should be casting on more stitches. If your yarn is thicker than the original pattern, you should be casting on fewer stitches.
Incidentally, the same concept is true for stitch patterns. If you are making something with an intricate knit-and-purl pattern, and you are going from a heavier yarn to a lighter yarn, then your pattern will appear smaller. You will also repeat the pattern more often if you are going to a thinner yarn.
After figuring out the new number of stitches for the pattern, do the same thing for the number of rows. For the example above, divide 23 by 18 to get your multiplier for the number of rows, and then use a pencil to change the number of rows you will need when you use your yarn throughout the pattern.
This math lesson will get you through any project without shaping that requires counting, so you will be able to make fingerless mitts, scarves, bags, blankets, and boxy tops using this math. Next week, we will get into the most complicated math of all, which is the math used for shaping sleeves and other rounded pieces.
If you have any trouble with the math portion, there are plenty of resources on the internet including a lesson on cross-multiplying to figure out your stitch count. You can also use this conversion website if math is not your thing, and it even gives you a sneak peek into next week’s lesson. Either way, just remember that using a different yarn is almost always a possibility; you just have to be willing to put the work into making sure the item will be the size you want.