Typically when you need to convert inches to cm, you can just multiply the number of inches by 2.54 because that is the number of centimeters in a single inch. But if you follow this standard math rule in your knitting patterns, it’ll lead to some trouble.
It’s an issue I’ve run across a lot when tech editing, especially when working with a new designer for the first time, because it’s just not something we may naturally consider. So I figured I’d write a blog post about it!
In knitting, we need to use a conversion rate of 2.5 instead in order to get the right numbers. Why is this? Well, think of how gauge is listed on a pattern. Gauge is most often given in a 4 x 4″ or 10 x 10 cm square. But actually if you were to convert those 4 inches to cm using the 2.54 rate, you’d end up with a gauge swatch that is 10.16 x 10.16 cm. Not exactly easy to count stitches/rows to the decimal like that. So rather than making knitters grab their magnifying glasses to measure how many stitches/rows are in 10.16 cm, we round down to an even 10, and then have to alter the conversion rate to match a ratio of 4:10, which is 2.5.
You might think that a .04 cm difference won’t cause too much trouble, but when you multiply that over multiple stitches and rows, it adds up. It might not be as big of an issue in your smallest sizes, but as the size of the garment increases, the difference between a cm measurement calculated with 2.54 versus one calculated with 2.5 will grow.
Let’s go through an example:
We’ll take a 60″ bust. If you’ve listed the bust measurement in cm based on a 2.54 conversion rate, it would come out to 152.5 cm (60 x 2.54). If instead you multiply 60 by 2.5, you get 150. So if you have 152.5 cm on your pattern and the sweater actually comes out to 150 cm, that’s an extra 2.5 cm of ease that the knitter wasn’t expecting.
It might not sound like a huge difference, but it does affect the larger sizes more than it will the smaller sizes, and with all the other issues with sizing and fit that fat knitters have to deal with in a lot of knitting patterns, this is a simple change you can make to get your measurements a little more accurate across measurement systems.