There are so many patterns out there that call for many beautiful yarns, but sometimes you want to use a different yarn than the pattern calls for. So what do you do? 1. You should try match up the wpi (wraps per inch) so bring your ruler when you are shopping for yarn. Ravelry is a very helpful site when looking for yarns because they often list the wpi on the yarn info page. 2. Find a similar fiber to ensure proper drape. 3. Finally create a gauge swatch according to pattern, and change needle size to reach the recommended gauge. I can't say I have always been the best at swatching, but I am trying to be better.
As many of you may know, two DK weight yarns are not necessarily the same weight. I recently realized that when selecting a yarn for Hannah Fettig's "Effortless" cardigan. The pattern calls for Madelinetosh DK, but she suggests to select a worsted weight yarn as a substitution. I went Ravelry cruising and found that Malabrigo Rios would be a suitable sub. Once I swatched it, I realized that it measured the same as the pattern recommendation. The yarn is luscious and the drape is perfect, not to mention that it is machine washable. Here is my swatch. Aren't the colors gorgeous?
Weight charts are helpful to determine weight, but I highly recommend to use the yards to weight ratio and wpi when trying to substitute yarn for a pattern.