One of these days I’ll get around to doing more project posts. (Or, y’know, more posts in general?) Really, I’ve got to get pictures of finished objects before they’re given away, as most of what I’ve been doing lately is baby gifts. Everyone around me seems to be procreating. ;)

Anyway, this is something I’ve explained more than once over at LJ’s knitting community – how to retool a pattern to work with a yarn that doesn’t match the given gauge.

I’ll start by saything this isn’t always the greatest idea in the world. While you certainly could take a sturdy-looking aran sweater with chunky cables and work it with a fingering-weight yarn… well, why would you? In my opinion, this works best if you have a yarn that’s not substantially different from the prescribed yarn, especially if you’re making an object that has a lot of detail in it. If the detail is in the shaping, as in the hourglass sweater from Last-Minute Knitted Gifts, you can get away with a drastic change in gauge more easily than if it’s an intricate twisted-stitch garment.

I am one of those bad knitters, by the way. I rarely check my gauge, either out of laziness or impatience. But for this process is critical. Pick the yarn you want to use and work up a swatch, probably using the needle size recommended on the ball band. This swatch needs to be fairly sizable, because you want to be able to judge the behavior of it as a piece of fabric – how does it look? Is it comfortable to work with at this size, or are you having trouble keeping a consistent gauge? What’s the drape like? What kind of stretch do you get? If you’re doing something with a stitch pattern other than a simple stockinette, make sure you swatch in that too, whether or not the pattern gives the gauge in that stitch pattern as well. Sometimes a needle that works well for stockinette or garter or seed stitch is exponentially harder when you’re working with a more complicated pattern. You really don’t want to kill yourself over a sweater, so make sure you pick something reasonable.

For the purpose of this post, I will be using Elizabeth Morrison’s Lucky at Knitty as my example. It’s a classic men’s sweater, but at a finer gauge, it could easily be a beautiful, feminine garment. So how do you do it?

The gauge of the sweater in the pattern is 16 stitches/26 rows over 4 inches on US 7 needles. That’s on the thick side of worsted, so what if you wanted to make it with, say, a sport weight?

Let’s make it easy on ourselves and presume we want to make the small size. The instructions say to cast on 83 stitches for the back. Divide that by the number of stitches per inch in the original gauge:

83/4 = 20.75

That’s the width of the hem of the front panel of this sweater, and since you work it straight up, there’s no shaping to worry about on this one. That’s the width all the way up to the armholes. So let’s figure that you want to work with a yarn that’s 6 stitches to the inch. Multiply your dimension by your gauge:

20.75*6 = 124.5

Sometimes when you’re doing this, you get lucky and land right on another number in the cast-on directions. Most of the time, however, you won’t. Depending on how many sizes are available and how much difference there is between the original gauge and your gauge, you may have to start extrapolating sizes. In this case, the largest size on the example pattern only casts on 115 stitches. However, as there’s a difference of 8 stitches between each size, you can easily size up to 123 stitches. At that point, you’re close enough to the number our arithmetic gave us that you can probably use that without a problem. This isn’t an exact science, and generally a stitch or two in one direction or the other isn’t a problem.

If you were lucky enough to land on a number given in the pattern, go forth and knit. There are still a few more things to consider, but you won’t need this next part. If you’ve had to extrapolate your size, you may want to take a moment to go through the pattern and write out your extrapolated instructions throughout. Might just be easier on you down the road.

Now, sometimes patterns don’t size up evenly the way this one does, but usually there’s some rhyme and reason to it. I’ve seen some patterns where the difference between the smallest and next-smallest size was the same as the difference between the next-largest and largest size. If that’s the case, I would look at what the pattern of increase is to extrapolate beyond the given sizes. (For those of you who know something about calculus, this would be like looking at the derivative or rate of change.) For example, if you were told to cast on 20 (30, 38, 44, 48, 54, 62, 72) stitches and you need one size bigger, look how the sizes vary. The differences run this way: 10, 8, 6, 4, 6, 8, 10. They’re increasing or decreasing by two stitches with each size. So if you need one size beyond the pattern, add 12 stitches – cast on 84.

This, of course, is the kind of math you need to do throughout the garment. And here’s where those who were lucky enough to land on a number in the pattern have to pay attention again. You have to do the same thing with your row gauge, or you’ll end up with something very wonky.

On a pattern like our example, this isn’t a huge deal in the body of the sweater. It says to work in pattern until the piece measures 15″. When you have a garment that tells you to work until the piece measures a certain length, frequently each size has a different length. In that case, follow the size that fits you, not the size you’re working. Otherwise you’ll end up with a bare midriff or a really baggy sweater. This may or may not be the look you’re going for.

Of course, some patterns give you a number of rows to work. This is especially true of sleeve patterns, and we’ll be looking at Lucky’s sleeves next. Figure out your row gauge. This pattern says to work 16 rows straight before you begin your shaping, so how much should you do? The pattern’s original gauge gives you 6.5 rows per inch. Row gauge can vary wildly (I’m a combined knitter, and my row gauge never matches patterns), so let’s just say you’re getting about 8 rows per inch. How long is 16 rows in the original gauge?

16/6.5 ≈ 2.46

So that, in our hypothetical gauge, would be:

2.46*8 ≈ 20

You would work 20 rows before starting the shaping, and would repeat this process throughout the sleeve.

ETA: Robin Dodge reminded me of a shortcut involving proportions. Say the pattern gauge is 5 stitches per inch and theirs is 4 – divide theirs by yours and multiply that by the number of stitches throughout the pattern. Most of the time, it works pretty well. But if it’s your first time going through this process, I would recommend doing it the long way, just to make sure you really understand the process and how everything interacts. Just my opinion, though.

One more thing: sometimes this takes some experimentation. For example, if you follow the directions for the raglan shaping exactly on this pattern, you could end up with something wildly misshapen. (Knitty’s Mariah, for example, had an enormous yoke on the larger sizes because the designer didn’t originally take into account the fact that there isn’t nearly as much variance in neck size as in bust size.) You have a lot more stitches on the needle than the pattern thinks you do. You may need to rethink the decreases in order to get a proper fit up top. This may include decreasing more stitches at a time or decreasing on every other right-side row, depending on how your stitch and row gauge interact.

As a side note, there’s an interesting byproduct of this process. You can resize a garment entirely with gauge. Holiday Knits has an adorable little baby sun hat in it. It’s about the size for a sixth-month-old baby, and is made with a worsted yarn. I wanted to make it for a younger baby, so I used a sock yarn on a smaller needle. I didn’t have to do any math at all once I had my initial gauge and knew the circumference that would give me. (And made it out of a self-striping sock yarn, so it’s super-cute.)

Comments and questions more than welcome. I’ve composed this over a few days, so this may not be the most coherent explanation ever.