There are many different reasons you might want to resize a knit garment. Maybe the sizes that are offered won't fit you, or you're using a different yarn where the gauge is not quite the same.
The principles of garment resizing are the same regardless of whether you're knitting a scarf or a sweater, but for the purposes of explaining the basics, we're going to look at a very simple pattern.
My Convertible Knitted Bodywarmer is just a ribbed loop worked in the round, but the steps you would go through to make this project larger or smaller are the same steps you would use to adjust any garment, you just have to do more adjusting when there are more numbers involved.
I know most people don't like to make swatches. It seems like a lot of extra work that doesn't necessarily guarantee that your garment is going to fit anyway.
I felt the same way until I started designing. A swatch is the only way to know what the yarn is going to do before you start working on your project. And when you are combining yarns, as I did with this project, you need to know what the yarns are going to look and act like together and make sure you like the result.
Ultimately swatching saves you a lot of time if you judged incorrectly and have to rip out your work and start over.
With your selected yarn or yarns and a size 15 US needle, cast on 12 stitches. Work in k2, p2 ribbing for 12 rows, or until the swatch is about square. (Yes this project is worked in the round, but I didn't knit a circular swatch, which I usually do.) The number of stitches per inch is more important than rows per inch on this project. Bind off.
Soak your swatch for 10 minutes in lukewarm water. Gently squeeze out the water, then stretch your swatch only as much as is needed to make the sides straight. Measure the width from the cast-on side. I got four and a half inches/11 cm.
The Math Part
The math required for this pattern is easy. First, you need to figure out how many stitches fit in an inch/2.5 cm by dividing your width into your number of stitches. In my example, that's 12 divided by 4.5, which gives you 2.6.
Then you need to measure how big around you'd like the finished project to be. Use a piece of yarn or a flexible measuring tape to circle your arms just below the shoulders. Shrug or stretch a little bit, so there's a little give built in. I got 43 inches/109 cm; then I added an inch/2.5 cm just for extra flexibility. You can do that or not.
Then multiply your number of stitches per inch or 2.5 cm (2.6) by the number of inches around (44), which got me 114.4.
Now comes the only tricky part. The pattern used is double rib, which requires a multiple of four stitches. So instead of 114 stitches, we need 116 stitches. Because we're knitting this in the round, that will allow you to knit straight, K2, P2 all the way around the project.
Say you wanted to make yours for a child, so you needed it to come out about 20 inches/51 cm around instead. Using the gauge measurement again, you'd get 52 stitches, which just happens to divide evenly by 4.
Reviewing the Steps
Whether you're reshaping a simple garment like a scarf, cowl, shrug or poncho or something more complicated, these same steps will help you work it out.
- Make and measure your gauge swatch.
- Determine the number of stitches per inch.
- Measure the person you're knitting for in the places the garment should fit.
- Multiply those measurements by your gauge to determine the number of stitches you need.
- Adjust for stitch pattern multiples as needed.