I hope you read that title like the "duh"s were a scary music introduction to the word Math (not like I was all, "Oh, math? Duh.")
I know that both the fear of math and the simple lack of understanding of math are two main barriers that keep people from tweaking knitting patterns to better fit their needs. I can help you with the second.....the first problem is one you'll just have to get over. (I get it...I have a close family member who claims to "not be good at math". Yet she's quite intelligent in all other areas of life, so I refuse to believe it's anything other than a mental block.)
So, let's talk about basic resizing. Michelle, this one is for you!
We'll start with a hat because that's just about the easiest thing you can resize. (Well, maybe a scarf is easier, but scarves don't really have to "fit" to begin with, so we'll just jump to hats.)
Let's say you have a hat that was written for head sizes of 19" (21", 23").
Typically, hats include a little negative ease, so the finished size might be something like 18" (20", 22"). (More on this in a minute.)
The gauge is 20 sts x 24 rows....maybe it's worsted weight yarn on size 5 needles. (Ignore the ball band people....always start swatching with the needle size in the pattern. In the case of winter hats, the needle might be significantly smaller than what you might expect because the designer wants your hat to have a structured fabric that will keep the wind out. Not a drapey fabric that is going to smoosh around all over the place and be full of holes that will be very un-cozy.)
Ok, first things first. You already know that to figure out how many stitches per INCH you're using, you just divide the gauge numbers by 4, right? So this hat should have 5 stitches per inch and 6 rows per inch. (Stitches are horizontal, rows are vertical.)
Remember how I said we'd come back to figuring out the finished size of the hat? Well, some patterns don't include this information, so here's how you figure it out. If it's a bottom up hat, look at the cast-on stitch counts. (If it's top down, look at the "final stitch count" numbers before you get to the brim.) Let's say that in this case, it's bottom-up and the cast-on numbers are 90 (100, 110) sts. To figure out how many inches around these hats are, jut divide the stitch counts by your stitch gauge per inch. So this hat's finished sizes are indeed 18" (20", 22")!
But let's say you want to remake this hat for a newborn. Her head is only 15" around. By looking at the numbers this pattern already gives you, you can figure out that each size includes about 1" of negative ease. So for a 15" head, you actually want to make a 14" hat. Just multiply 14 x 5 (your stitch gauge), and you'll find that you need to cast on 70 stitches. Obviously, you'll have to knit the hat to a much shorter height, and unfortunately there's no "conversion" rules for this. Designers just get this info from sizing charts or measuring appropriately sized humans. But if you spend a moment on the interwebs, you can find charts like this great one from Tot Toppers. Or, if you have that baby in your possession, just keep trying the hat on while it's in progress and knit it to the length that seems right.
Pay attention now. This is going to be on the quiz.
The one caveat you'll have to anticipate for resizing hats is that the decrease instructions may no longer work. For a hat like this, the decreases are probably based on 10 stitches, since all the stitch counts are evenly divisible by 10. (So the first round of decreases would be *(K8, K2tog).) But let's say you have a pattern where the stitch counts are 90 (100, 110) and you decide to cast on 75 stitches because you want a hat that will fit a 16" head. You may want to refigure decreases entirely (although 75 doesn't give you any great options for evenly decreasing....it's either 5 stitches or 15), but the easier route would be to decrease 5 stitches in the first round [*(K13, K2tog)] so you go into the next round with the correct multiple (you're now down to 70 stitches which is again a multiple of 10.)
Another thing that can be a little tricky is if the hat uses a stitch pattern that won't allow you to cast on the exact number of stitches that your gauge tells you to. For instance, imagine a hat that is done entirely in 2 x 2 ribbing. All stitch counts will have to be a multiple of 4 in order for the stitch pattern to work. In our earlier example, only the 21" size is divisible by 4. The stitch counts, adjusted for the pattern, would not be 90 (100, 110), they'd have to be 88 (100, 112). And when you decide to make your newborn hat, 70 is not divisible by 4, so you'd have to decide if you want to cast on 72 stitches or make it a little smaller and cast on 68.
You with me so far? Nothing hard, just a lot to think about!
Ok, let's talk about something slightly more complicated. (Michelle....wake up!!) We're gonna resize some socks!
Let's say we have a pattern that is written for Men's 7/8 (9/10, 11/12). They're toe up socks that begin with 18 (22, 26) stitches at the toe and ask you to increase to 66 (74, 82) sts. The gauge on the pattern is 32 sts x 44 rows, which makes the stitch gauge 8 (32 divided by 4 = 8). So if you divide the stitch count numbers by the stitch gauge [66 (74, 82) divided by 8] you can see that these socks have a finished circumference of 8.25" (9.25", 10.25").
But alas, your husband has the Biggest Feet on Earth. He's now retired from the NBA, but he got to keep his size 14 hooves. So you want to figure out how to resize this pattern for Mr. Big(foot).
First, look for patterns in the numbers the pattern already gives you. Each of these sizes are already 8 stitches apart. Each size is 1" larger than the last. If we continue the sequence of sizes already given [7/8 (9/10, 11/12)], you would say that the next size up would be for size 13/14 feet. Perfect! So, in this example you need only add 8 stitches to the largest size to get the correct stitch count....90 stitches it is!
But that's not the cast on, is it? Again, just follow the sequence. The cast-on numbers at the toe were 18 (22, 26), so just continue the sequence. You'll cast on 30 sts.
When you get to the short-row heel, you'll just work back and forth on half the stitches, regardless of what your stitch count is. I wouldn't make a re-sized pattern your very first project of any particular type (for example, toe-up socks with a short-row heel), but if you're familiar with how to work a short-row heel, then doing one on any number of stitches is pretty darn easy.
Once you're past the heel and on to the leg, your stitch count probably remains the same as before (unless you're working an extra-fancy pattern that requires a stitch multiple that your foot stitch counts won't work for....if that's the case, the patterns may have you working a row of increases as you begin the leg in the round.)
And that's really all there is to it!
• Know how many stitches per inch you need
• Know what you're "target measurement" is - either by measuring the body part you wish to clothe, measuring a similar article of clothing you already own, or by using a sizing chart you found on-line
• Follow the sequences in a pattern to size it up or down and figure out stitch counts
• Be aware of pattern features that require that you use a certain multiple (such as a stitch pattern other than stockinette, or the decreases at the crown of a hat.)
• Use multiplication and division to convert inches into stitch counts or stitch counts into inches.
It's all pretty basic math, you just need to know which numbers to look for and how to use them!