It's my professional obligation as a designer to tell you that swatching is important because I want you to swatch. I want you to check your gauge when you knit one of my designs so your project comes out the size you want and you're a happy customer!
My work has changed my relationship with swatching. We designers have it pretty easy, in that any patterns we write are based on our own personal gauges. Once we have a fabric we like, we're golden! Let's go do some math! Knitting from other people's patterns (OPP from here on out), however, means I have to (or at least am supposed to) swatch for realsies. If my gauge doesn't match, I have to keep trying if I want to knit that pattern.
Such was the case with Tinder, my first planned OPP project of the year. I've had 11 balls of Valley Yarns Greenwich stashed away for...several years. A friend of mine on Ravelry gifted me the pattern in December 2013. You do the math!
Like a good, dutiful, knitter I started my merry swatching with a US 8 as called for in the pattern. I felt confident, and excited. Finally I would have my grey sweater I had been dreaming of! The swatch was....too big. Like 1 st PER INCH too big. I dropped down to size 7 needles and tried again. Still too big, but getting closer--16 sts over 4" vs the called-for 18 sts over 4". The fabric looked great, much better than on size 8's.
I was cranky at this point, though my troubles made sense. Shelter, the yarn called for in the pattern, is technically a worsted weight and Greenwich is technically a slightly thicker aran. Not really apples to oranges in my book....clementines to oranges?
I tried to be good and swatch once more on size 6 needles, but quit about 3 rows in when I realized I hated the knitting experience of using small-ish needles with that yarn. Ultimately, while I really really want my awesome grey sweater, I need to enjoy knitting it too!
So I wound up cheating and casting on for a smaller size than I would normally choose, because with my gauge on US 7's it will come out to be the size I want.
There is a way to do this with more certainty though, as opposed to just guessing. Don't guess, especially not with a garment!
I looked through the pattern and found the stitch counts of each piece at the bust area. I'm not going to list the actual numbers here to protect the integrity of the pattern, but for example, let's say each front was 30 sts for the smallest size and 60 sts for the back piece. Subtract 2 sts from each piece to account for seaming the sides together and picking up sts along the front for the button bands--28 and 58, respectively. 28 + 58 + 28 =114 sts. If the pattern gauge calls for say, 3 sts per inch, that would turn out to be 38" around. Add in a 1" buttonband and you've got a 39" finished bust measurement. But if your gauge is 3.5 sts per inch, you'd wind up with a 33.5" bust (32.5" body + 1" buttonband). Big difference! That's why gauge is so important.
While I would have aimed for the 34.25" size if I was on point with my gauge, with my new gauge I'm going to be following the directions for the 31.5" size. If my math is correct, I'll wind up with a 34.25" anyways, which will give me just enough ease with my 33" bust. Yay!
You can use this technique even if your gauge is on point, but maybe your personal measurements don't jive with the sizes offered. If it's a sweater worked in pieces and seamed, you can even combine pieces from different sizes to get a custom size, like following the directions for size M for the cardigan fronts and size L for the cardigan back. Some adjustments might need to be made in length-based areas, like the body length of the piece or the armhole depth, since those measurements don't always stay consistent over multiple sizes. You'll just have to make some design choices and maybe rework some increase or decrease sections to fit your new franken-garment. If you're used to modifying patterns, you have the skills to tackle this technique!