It always goes back to gauge and stitches.
From my previous post:
"Now, what's the next step? Back to gauge and stitches. It always goes back to gauge and stitches, doesn't it?? The next installment will be short and sweet. I will briefly cover how to translate all of those measurements into stitch and row counts, making adjustments depending on stitch patterns, and simple ways to work stitch patterns in to an ever-changing landscape. "
To begin, yes, we need to know what stitches we want to use, the yarn, the needles size, and we need to swatch. Just as we covered in the post, Stitches, gauge, and dreaded maths, you will first need to know how many stitches per inch are in the selected stitch pattern and yarn along with the rows per inch. A quick word about rows per inch though because this is undoubtedly the hardest thing to match for one simple reason... needle materials. Stitch counts can vary also, but for me, row count is definitely more noticeable. Please go read THIS post over at Knit Darling... it may save you hours of swatching just in your regular, every day knitting!
Now, once we have our gauge for both rows and stitches, we can begin calculating. Take your measurement - in our example from the previous post, 36" circumference - multiply that by the stitches per inch (for this example let's say 5 sts/inch) equaling 180 stitches. This is the number you would cast on.
However, we also need to take into consideration our selected stitches and how they fit into our cast-on number. Maybe we are starting with a 1x1 rib hemline in which case there's no problem, but say we are changing to an all-over textured stitch that is worked in multiples of 7+2. If the +2 are edge stitches and the garment is worked in the round, we can omit them and divide 180 by 7 which equals 25.7 repeats of the 7 stitch pattern. When this happens, you have 2 options: take 7 and multiply it by 25 which equals 175 and decrease after the hem (or simply adjust from the cast on in which case you need to be sure you have a stitch that will work for an odd number cast on) OR you can just bump the 25.7 to 26 and multiply by 7 giving you 182. Again, you can increase a couple stitches after the hem or simply cast on 182 from the start. When you go back to your schematic though, you will need to make adjustments based on these changes, so if you decided to simply bump up to 182, the measurement will be 36.4"; if you decided to go down to 175, the measurement is now going to be 35".
Row measurements are going to come in handy in two places in particular: sleeves and necklines. It's normally pretty easy to throw out a line like, "work even in pattern until piece measures 14.5 inches from the cast on edge, ending on a WS row" however, as I mentioned in the previous post, you will need your row gauge to calculate sleeve decreases, even if you use one of the calculators linked to. If you have 7 rows per inch, you will simply plug this into the calculator (or if it asks for total number of rows you multiply 7 by the sleeve length before the cuff; for example 14.25 multiplied by 7, equals 99.75 rows rounded up to 100).
For necklines, it's often going to come down to just a few rows in the neckline shaping. You can instruct to knit until the armhole measures 4 inches and start the neckline shaping, but then it's best to go in row by row with specifics from there.
There will be times, too when calling out rows is necessary because of a particular detail. When I wrote the Patou Pullover pattern, I had initially instructed to knit to a certain measurement and my sample knitter was running out of room for the eyelet "V" around the neckline. Why? Because even though she had knit to the desired length, she was running out of stitches due to the neckline placement against the "V". I decided to call out how many rows needed to be knit from the underarm break instead to ensure there would be room for the decreases.
Now, in regard to size grading, you will have to make a few decisions as a designer about making adjustments to fit the stitches you intend on using, and there are a lot of ways to get around a whole bunch of weird math situations. The easiest way to size grade without mucking up the stitches is to grade according to your stitch repeats. Say you have a 6 stitch repeat and you're working at a gauge of 5 stitches per inch. Rather than strict 2 inch size increments (10 stitch), do closer to 2.5 (12 stitch) increments. Your sizes would go from 34 (36, 38, 40)" to 34.5 (37, 39.5, 42)". It's not a huge difference and it's going to save loads of time in the long run. Another easy way to add stitches is by placing them into vertical panels. Each size may change stitch count on either side, but the panel can stay the same. This will allow you a lot more freedom in sizing without the headaches of adjusting the pattern for each new measurement.
This is a series aimed at budding designers, so be mindful that you aren't biting off more than you can chew in these early stages. There's no shame in working up Stockinette or garter stitch patterns as you get your bearings! I'm a big fan of the "work smart, not hard" motto and I will simplify patterns any way I can so that when it comes to writing the instructions, it's as streamlined as possible.
I leave you here with this as always...