I am an avid and primarily self taught knitter. I learned most of the techniques I know by trial and error as I worked a pattern that was probably too advanced for me at the time. Collecting these tricks and techniques into one place is hopefully as helpful to others as it is to me.
One of my most frustrating moments of knitting was when I finished an elaborate sweater after hours and hours of work only to find that it didn't fit, at all. I was so angry about it that I unraveled the whole thing and made a simple scarf with the yarn.
Since then I've learned better how to make sure that garments that I knit will fit before I even cast on. The first thing you absolutely must do is measure yourself. It's hard to make something that fits without knowing what you're aiming for.
You can probably guess the vintage of the book I found this diagram in. Nevertheless, it is still a good guide to what measurements you may need to know. The only ones I would add is the length between your bust, waist, and hip measurements. We do not all have the same length torso.
Once you have these measurements you can look at whatever sweater pattern you want to make and check them against what the designer lists are the finished measurements of the garment. It they all match up, you might think "Hooray, this will fit perfectly!" You might be right, but probably not. The designer's measurements for the finished garment are based on their stitch gauge not yours, and also may not take wearing ease into account.
So, what next? Determine how much wearing ease there is supposed to be/how much you want. Since knitting is so stretchy, many garments have negative ease, meaning the garment actually measures a little smaller than you, and fits snugly. Others have an inch or two of positive ease to make them fit more loosely. Decide how much ease is needed for the look you want, and add or sutract that from your body measurements. If you need more length in the sleeves or body, take note of that too. Now you have the measurements you want your final garment to be.
The next step is to make a gauge swatch. Using the same yarn and needles you intend to make the sweater from, knit up a square that is at least 3-4" on a side. Be sure to use whatever stitches are used in the final garment. Once you have your square, carefully measure how many stitces and rows per inch. This is your gauge. If it matches what the pattern calls for you may think "Hooray, this will fit perfectly!" Again, you may be right, but only if your desired garment measurements match the designer's finished measurements. If not, then you still will need to adjust the pattern. Beware there be math ahead in these waters :)
This is from the same 1950's era book, but jus because it's a little out of date fashion wise doesn't mean it's wrong. This is a diagram of the back and front pieces of a very basic waist length sweater. No matter the particulars of the sweater pattern you want to make, it will look similar to this, and these are the measurements you will probably need to adjust. This diagram shows the waist and bust measurements halved for doing the front and back separately. Whether you work your garment this way or in the round will depend on your pattern. For this example I'm going to assume the garment is worked in the round, and use the full measurements.
So how to go about doing the adjusting? Well lets start with the waist. Lets say your gauge from your swatch and pattern both are 5 stitces and rows per inch. The pattern says it's for a 29" waist, but you need 32." 29" at 5 stitches per inch is 145 stitches (5 sts per inch x 29"). By the same math, 32" is 160 stitches. So for example, working a simple sweater from the waist up, you would cast on 160 stitches for the waist insead of the 145 the pattern calls for.
Moving on to the bust. The pattern is for a 34" bust, but you need 38." Same math as before: the pattern has 170 stitches at the bust, you need 190. This is also where the length measurements come in. The pattern says to work 9" between the waist and bust, but you have a longer torso and want 10." According to your gauge, that means you need an extra 10 rows between your waist and bust, or 50 rows total (5 rows per inch x 10").
So you need to increase from 160 stitches at the waist to 190 stitches at the bust over 50 rows. Your pattern will have increases written into it, so follow the instructions there for how to spread the increase stitches across the rows, all you are adjusting is the frequency of the increase rows. Usually increases are done at the side seams in pairs to keep things balanced. So in one increase row you will most likely add 4 stitches. You need 30 stitches increased, which is not divisible by 4. This is where a little extra adjustment comes in. You can either add 28 total stitches in 7 increase rows, or 32 total stitches in 8 increase rows. Let's say you are ok with it being a tiny bit smaller and you choose 7. So, you need 7 increase rows evenly spaced in 50 rows total. So 7 x 7 is 49. If you do an increase row on every 7th row 7 times, you are at 49 rows. One more row without any increases makes your 50.
If you'd chosen 8 increase rows, 8 x 6 is 48, you'd do an increase row on every 6th row, 8 times to get 48 rows, and two more without increases to get to 50.
These same steps can be used to adjust the width and length between waist and hip, between bicep and wrist, bust and shoulder, wherever you need to change the pattern to fit.
Obviously this is going to be easier on patterns without complicated stiches like cables or lace. If you are trying to adjust a pattern with stitches that have set numbers like that you will have to work your adjustments around them or adjust your stitch numbers up or down to keep even multiples for the stitch repeats used in the pattern.
Any adjustments to stitch counts in the bust is likely going to affect your neckline and shoulder instructions. Use as much of the pattern instructions as you can to keep things easy on yourself. It's a good idea to try out adjustments like this on an easy pattern before trying a more complex one. If you can, try on the garment as you go. If you need to take something out and rework it it's better to catch it early than once everything is finished.
If you're curious, this is the sweater I tried and failed at. The yarn made a nice scarf at least.
