In today's knitting culture, there is a strong emphasis on swatching - a practice that makes great logical sense, as it helps the knitter determine what tension / size of needles to use in order to obtain the fabric density and garment size they are aiming for.
Unfortunately, swatching does not work in all circumstances, and it does not work for everyone. All too often, a knitter will impeccably knit, block, and measure a swatch and make careful calculations based on it - only to get substantially different results in their finished piece.
There are many reasons why this might happen. But all of them can be summarised as follows: Swatching does not replicate all the factors at play in the making of your actual piece.
If you swatch flat, then knit your project in the round, this could make a difference. If you swatch on needles from Brand X, then knit your project on Brand Y, this could make a difference, even if those needles are identical in diameter. And those are just a couple of the more obvious ones. Everything, from the size of the piece you are knitting (smaller objects cause some people to tighten their tension unconsciously, as their brain reacts to the proportions), to the colour of your yarn, to the air temperature, to your mood, to whether you happened to use hand cream that day, can influence your tension.
Like handwriting, knitting style is highly individual. And for some people these types of inconsistencies play up more than for others. In short there are knitters who find swatching useless.
If this describes you, there are other things you can do to ensure gauge. Namely, take the time to get to know your 'knitterly handwriting' for a wide variety of items.
Gather a pile of items you have recently created with yarns of different weights. Lay each item flat. Get out a gauge ruler and measure your stitch x row count, in stockinette, over a 10x10cm area (or 5x5cm in smaller items). Then jot down the following information about each piece:
. the stitch x row figures
. type of item it is (hat, sweater, socks, etc),
. was it knitted in the round or flat
. yarn weight (fingering, DK, etc)
. size and type of needles used, assuming you have this info
When you have a decent portion of a notebook filled with these entries and look over your 'data', you will start to see patterns, which will eventually enable you to get a sense for your individual gauge & tension tendencies. Doing this myself, I was eventually able to put together a rough guide to help me get a sense of what to expect / aim for, with the yarns I tend to work with.
Before I start knitting, I make my initial decision as to stitch count and needle size based on this chart and on how dense/open I want my fabric in this particular knit to be. Then, when I get far enough to have sufficient fabric, I steam block a section of my work and take a measurement. If it deviates from the gauge in my chart significantly I start over, with the necessary adjustments. If not, happy days and I keep going.
There are more nuances to this approach, of course. For instance, working with cables, lace, slip stitches, chevrons, and various other stitch patterns can affect the gauge in various ways, and I account for that when making my calculations. But the chart is the starting point. And for the most part, this method actually works pretty well for me: I only start over maybe 10% of the time.
To swatch as you work may seem unorthodox to some. But there are knitters who function in this manner, and find it more useful than testing gauge on 10x10cm squares.
This is by no means a post telling you not to swatch. But if swatching in the traditional manner isn't working for you, I thought it might be helpful to read about alternatives.