If you want to know how wide a piece will be when knitted with x thickness yarn on y sized needles, you will need to measure gauge. Many patterns when knitted with different yarn or needles will work a lot better if you can get an accurate gauge measurement.
Gauge is important! It may take time to knit a few gauge swatches to find the perfect needle size but it takes much more time to finish a garment, find that it doesn't fit, pull it apart and re-knit it.
Considerations for accurate gauge swatches
To accurately measure your gauge you need to knit a gauge swatch of about 5" (12.5cm) square.
Consult your pattern to see what stitch you should use to knit your gauge swatch. Usually this is either the stitch pattern used for the majority of the garment or stockinette stitch where it would be hard to count the stitches if the garment's stitch pattern were used.
At first you should try the needle size recommended by your pattern and then adjust based on your first gauge swatch. See Adjusting needles size to obtain the correct gauge
The swatch should always be knitted the way you intend to knit the garment. If you are going to be knitting in the round you should knit your gauge swatch in the round and if you are going to be knitting flat you should knit your gauge swatch flat. This is because many knitters find that they knit and purl at slightly different tensions and knit stitches are changed to purl stitches and vice versa when changing to knitting in the round. See the instructions for knitting stockinets stitch flat and in the round for an example. Knitting a gauge swatch when working fair isle or stranded colour work is also important. Colour work distorts the shape of the stich making it more square and less rectangular.
This swatch should then be washed and dried the way the finished garment is going to be laundered and blocked if it is lace. Some yarns change gauge dramatically when laundered and it would be terrible to knit a sweater that fits perfectly before washing but stretches or shrinks after the first wash.
Counting the stitches
Place pins 4" (10cm) apart horizontally and carefully count how many stitches and half stitches there are between the pins. Then move the pins to be 4" (10cm) apart vertically and count how many knitted rows there are between the pins.
This gives your gauge in stitches and rows per 4inches (or 10cm). To get stitches and rows per inch divide your results by 4.
Adjusting needles size to obtain the correct gauge
If the gauge required by your pattern has fewer stitches per inch than your swatch with the recommended needle size, you need to increase your needle size by one or two sizes and knit and measure another gauge swatch. Whereas if the gauge required by your pattern has more stitches per inch than your swatch, you need to decrease your needle size and try again.
If you knit very tightly you may have to go up a few needle sizes and if you are a loose knitter you may have to go down a few needle sizes.
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