No two people hold the exact same amount of tension when they crochet, therefore you could use the same yarn and hook size that the designer specifies in the pattern, and end up with a finished product that is a different size.
Sometimes this difference is inconsequential. It may not bother you if your crocheted blanket is 1 inch smaller than intended. However, in any project a gauge that is very far off can change the look of the stitches and overall project. When it comes to garments, the correct gauge is imperative if you want your item to end up fitting. Gauge swatches can also be helpful if you want to use a different yarn than suggested in the pattern. So, lets go over how to make a gauge swatch.
Reference the Pattern
It may say something like 4"= 16 stitches and 18 rows in pattern
We will use this example for the rest of the instructions.
Use the same yarn and hook you will use for the pattern...
and chain the amount of stitches indicated in the pattern. In our example, that number is 16. If you need to add stitches for a turning chain, add those to your chain as well.
Follow the pattern to understand what stitch or pattern you are using for the swatch.
For our example, it indicates "16 stitches and 18 rows in pattern". What that means is to follow the pattern for rows 1-18.
Other times, you may see a certain stitch indicated in the gauge swatch instead. For example, 16 stitches and 18 rows of single crochets. In this case, you would crochet 18 rows of single crochets.
Once you reach the indicated amount of rows, measure your square.
It should be 4" x 4", but that's not always the case. If your swatch is smaller than 4", go up a crochet hook size and try the gauge swatch again. If its larger than 4", go down a crochet hook size and try the gauge swatch again.
How Do I Use a Gauge Swatch to See if a Different Yarn will work with a Pattern?
Make your gauge swatch like normal, but with the different yarn. Measure at the end to see if it is the right size. If it is not, you can try adjusting your hook size. If that does not work, the yarn you are trying may not work with the pattern. Remember that it usually only works to substitute yarns that are the same yarn weight.
What if the Pattern I'm Using Doesn't include a Gauge Swatch?
The designer is indicating that its not overly important and you can skip it.
What if my Gauge Swatch is Too Big?
Go down a hook size and try the gauge swatch again.
What if my Gauge Swatch is Too Small?
Go up a hook size and try the gauge swatch again.
What if its the Right Size, but the Stitches are Too Tight or Loose?
This would also indicate that your gauge isn't quite right. Try using a different yarn or hook size and see if you get better results. This problem most commonly occurs when attempting to use a different yarn than specified in the pattern.