''To save time, take the time to knit a swatch''

There are different levels of accuracy if I may say. The one I am showing now is the easy and take only a few minutes. It is suitable for basic stockinette stich projects that do not involve a massive surface. If you plan on a very intense and time-consuming project, I suggest you do a bigger swatch and block it before measuring, so that it is as accurate as possible. Over a bigger surface, it can make quite a difference...

You will need:

Knitting needles
Yarn
T-pins or sewing pins
Ruler

Step 1: Knit Your Gauge Swatch

Cast-on a bigger amount of stitches than what the label tells you.
(i.e.: Labels says 18 sts/ 4'', I will cast on 18 sts or more)

In the case where you don't have a label, than cast-on and start knitting you will knnow pretty fast if it is big enough. You want your swatch to be at least 3 inches so that you can have a minimum measuring surface of 2 inches.

Once your cute little knit piece is about 3 inches high or more, bind off loosely, but do not cut the yarn. You will be able to simply unknit the piece and reuse the yarn in your actual project.

Step 2: Measuring Your Tension

So that you don't have to hold the ruler and count at the same time, make this very easy for yourself.
Place one t-pin at each end of the 2 inches measurement (Like on the picture).

Then you only have to count the stitches in between!
As most gauge swatches are in stockinette stich, I won't go into the details of reading your knitting. But basically, a knit stitch consists of what seems to be a ''V'' shape. (In yellow on the picture) If you have a hard time finding them, practice looking for them on a swatch made with very thick yarn

Make sure you pay attention to the half stitches, they do count! Here is an example...

My gauge is 9.5 sts/inch, but I omit the half stitch and only count 9sts/inch
If the hat circumferenc should be 20'', this means : 0.5 stitch/inch = equivalent of 10 sts over the whole hat circumference = about 1 inch of difference.

This means my heat would be 1 inch bigger than what I had planned...