Your answer depends on what you are trying to accomplish.
If you have purchased a new yarn and you want to figure out what needle will give you the best fabric for your project, then you will definitely want to knit a swatch. Start by looking at the ball band to find the needle the company who made this yarn recommends. Many times they will have a range of needle sizes. The biggest needle will give you a fabric closest what the manufacture believes is suitable for a sweater. The smallest number is what is best for the ribbing bands. Also be aware that if the yarn is made in a European country such as Italy or Germany, most likely the needle is labeled in millimeters. This is a different sizing system to our US needle size system. For example a US8 needle is a 5mm needle. Some European companies do not print the “mm” with the number, but instead only print a number 5. In these rare cases it will be important to look at the whole yarn label to notice if most of the information on the lable is written in a foreign language, if so, you can deduce the needle number is a millimeter not a US needle size.
Want extra loose or firm fabric
When you want your fabric to be extra loose or firm, you need to knit a swatch to not only discover the feel of the fabric, but to determine which needle size gives you the fabric drape you want.
So, your a new knitter. That may mean you still don’t know if your tension is tight, loose or average. Most new knitters do not have consistent tension until they have completed two or three projects. Even still you many not have confidence in how you will knit with a new yarn, so making a swatch will give you practice and confidence before you begin your project.
Special stitch is used in the project
When a special stitch design is used for your project it may be necessary to knit a swatch in this stitch design to determine if your execution of this stitch will match the tension of the pattern. Making this swatch will also give you a chance to practice this stitch and work out any bugs or difficulties you might have when knitting this stitch pattern. It is important to note here that the tension written on the yarn label is always in stockinette stitch, and never in a special stitch design. Stockinette stitch is a fabric that has all the knit stitches on one side of the fabric and the purl stitches on the opposite side. Stockinette stitch tension is the standard for all tensions written on yarn labels.
Your yarn does not match the size of the yarn in the pattern
This is probably the most important reason you will want to knit a swatch. If the yarn you have chosen is bigger or smaller than the yarn suggested for the pattern, and you use it to make the sweater, then the end result will also be a bigger or smaller sweater. It is ok to choose a different yarn than the pattern, I change yarns all the time, but be prepared to adjust the stitches and rows in the pattern. Sometimes, when the yarn is just slightly different to the pattern, you can simple choose a larger or smaller size to knit and that will solve your problem. No matter what your approach is, if you have a different yarn, you will want to calculate how many stitches you will need to achieve the widest part of the sweater. Usually this is the bust measurement. This requires a few mathematical steps, and if you are afraid of math, here is your chance to make math fun. It is knitting after all.
How I make my swatches
The bigger you make a swatch the more accurate the tension calculation will be. I recommend making a 26 stitch swatch. Cast on 26 sts, knit 3 rows, Row 4 – K3, p20, k3. Row 5 – knit. Repeat rows 4 and 5 nine times more, for a total of 20 rows of stockinette stitch. End with 3 rows of knit (garter st.) and cast off loosely. Yes – you need to knit this whole swatch and not part of it. Once it is off your needles you will measure across the center of the swatch, just inside the garter stitch boarders and write down this measurement. The garter boarder is helpful to keep the edges from curling to make measuring more accurate.
But wait… the yarn tension square is not 26 stitches
You might be wondering at this point if all the swatches I make are 26 stitches. You could be wondering this since the ball band tension that is printed on the yarn label has a different number such as 20 or 22 or 18 stitches equals 10 cm (4″) square. The answer is, yes, my swatches are all most always 26 stitches. The only exception is when there is a stitch pattern that does not go into 20 stitches. The swatch should be a good representation of what your project will look like. If I need more that 26 stitches to see the pattern repeat than that is what I will make.
The 10cm by 10cm square swatch, which to americans is a 4″ by 4″ swatch is a standard all yarn companies use to communicate stitch size to the consumer. It does not mean you need to make a swatch that is 4″, rather it is the standard calculation you use to determine how many stitches are per inch when using the recommended needle on this yarn. For example; when a label states 18 sts = 10cm. that means this yarn will knit 4.5 stitches per inch. That is 18sts divided by 4″ = 4.5 spi.
Calculating your stitch gauge.
Yes this requires some math, but it is knitting math, and that is much more fun than regular math.
Lets say the center 20 stitches of your swatch measures 5.25″. We will divide 5.25 by 20, which equals, 3.8. That means you are knitting 3.8 stitches per inch with the selected needle. Now that you have this information you need to see how this tension compares to the tension listed on your pattern.
Lets say the pattern requires a tension of 3 sts per inch. And lets say you chose the size small (36″), the back, which is half of the whole sweater, asks you to cast on 54 sts. If you cast on 54 sts in your gauge of 3.8, the back will measure 14.2″ for a total chest of 28.4″ and the sweater will be too small. But if you look at the x-large (45″) of the pattern, the back cast on is 68. If you follow this size for your sweater, it will measure close to 36″. Take 68 sts divided by 3.8 equals 17.8″ times two is 35.7″ finished sweater.