First, a definition: A Helpful Swatch is a swatch that gives you, the knitter, information that helps you in the planning, preparation and construction of your knitting project. It tells you a number of things that are essential for successful completion, among them:
- Your stitch and row gauge with the yarn and needles you have selected for your project
- The type of fabric produced with your yarn and your gauge.
- What happens when the yarn is washed and dried.
- What adjustments are needed to make a garment that fits the way you want it
- How a given stitch pattern looks when knitted using your selected yarn.
In addition, you can use your swatch to test out different techniques such as decreases or increases that you might want to use during construction. Full disclosure here: You’ll need to knit more than one swatch to get a helpful swatch. This is why most experienced knitters will tell you that you should buy an extra skein of yarn just for swatching.
One of the first issues new knitters encounter when doing a swatch is not knitting a big enough swatch — they tend to cast on only enough to knit a 4 inch square. In most cases, this isn’t big enough to provide helpful information. So, how big does a swatch need to be? The short answer is: Big enough to get a good sense of how the project will knit up. Elizabeth Zimmerman wrote that “a hat is a good swatch” – which is true if you are planning to knit your sweater in the round. Some knitters use a sleeve as their swatch, others knit a swatch that is at least 6 inches wide by 6 inches high. I would say that the 6 X 6 swatch is a good starting point – but you made need to make one that is even wider.
GAUGE: Here’s one inescapable truth about yarn, needles, and gauge: “Your Mileage May Vary”. Meaning that gauge is a very personal thing – it’s yours and yours alone. It is a measurement of the fabric YOU produce when you knit. So doing a swatch to identify your gauge is important to your success.
Before you start your first swatch you need to decide what size needle works best for the project. Yarn labels usually have a recommended needle size and gauge listed. Some manufacturers state a range on the label, others use a symbol to indicate the recommended gauge. Either way, this is just the starting point. A pattern may also include information about the yarn, needles and gauge the designer used to create the pattern. Both sources provide you with a starting point for selecting the size needle you need to use to obtain the knitted fabric you want to make.
Gauge written on label 
Symbol showing gauge
Here are the steps I follow to make a Gauge Swatch:
- Select 3 sizes of needles – if the recommended needle size on the yarn label is a US 7, I use a US 6, US 7, and US 8 for this swatch.
- Cast on enough stitches on the smallest needle size for a width of at least 6 inches. (To estimate the number of stitches to cast on, I use the recommended gauge on the yarn label. For example, the label states 20 stitches in 4 inches, that’s 5 stitches per inch, so I’d cast on 30 stitches for 6 inches.). Knit in stockinette stitch for at least 3-4 inches ending with a right side (knit) row.
- Knit the wrong side row to indicate where the change in needle size occurs.
- Change to the US 7 and knit in stockinette stitch for another 3-4 inches.
- Repeat steps 3 & 4 with the US 8. Bind off loosely. (You can also just put the live stitches on waste yarn).
- Take stitch and row gauge measurements for all needle sizes and make note of these before blocking per step 7.
- Block this swatch using the care instructions on the yarn label.
- Once the swatch is dry, take stitch and row gauge measurements for each needle size and write them down in your project notes. These are your “finished gauges”.
- Select the needle size by looking at the fabric produced by each size needle. Which one gives you the best fabric? Do you like the stitch definition? Do you like the way it drapes and feels? Is one better than the other two? It’s your choice – but this helps you select the needle that works best with you and your choice of yarn.
Measuring the Swatch for Gauge:
Stitch gauge is expressed as “number of stitches in number of inches (or centimeters)”. You may see this as “20 sts/4 in (10cm) or 5 sts/in. To determine your stitch gauge you need to count the number of stitches in a horizontally measured row of stitches. This measurement should be taken using a ruler and marking the starting point and ending point away from the edges.
Row gauge is expressed as “number of rows in number of inches (or centimeters)”. You may see this as “28 rows/4 in( 10cm) or 7 rows/inch. To determine your row gauge you need to count the number of rows in a vertically measured column of stitches. Again, the measurement needs to be taken using a ruler and away from the top and bottom edges of the swatch.
Note that it is not necessary to measure just 4 inches horizontally or vertically. If you measure more than 4 inches, you may get a more accurate gauge because it will give you a better average – yes – gauge is actually the average number of stitches or rows per inch. And, math is required to determine gauge.
There are any number of YouTube videos available on measuring gauge swatches to determine stitch and row gauge. I recommend the following ones for more detailed explanation: