What is Gauge? Gauge is simply a measure of the number of stitches and the number of rows in a certain distance (most often over 4 inches or 10 cm) .
Why Does Gauge Matter? Checking your gauge will ensure the item you are making, will turn out to the correct size as intended by the designer. If you miss this critical step, you may very well end up with a project that is far too big, or far too small. This is especially important when constructing garments, where sizing is pertinent for the correct fit.
Gauge depends on a variety of important factors including:
1) Crochet hook size or knitting needle size
Not all hook and needle sizes are created equal between brands. For example, a 6mm crochet hook in one brand, may not be identical in size to a 6mm hook from another brand. Even though a pattern calls for a specific hook size, this is just a recommendation as a starting point (It is the hook size that the designer used to achieve gauge), which may not be the case for you. When you complete your gauge swatch, if gauge measurements are not met, change your hook (or needle) size accordingly and repeat the gauge swatch until the correct gauge is met.
2) Yarn size (weight)
Not all yarn weights are created equal. A worsted weight #4 yarn, in one brand could very well be twice as thick as a worsted #4 in another brand, or even within the same brand of yarn. Again, this is why checking gauge is so important.
3) The person who is crocheting or knitting
Everyone crochets and knits a little differently, some tightly, some loosely. This can result in gauge differing vastly from one person to another, even if they are using the same exact same yarn and identical hook or needle.
How to Make a Gauge Swatch: Most crochet and knitting pattern instructions will tell you to measure a 4 inch area in a specific stitch. Make sure you are using the exact same yarn you will be using for your project as well as the exact same crochet hook. Not all yarn is created equal and not all crochet hooks and knitting needles are created equal. There is often variance in hooks or yarn labelled the same between brands or even within the same brand. After you make your swatch as directed in the pattern (for accuracy a swatch of at least 6'' by 6" will result in better accuracy when measuring), measure it to see if meets the gauge requirements. Make sure to measure in the center of the swatch, do not line your measuring tape up with the edges of the swatch.
If your gauge does not meet the pattern requirement, do not fret, you can easily make adjustments to correct this. If you have too few stitches in your gauge swatch, it means your item will turn out too large and you need to go down a hook or needle size or more until you meet the gauge requirement. If you have too many stitches in your gauge swatch, it means your project will turn out too small and you will need to go up a hook or needle size or more until gauge requirements are met.
Checking your Gauge Swatch: To check the stitch gauge, most often your pattern will require you to measure the number of stitches in 4 inches. Take your ruler or measuring tape and count across, how many stitches there are within 4 inches. Make sure to line your ruler up at the beginning of the stitch. As you can see in the example photo, there are 14 stitches in 4 inches.
Note: Do not measure from the edge of the swatch, measure in the middle.
Row Gauge is also usually measured by the number of rows in 4 inches. In the example photo below, there are 11 rows of half double crochet's in 4 inches.