When you go to buy your first set of circular knitting needles, you'll notice that the wall at your craft store is filled with all kinds of needles, more than in the straight or double point needle section. That's because with circular needles, there are two different sizes to consider. First, there's the standard needle size. Then there's the length factor. Experienced knitters will often have 3 or 4 different sets of circular needles in any given needle size because they need the cord that connects those needles to be particular lengths for specific sizes.
So when a pattern calls for Size 7 16 inch needles, it's first referring to the size of the needle head and then to the length of the needles from tip to tip.
Let's take a look at the standard sizes for circular knitting needles:
1. The shortest length you'll ever see is 9 inches. A 9 inch circular knitting needle will create the smallest projects you can work on circular needles-- perhaps a baby hat. Anything with a smaller circumference will need to be worked on double-points because it just isn't possible to have needles on a shorter cord come together to work stitches.
2. 16 inch needles are generally the next step up. This is the size needle you'll use for most hats. Note that the circumference of a hat is usually more like 20-22 inches. This is because any knitting project's true dimensions will be a little bit more than its dimensions while on the needles.
3. Next up are the 24 inch needles: the most common length. Most sweaters can be knitted on this length of needle, like this one.
4. 29 inch needles give you even more room and can work well for bigger flat projects, like baby blankets or sweaters worked in pieces.
5. The last standard size is 36 inches, which is also often the recommended length for bigger flat projects. When working on a circular needle this long, you generally aren't knitting in the round. You just need that cord length to accommodate the project that would not fit on straight needles. (Unless the straight needles were two feet long, in which case woe unto anyone trying to sit near you while you knit.)
Circular needles can also come in even longer cord lengths. 40 and 47 inches are the next sizes up. I have one set with a 60 inch cord. This length would be necessary to knit something like a bed-size blanket.
One great way to build a collection of circular needles with every size you could need is to buy a kit. A kit comes with a whole slew of needle sizes. Mine includes everything from 5-15. Then it has 5 or 6 interchangeable cords. So if you need a 16 inch cord with size 7 needles, just snap those needles onto the right cord and you're good to go. These kits also come with extenders that allow you to join two cords together. They're also tremendously handy if you're working on a project that requires you to change needle size partway through. Just snap off the first set of needles and put on the new needle size. Truly, receiving my kit has made my knitting life so much easier.
As you're building your circular knitting needle collection, it's a good idea to keep a chart of what you already own so you don't wind up with two Size 10 16 inch needles. (No, I don't know anyone who would do such a silly thing...) Now put your circular knitting needles to use in Stefanie Japel's online Craftsy class Hats Four Ways Circular Knit Lab!