If you are going to spend 15-20 hours knitting a pair of socks, it is important to choose a yarn that you love to look at, feels good against your skin, will be durable, suitable for your climate, and requires upkeep that is compatible with your lifestyle.
Here are some factors to consider when choosing yarn for socks:
Most sock yarns are contain some degree of animal fibre. Animal fibre must be hand washed (or machine washed on the hand wash setting) and can *never* see the inside of a dryer or it will felt (read: shrink to fit a toddler). If you have no time or interest in handwashing socks, look for sock yarns with 'Superwash' on the label. Superwash yarns have been processed to prevent the felting process and can be machine washed with your regular laundry. Always check the washing instructions on the yarn label before you buy.
No article of clothing takes the same beating as a pair of socks. It’s important to pick fibre(s) that are durable and long lasting. The longest wearing sock yarns have some degree of nylon re-enforcement. Sock yarns typically have a nylon content of 10-25% to make them more durable as well as more elastic.
A 100% acrylic sock might be really durable and long-lasting, but will lack the warmth of wool and other animal fibres.
Socks need to have good elasticity and memory to bounce back to shape after we stretch them by pulling them on and off over our heel. Wool is naturally more elastic than other natural fibres like alpaca, angora or cashmere. In other words, a 100% cashmere sock will not retain its shape very long. A cashmere blend of 75% wool, 10% cashmere and 15% nylon will let you have the feel of cashmere with the durability and elasticity of wool and nylon.
This being said, there are knitters who are staunchly against knitting with any man made fibre or superwash (processed) wools. If this is you, it is important to choose a wool that has a high twist, long staple length and high degree of natural elasticity; corriedale and BFL come to mind.
Consider your climate. If it is usually warm where you live, a cotton/nylon or completely acrylic sock yarn might be best for you. If it gets cold where you live, a wool blend will be the warmest option for your feet.
Most sock yarns are fingering weight. This weight is ideal if you want to be able to wear your socks inside your everyday shoes. Light fingering and sport weight also make good everyday socks.
Thicker yarns, like worsted weight or bulky make ideal house socks and boot socks
It typically takes 100gram/ 3.5 ounces (400 yards/ 365 metres) of fingering weight yarn to make an adult pair of socks. If you are knitting for very large feet or if you like the leg of your socks to be super long, you may need more yarn than that. Keep this in mind when you are shopping as some yarns are sold in 100gram/ 3.5 ounce skeins while others are sold in 50gram/ 1.75 ounce or even 25 gram/ .875 ounce skeins.
If you are buying your yarn in person you have the advantage of being able to feel the yarn. Consider how it will feel against the skin of your feet and calves. The skin of your feet and calves may be more or less sensitive than the skin on your hands and face.
Light coloured yarns will show dirt more easily and be more difficult to clean. Dark coloured yarns can be a bitch to knit with (especially if you drop a stitch!) so go for something in the middle. Self-striping and self-patterning yarns are a great choice because they are fun to knit with and non-knitters are always super impressed and think you did something magical to make the stripes/pattern when really it was the yarn that did all the work.