Suppose you are knitting a pair of socks as a gift, but you don't know the recipient well enough to break out your tape measure and grab a foot to measure it. If you guess or try to estimate, you are risking a floppy sock that bunches or one that stretches too much and cramps up the toes. Knitting socks is a labor of love, requiring up to 20,000 stitches for just one sock. With that much effort, you want the socks to actually fit. You can calculate the sock's measurement just by knowing what size shoe he or she wears.
Obtain the shoe size of the person for whom you want to knit socks. Try a surreptitious glance at the potential sock owner's shoes the next time you're together. Alternatively, you could guide the conversation to shoe sizes to find out what size shoe this person wears.
Consult an inches-to-shoe-size conversion chart like the one included in the resources. Be sure to use the right chart. For example, a sock for an adult woman who wears a size 6.5 shoe would need to be about 9 inches long from toe to heel. A man who wears a size 6.5 shoe would need his socks to be 9.5 inches long.
Consider the type of yarn you intend to use. Some yarn is stretchy and creates a fabric with a lot of give. In this case, you will knit your sock foot slightly shorter than the foot's length. Other yarn is less stretchy and results in a more rigid fabric without much give. For this type of yarn, you will need to knit the sock's foot to the exact measurement.
Try on the sock as you knit. Ideally, the best-fitting socks are those that can be tried on as you knit them. If this is not possible, rely on the measurement you obtained from the shoe-size conversion chart.