Here is some information for those who knit socks without having the recipient's foot nearby...
shoe size and foot length (including length of heel flap)
EU and American conversions
Since I knit with a pretty consistent type of yarn (4m/g or thereabouts) I always knit mens' socks with 72 sts in circumference, and ladies' socks at 60 sts or so (give or take 4 sts for pattern alignment). The only variable is then foot length (where to start the heel for toe-up; where to start the toe for top-down).
To find where you need to start your heel for toe-up socks, you'll need to do some math:
My husband has a size 10 - which, according to the link, means a 10.75 inch foot length (27.3 cm). I start heels at 20cm from the tip of the toe ("forethought" or short-row heels).
So if you want a men's size 9: the foot length is 1/2 shorter, so start the heel 1/2 inch sooner (1.3 cm).
For flap-style heels, start the gussets at 3-3.5 inches short of total foot length. I have a harder time getting a good fit with this style of heel, as it's harder to judge when to start the heel. For this reason, my current favorite heel is the "forethought", for toe-up models.
Here's a post on using a paper template to get great-fitting socks!