Sock Rule! Ruler
Slip this ruler right into your sock while it is still on the needles to easily gauge when to start the toe (or the gusset/heel if you are knitting toe-up socks). With inches, centimeters, US Men/Women, and Euro shoe sizes etched right on, you'll always get a perfect fit!
This handy ruler can be used in a few different ways. The length of a foot can be measured by standing on it. It works well to do this against a wall so the part of the heel that sticks out the most is in line with the edge of the sock ruler. Adult socks typically have 10% negative ease, so depending on how much negative ease you like in your sock you can either use the corresponding shoe size or add in or subtract the percentage of ease you'd like.
Once you're knitting your socks you can use the inch/cm ruler to measure the height of your cuff for either toe up or top down socks. You can stick the ruler into top down socks, with the zero edge up against the heel of the sock, to measure what shoe size or inch/cm measurement you're approaching in your knitting. If you're knitting toe up socks you can hold the sock up against the ruler to measure the shoe size or inch/cm measurement.
Adult socks typically have 10% negative ease and measure approximately 1/2" or 1.3 cm shorter than the foot length and 1" or 2.5 cm smaller than the leg or foot circumference. This information is etched onto the sock ruler as a reminder.