If we haven't, then I get to take a good look at their arms at Parent/Teacher Conferences. One of the first sock machine (sock knitting in general, I suppose) trivia I learned was that the size of a person's foot was the same length as their arm from elbow to wrist (that's the outside elbow joint, not the crook on the inside).
Or, if you know the person's height, their foot size is usually 1/6th their height.
There are other interesting body-relationships I've learned as well, helpful for knitting gifts and for skeining yarn ...
- The arms, outstretched, from finger tip to finger tip, match the height of the person (for the most part). If you're six feet even, this means you can measure a 2 yard length of yarn just by stretching out your hands at maximum draw. Handy for nalbinding or other tasks needing estimately measured lengths of yarn.
- Average head circumference -- 22.75" for a man and 21.625" for a woman. Knit a 22" circumference using a K2P2 or K1P1 ribbing for a few inches to give yourself flexibility either way, and you've covered almost all adult heads and quite a few childrens' as well.
- Distance from the wrist to the top of the middle finger is 1/10th the person's height. Now you can knit mittens and gloves ... just do the cuff in ribbing for maximum fit-ability range.
- Most adult scarves are about 5-6 feet in length ... longer for tall people, shorter for short people. A good-looking scarf length matches the height of the person. If we were to put one on Vitruvian Man (the Da Vinci picture), then it would hang down to about his knees (I think), as the head from chin to crown is 1/8th the height of the person. That gives you plenty of running room to wrap it around your neck for warmth. Dr. Who's scarf is quite a bit longer, 12 feet or so -- thus the extra wraps and dragging tail.
What sizing tips do you have? Post them in the comments to help your fellow readers, and thanks!