16 inch (41cm) or shorter

Women: Lies on collarbone and is visible even with a quite high neckline. Popular for fancy necklets.
Men: Too short unless very slender build
Our model is wearing a 16inch chain

17 to 18 inch - 43 to 46cms

Women: The most popular length. Still visible with most necklines. Best one to pick if not sure!
Men: Shortest length suitable for a man, lies on collarbone
Our model is wearing an 18inch chain

19 to 20 inch - 48 to 51cms

Women: A mid length. Good to wear with a larger pendant
Men: Most popular length, pick this one if not sure
Our model is wearing a 20inch chain

21 to 22 inch, 53 to 56cm

Women: Can be worn over a high neck top.
Men: Another good choice
Our model is wearing a 22inch chain

23 inch (58cm) and longer

Women: 24 inch is the shortest length that can be put on straight over your head without undoing the clasp
Men: Another good choice
Our model is wearing a 24inch chain

Specials

If you're looking for more than a chain, it's here.  Gemset or unusual necklaces, T bar fastening or lariats,  Lots of ideas here.