You can take a ribbon instead of the measuring tape and wrap it around the base of your neck. On each end of the ribbon mark a point where the ribbon ends crosses. Then take the ribbon off and use the ruler to measure the ribbon length between those two points you've marked.
Now that you know your neck size let's find out what the standard chain sizes are and how to apply them to your neck size. There are chains 16'', 18'', 20'', 22'', 24'', 26'', 30'' long and so on. The popular rule in choosing the chain size is to add a couple of inches to your neck size (you've just measured).
The standard chain lengths show you where the pendant/locket or the lower side of your chain/necklace will appear after putting it on. In other words whether it'll be at the collarbone or at the navel or somewhere between them.
16'' chain is quite short so be careful with this size. 20'' is the most common size so it is recommended either for men or for women. 22'' is the safe size for men with particularly strong neck in case you want to buy him a present and do not know his exact neck size.
Sizes from 20'' to 26'' are the matter of your individual choice and clothes preferences. Though it is quite common for women to wear more than 30'' long chains or necklaces, such a long chain on a man expresses a particular style of his.
You see that it's simpler than it was with the ring sizes. You can buy a chain as a present without even knowing your friends neck size. There are safe chain sizes to choose (20'' and more) and all you need is to consider the style and the price of the chain or necklace. You can also try to find a necklace with an adjustable back.
These are the common rules of chains & necklaces length choosing:
Ã¢â‚¬¢a choker accents your neck and a long chain with a pendant makes people stare at the pendant not at your neck
Ã¢â‚¬¢a choker visually shortens your neck and a V-shaped chain with a pendant makes your neck look longer
Ã¢â‚¬¢a turtleneck demands a long chain or necklace
Ã¢â‚¬¢the lower side of your chain shouldn't touch the very edge of your dress neckline (the chain should be shorter or longer then)