How to measure internally threaded and threadless labret posts
The length measurement is just the part that goes through your piercing.
If you measure with calipers remember that you don't want the labret to be too tight.
If you have any labrets or barbells you can measure the post or depending on the position of the piercing you might be able to measure your ear or how much of another stud goes through the piercing.
Millimetres to Inches Converter