When your looking to buy new jewellery you will need to know 2 measurements. You’ll need the thickness (size) of the jewellery and the length/diameter of the jewellery.
A common thickness is 1.6mm (millimetres) and this will be the usual size of a naval/tongue/tragus bar etc. Another common thickness is 1.2mm and is usually used in cartilage/eyebrow piercings. Nostril jewellery is commonly 0.8mm to 1mm
The length of the jewellery enables a comfortable fit for your piercing. Generally a piercer will use jewellery of a longer length for the initial piercing, this is to accommodate for any swelling that may occur. It is recommended that after the healing period to return to your piercer to have the correct size fitted. Which is a perfect time to find out what size & length they would recommend you wear.
You can also measure your jewellery using vernier calipers or by asking your local piercing professional.
Take a look at the images below for help on measuring your jewellery.