Hi,
I think you have a couple of things mixed up.
First the needle is not marked, the needle comes in sizes that measure the length in inches and the width in a gauge i.e. 21. A sample needle will be 21g X 1 ½ inch.
The syringe (barrel) is marked in cc’s.
The gear you use comes in different volumes and concentrations.
For instance you can have 10 cc, 20 cc or 30 cc of total gear. This is the volume
The concentration can vary for each type of gear, remember 10cc=1ml
Normally it is 100mg/ml or 200mg/ml or 250mg/ml.
What does all this nonsense mean. Simply put the amount of mg per cc will vary with the gear that you use.