The outside diameter, it is 5/8”. Generally this is used inside an existing hole as a barrier to seal out liquids and gas. Also, they help cushion the nut or bolt head. Could you please provide more detail regarding your question?
The dimensions are as follows: ID = 3/8”, Groove width = 1/16”, OD = 5/8”, Thickness = 1/4”, and Groove Diameter = 1/2”.
For this grommet, the OD is 15.87mm.
3/8 is the bore / Inside diameter. The outside diameter is 5/8 inch / 0.625 inch.