I ended up measuring it at the store. It is 1/16".
The 1/2 in. Cork Cupboard Door Discs (24-Pieces) has a diameter of 1/2" so this may not work for your application, depending on the size of the coaster.
The Cork Cupboard Door Discs are called out as fire-retardant and intended to be used as cabinet bumpers. This is not a recommended use for the product, as the product will be exposed to the high heat of the grill for a long time and product safety would be compromised.