There's no single answer to this question! The thickness of a pencil can vary depending on the type of pencil:

* Standard wooden pencils: These usually have a diameter of around 7-8 millimeters.

* Mechanical pencils: These can have a range of thicknesses, with lead diameters typically from 0.5 mm to 2.0 mm.

So, it depends on what kind of pencil you're talking about!