What does this number measure?
One of the constituents of atomic nuclei. The proton carries a positive electric charge. Its structure involves quarks and gluons; the stated diameter derives from its charge distribution, without a solid boundary like that of a marble.
Compare the stated dimensions
The comparison uses the number in each fact sheet: altitude, height, diameter, length or distance, depending on the reference. Two values in metres do not necessarily describe the same property.
Source and limitations
Catalogue order of magnitude; a source is provided, but this measurement has not been individually audited.
Proton: a conceptual model of its charge cloud and internal structure; colours, shapes and positions are interpreted, without photographic observation. Approximate charge diameter.