The fillet means a measuring line; the pillars were 12 cubits, i. e., 18 feet, in circumference, and thus the diameter would be 5 feet 9 inches. As the brass was four fingers, i. e., scarcely four inches thick, the hollow center would be more than five feet in diameter.
Related Commentaries of Jeremiah 52:21
Jeremiah 52:21
21 And concerning the pillars, the height of one pillar was eighteen cubits; and a filletf of twelve cubits did compass it; and the thickness thereof was four fingers: it was hollow.