Albert Barnes Commentary Jeremiah 52:21

"And as for the pillars, the height of the one pillar was eighteen cubits; and a line of twelve cubits did compass it; and the thickness thereof was four fingers: it was hollow." — Jeremiah 52:21 (ASV)

The fillet means a measuring line; the pillars were 12 cubits, that is, 18 feet, in circumference, and thus the diameter would be 5 feet 9 inches. As the brass was four fingers, that is, scarcely four inches thick, the hollow center would be more than five feet in diameter.