John Gill 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)

And [concerning] the pillars, the height of one pillar [was]
eighteen cubits
As in (1 Kings 7:15); said to be thirty five, (2 Chronicles 3:15); of the reconciliation of which, (See Gill on 2 Chronicles 3:15).

and a fillet of twelve cubits compassed it;
a thread or line of that measure encompassed each of the pillars, (1 Kings 7:15).

and the thickness of it [was] four fingers;
either of the pillar, or the fillet about it; that is, the brass of it was four fingers thick.

[it was] hollow;
that is, the pillar was hollow.