Albert Barnes Commentary 1 Kings 18:32

"And with the stones he built an altar in the name of Jehovah; and he made a trench about the altar, as great as would contain two measures of seed." — 1 Kings 18:32 (ASV)

He built an altar in the name of the Lord - that is, calling on the name of Yahweh as he built it, and so dedicating it to His service.

Two measures of seed - literally, “two seahs of seed.” The seah contained about three gallons.