In sum, utmost is the adjective that indicates that something is of the greatest degree, number, or amount or farthest or most distant. Upmost is the adjective for conveying that something is at the highest or in the most prominent position. As its spelling suggests, upmost describes things that are up high.

Utmost is typically used in contexts that involve things being at the utter limit or the outer limit. When you do your utmost, you go all out. The utmost point is the one that’s farthest out. Upmost, on the other hand, is usually about things that are the highest up—often in terms of physical height or priority (think of the top item on a list).

When you need an adjective meaning (1) of the highest or greatest degree or (2) most extreme, the word is utmost. When you need an adjective meaning situated at the top, highest, or most upward position, the word you’re looking for is upmost. The latter is an old, almost archaic word that now mostly appears where writers obviously mean utmost.