Unmeasurable vs. immeasurable

Unmeasurable means, specifically not able to be measured objectively. We use this one in the literal sense: “the immense lizard was unmeasurable with the tiny ruler in the standard lizard measuring kit”; “the sea trench in which the angler fish lived was of an unmeasurable depth”.

Immeasurable means, a bit less specifically, too large, extensive or extreme to measure. We should reserve this one for the more figurative sense (otherwise there’s no point in having two different words): ” the immeasurable mercy of their god”; “he had immeasurable ambition”.