The widely accepted rule is to use farther when being literal and discussing a physical distance, as in “He went farther down the road.” Further is used when discussing a more symbolic distance or to discuss a degree or extent, as in “I wanted to discuss it further, but we didn’t have time.”

Both farther and further are correct. You might think they’re synonymous, but they have different meanings and uses in English. The standard quick answer is “ farther is for physical distance, while further is for figurative distance.”

Farther = (adverb/adjective) at or to a greater distance or more advanced point. Further = (adverb/adjective) to a greater degree or extent.