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’ refers specifically to measurable spatial differences. ‘Further’ refers to figurative or non-physical things (such as in ‘further thought’) as well as physical distances. As such, ‘further’ is still the safer option as it is more flexible that ‘farther’.

Contrary to popular opinion, further and farther don't have the same meaning. Basically, further refers to something that is additional and farther refers to actual distances between objects.

Both can refer to distances, but further has some additional senses for which you can’t use farther. Farther can be used as an adjective and an adverb. It’s used to mean “at a greater distance,” whether literally or figuratively. Further may be used in a similar way to mean “at a greater distance” (though “farther” is more common).