Although they are often used interchangeably, further and farther don't have exactly the same meaning. Basically, farther refers to actual distances between objects while further also refers to figurative distances or something that is additional. Here are some easy-to-understand definitions and examples of how to use the two words.
Farther refers to a greater physical distance, a distance that has been measured.
Examples of farther:
As an adjective: describes when one object is more distant than the other, requiring a measurement of the distance from one common point to both objects.
As an adverb: when an action results in a greater distance.
Further is defined as something that is additional or more, as well as referring to distance. It is used when there is no knowledge of the actual physical distance or time difference.
Examples of further:
As an adverb: expresses relationship to a place or time, something additional or to a greater degree.
As an adjective: describes a distance or something that is beyond or additional.
As a verb: an action of helping something forward.
One of our YourDictionary readers shared a great way to help you use the two words correctly:
"If you can't replace "further" with "additional" or "more" in a sentence, you are probably using it incorrectly."
So, remember, you need to measure to use "farther," but you use "further" in all other situations. However, if there is some confusion between it being a physical or figurative distance, it is now considered fine to use either word.