The basic rule
Farther is traditionally used for literal, physical distance: "The store is farther than I thought." Further is used for figurative distance, degree, or extent, and also means "additional": "We need further discussion." "Nothing could be further from the truth."
The trick
Farther has "far" in it — think of physical distance you could measure with a ruler or a map. Further is broader — used for anything abstract, or when you mean "more"/"additional."
Examples
Farther: "He can throw the ball farther than me." "How much farther is it to the beach?"
Further: "Let's discuss this further." "For further information, contact us." "She took her argument further than necessary."
In practice
Many English speakers (and even style guides) use "further" for both meanings, and it's rarely considered wrong. "Farther" is the safer, more precise choice specifically for literal distance.
Frequently asked questions
Can I just use "further" for everything?
In casual usage, yes, it's widely accepted — but if you want precision, reserve "farther" for literal, physical distance.
Which one means "additional"?
"Further," as in "further information" or "further questions."
"How much farther" or "how much further" to the airport?
Both are used, but "farther" is the traditionally correct choice since it refers to physical distance.
Does British English treat these differently?
British English tends to prefer "further" in most contexts, using "farther" less often than American English.