further
adjective and adverbmore, extra, or to a greater degree (also distance in British English)
Further = more: further information.
Both words are right. Traditional US style keeps them apart, while British English uses further for both.
Quick answer
In traditional US usage, farther is for physical distance and further is for more or extra. Further is also common for distance, especially in British English.

more, extra, or to a greater degree (also distance in British English)
Further = more: further information.
physical distance; in traditional US usage it is preferred for distance
Farther has far in it: distance.
| Word | Type | Means | Example | Common mistake |
|---|---|---|---|---|
| further | adjective and adverb | more, extra, or to a greater degree (also distance in British English) | Do you need any further information? | |
| farther | adverb and adjective | physical distance; in traditional US usage it is preferred for distance | How much farther is the station? | — |
adjective and adverb
Examples
Do you need any further information?
Let's talk about this further tomorrow.
Common mistake
Not: We need farther information.
Say: We need further information.
For extra information, further is the usual choice.
adverb and adjective
Examples
How much farther is the station?
We walked farther than we planned.
Read the sentence, then choose the word that fits.
Write or say two sentences, one with each word, in a real conversation.