Further and Farther

Writers are often unsure when to use further and when to use farther. Their confusion is understandable as the two words can often be used interchangeably. The quick answer is this:

• Use farther for physical distance.
• Use further for non-physical distance.

The clue is the stem far. This obviously relates to physical distance. The words can be used interchangeably for two reasons:

• It's not clear whether you're talking about physical distance or non-physical distance. This is quite common. When it happens, opt for further.
• The distinction between the words is disappearing. If you're unsure which to use, opt for further. It is slowly taking the place of farther.

Farther

If you're a stickler for the rules, then farther should be used with physical distance. In other words, if you're thinking about more miles, more kilometres, more inches, more centimetres, etc., then you almost certainly want farther.

Q: How much farther?
A: Four miles.

Q: How much farther?
A: Six hundred centimetres.

Q: How much farther?
A: It's just around the corner.
(This satisfies both criteria; i.e., it's both physical and non-physical distance. Further would have been okay as well.)

Q: How much farther?
A: Three more fence posts.
(This satisfies both criteria; i.e., it's both physical and non-physical distance. Further would have been okay as well.)

Further

Further should be used with non-physical distance. If you're not thinking about more miles etc. but more time, more effort (in fact, more anything that isn't distance), then you almost certainly want further. Further is more diverse than farther. Farther is limited to the idea of more distance. Further, on the other hand, covers all topics (even distance these days) and can mean furthermore, moreover, more, additional, extra and supplementary.

Q: Further?
A: No, I get your point.

Q: Further?
A: Two days.

Q: Further?
A: We still have over a 100 to make.

Q: Further?
A: It's just around the corner.
(Farther would have been okay too, but further is less likely to be frowned upon.)

Q: Further?
A: Three more fence posts.
(Farther would have been okay too, but further is less likely to be frowned upon.)

Q: Further?
A: Twenty-six pages.
(At a push, you could make a case for farther here, but further would be better as pages aren't physical distance. This overlap of ideas is common, so don't beat yourself up if you're struggling to make a clear distinction. When you're in that situation, go for further.)