‘Further’ and ‘farther’ are often (and sometimes incorrectly) used interchangeably.
Use ‘further’ when writing about additions or progress. Hint: ‘further’ is also a verb, meaning ‘to develop or help grow.’
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To further your career, get a college education.
To further your career, get a college education.
Use ‘farther’ when writing about physical distance. Hint: ‘farther’ is the comparative form of ‘far.’ If ‘far’ doesn’t make sense in the sentence, use ‘further.’
How much farther do we have to go before we arrive?
The boat drifted farther out to sea.
The boat drifted farther out to sea.
In practice, there are still plenty of instances where ‘further’ and ‘farther’ are interchangeable, but physical distance is always ‘farther.’ Otherwise, ‘further’ is usually safe to use.