While the words farther and further might appear interchangeable, certain rules govern the use of each, and knowing their respective, appropriate usage is sure to make your speech and writing sound more polished. Typically, if you are referring to a physical distance, you would use farther as opposed to further. You can remember this rule easily by bearing in mind that farther contains the word ‘far’, implying physical distance.
INCORRECT: In order to escape the ravenous squid, the orange crab must swim further than the white crab.
CORRECT: In order to escape the ravenous squid, the orange crab must swim farther than the white crab.
Use further when you wish to refer to an intangible or figurative distance or extent. You can remember this rule by bearing in mind that further is also a verb while farther is not. For instance, you can further your spiritual growth, but you cannot farther it.
INCORRECT: The artist progressed farther in coloring in the first crab than he did in coloring in the second crab.
CORRECT: The artist progressed further in coloring in the first crab than he did in coloring in the second crab.
If you are unsure whether to use further or farther in a particular context, it is typically safe to err toward further rather than farther. Whereas further does not immediately sound incorrect in either context, using farther in a figurative context is more ostensibly incorrect. However, the difference between the two has remained fuzzy for many years, and it is highly unlikely that anyone should call you out for incorrect usage.
INCORRECT: Before we quarrel any farther, we crabs should assess our tenuous relationship with the squid community.
CORRECT: Before we quarrel any further, we crabs should assess our tenuous relationship with the squid community.