Deciding which of these two words, further or farther, one must use in a sentence is quite easy to do.  Further and farther crop up quite a bit in everyday spoken English.  Most people do not think too much about whether the correct usage is further or farther.  However there is one and it’s not that difficult to understand.

further or farther – when to use

Both the words further and farther come from the word far
Further is the comparative form of the word far to mean “much.”
Farther  is the comparative form of the word for when you talk about distance as a form of measurement.

A quick search on Google reveals their definition of farther and further as:

Farther

Farther  has to do with physical distance.
at, to, or by a greater distance (used to indicate the extent to which one thing or person is or becomes distant from another).
“for some time I had wanted to move farther from Lynne”
“I live much farther from downtown Atlanta than you.”
Farther refers to length or distance. It is the comparative form of the word for when you are referring to distance as a form of measurement.
Synonymns for farther inlcude additional, more and extra.

Further

Use further with non-physical or figurative distance.  Further as an adverb means “to a greater degree,” “additional,” or “additionally.” It refers to time or amount.  Further is the comparative form of the word far when meaning “much.”

adverb
more distant in space than something else of the same kind.
“two men were standing at the further end of the clearing”
adjective
additional to what already exists or has already taken place, been done, or been accounted for.
“cook for a further ten minutes”
“I will have nothing further to do with you.” I have nothing additional to say to you
verb
help the progress or development of (something); promote
“he had depended on using them to further his own career”
as in promote, advance, assist and other synonyms indicating help, lending a hand to, etc.

Knowing these defintions should make it much easier to choose correctly between further and farther.  If you are finding it difficult to ascertain whether to use further or farther because you cannot easily make the distinction between physical and figurative distance, use further.