Most people make errors in the usage of 'farther' & 'further'.Here's explanation for the distinction between the two similar words

'Farther' vs 'Further':

Hurleen: “Reshma, I am tired. How farther should we climb?”
Reshma : “Some more distance. Hang in there”

After 20 meters

Hurleen: “How farther?”
Reshma ( clearly irritated): “If you talk any further, I will abandon you here and climb the hill by myself.”

While Farther is used for actual measurable distances, further is used for degree, quantity etc. which cannot be precisely measured.

Note : Remember the ‘far’ in farther. The difference is easy to remember.