Farther usually refers to measurable physical distance. If you could imagine miles, feet, or steps, this is often the better choice. For example, She ran farther than anyone else on the team.
Further usually refers to a greater degree, extra amount, added discussion, or a later point in time. For example, We need further evidence before we decide. You can also say, Let us discuss it further after lunch.
This is where people get confused: many modern style guides accept further for physical distance too. That means you will sometimes see sentences like They walked further down the road. Even so, many writers prefer farther when the meaning is clearly about real space, because it makes the contrast easier to understand.
- Use farther for physical distance.
- Use further for more, additional, or beyond.
- If either seems possible, choose the one that makes your meaning clearest.
A simple memory trick helps: farther contains far, so it is a strong fit for literal distance.