Farther is traditionally used for physical distance.
- Nairobi is farther from here than Nakuru.
- How much farther do we have to walk?
Further is commonly used for figurative, additional or non-physical ideas.
- We need further information.
- I don’t want to discuss this any further.
- Please contact us for further details.
In modern English, further is also widely used for both physical and figurative distance.
Simple guide
Use farther when you want to emphasise physical distance. Use further for additional information, discussion or progress.