It is 21 miles according to Google Maps.
It is 503.09 miles according to MapQuest.
72.8 miles via M20
Kent, England - 1,442 square miles.
103 miles
161 miles
39 miles
235 miles
162 miles
235 miles
4766 miles
Which Mansfield and which Kent are you asking about?
Assuming you're in the UK, Kent is 1442 square miles.