Rough rules:

4000 sq metres = 1 acre = 10 sq chaims
2.5 acres = 1 hectare
1 hectare = 10,000 sq metres (100 x 100)
100 nectares = 1 sq km
250 acres = 1 sq km
2.5 sq km = 1 sq mile
1 sq mile = 640 acres = 6400 sq chains

Fine detail (which everybody desperately needs to know):
35.315 cubic feet = 1 cu metre
2.2046 pounds = i kilogramme

etc., etc., etc.