640 acre is how big?

3 answers

1 square mile
1 acre = 0.4046856 hectares
640 acres = appr 259 hectares

or, it depends if you mean

a Scottish acre
an Irish acre
a Cheshire acre
or a Roman acre , each of which has a different value,
pointing out the silliness of the Imperial system of measurement
why is the Imperial system any worse than the Cheshire system? Fie on thee, sirrah - hie thee hence!