640 acres is how many miles?

The most useful answer would usually be 1 SQUARE mile.

I live in a rural setting, and the common joke is 'how many miles are in an acre' and the person answers with any number they want. This is because an acre is a SQUARE measument and a mile is a LINEAR measurment.

So 640 acres is 1 mile by 1 mile, or 1/2 mile by 2 miles, or 1/4 mile by 4 miles or ... you get the idea.

And for my Canadian friends, that's 1.6km by 1.6km.

Another useful measumerent: an acre is 208.7 feet by 208.7 feet.