Why is a section of land 640 acres?

I don’t know. 

But that is what was specified in the original instructions to the government surveyors when the State of Oklahoma was first laid out.

I do, however, have a neat fact about the chain that was used when measuring the original sections. It is called a Gunters chain and it was 66 feet long. It was that specific length for a reason; to calculate acreage of tracts of land.

80 Gunters chains placed end to end measured one mile (5280 feet). 80 chains x 80 chains = 6400/10 = 640 acres. So the product of the number of chains measured on each side of a tract of land divided by 10 equals the number of acres

Example:

5 chains x 5 chains = 2.5 acres

10 chains x 10 chains = 10 acres

20 chains x 20 chains = 40 acres

40 chains x 40 chains = 160 acres

How does that work? 

I don’t know.

But one acre of land would then, by default, be the sum of 66’ x 66’ (1 chain x 1 chain) divided by 0.10 = 43,560 square feet, which is the number used in all acreage calculations today.

Length of side (feet) x length of side (feet) / 43,560 = acres