An Acre as the Measure for Land Area
The unit of land area in the United States is the acre. An acre contains 43,560 square feet. Have you ever wondered why an acre is 43,560 square feet instead of a round number like 40,000 or 50,000 square feet? The story goes like this. When plowing with a yoke of oxen, it was standard practice to rest the animals (and the farmer) after plowing a furrow 1/8 mile (660 feet) long. An eighth of a mile therefore became known as a furrow-long or furlong; (a furlong is a nearly forgotten term for distance, except at horse racing tracks where it remains in common use).
The usual practice after plowing a furlong was to then turn the team around on a "land" and plow the other direction. Lands were laid out so the farmer would be able to finish a land every 10 rounds with a 10 inch plowshare (about 16.5 feet). One could imagine that perhaps farmers used a pole or rod that was 16.5 feet long when laying out lands because this measure of distance is still called a rod today.
Source: Dorn, T., Acreage & Small Farm Insights Newsletter from the University of Nebraska- Lincoln (UNL) Extension Acreage, April, 2006, http://acreage.unl.edu/Newsletter/NLS/April2006.htm