Large expanses of land, such as states and national forests, and large bodies of water, are often described in square miles. At other times, they are described in acres. Surveyors often give the dimensions of plots of land in feet, from which you can calculate an area in square feet. It is useful to be able to convert other units of area to square miles. To convert between acres and square miles, use the conversion ratio 1 square mile / 640 acres = 1. To convert between square feet and square miles use the conversion ratio 1 square mile / 27878400 square feet = 1.
The Area of a Body of Water
Look up the area in acres of a body of water, such as Lake Michigan. Lake Michigan covers 8,434,598.55 acres, according to the Michigan Department of Natural Resources.
Set up the equation using the conversion ratio. The area in square miles = 8,434,598.55 acres * 1 square mile / 640 acres.
Calculate 8,434,598.55 acres * 1 square mile / 640 acres = 13,179.06 square miles.
The Area of a Land Mass
Draw a picture of a rectangular land mass, such as a rectangular farm plot. Suppose that a surveyor found that the parcel is 2325 feet long and 1584 feet wide.
Calculate the area in square feet. The formula is area = Length_Width = 2325 feet_ 1584 feet = 3682800 square feet.
Calculate the area in square miles. The formula is area in square miles = 3682800 square feet * 1 square mile / 27878400 square feet = 0.132 square miles.
TL;DR (Too Long; Didn't Read)
Always keep units in your calculations. This will ensure that you use the conversion ratios the right way.