Find the dimensions of your land, city, town, state or whatever type of land for which you are attempting to calculate square mileage. You need to know the length and the width of the area of land you are trying to calculate.
Use basic multiplication to figure out the square mileage of your particular area. Using a calculator is the easiest way to calculate your square mileage, but you can do it by hand if you are good with math.
Multiply the length of your land by the width of your land. For example, if the piece of land you need to know the square mileage of is 2 miles wide and 5 miles long, you would multiply 2 by 5, giving you the result of 10. The square mileage of your land is 10 square miles. Length times width equals the area of that which is measured and is expressed in "square" terms. In this case, square miles.