If you're asked to calculate a large area – say, the area of a township, the amount of land between two long roads or the area of a large body of water – linear miles are the most likely unit of measure you'll be given. You can use the simple formula of length × width to approximate the area of any space that's roughly square or rectangle shaped. Or, if you've been given the space's area in a different unit of measure, you can convert from that unit into square miles.
TL;DR (Too Long; Didn't Read)
Apply the formula A = l × w for any rectangular-shaped area, where A is the area, l is the area's length in miles and w is its width in miles.
Calculating Square Miles
If you're given the length and width of the space you're measuring in miles, finding its area is as simple as multiplying the two measurements together, using the basic formula Area = length × width.
So if you're asked to calculate the area of a tract of land that measures 2 miles by 4 miles, you'd calculate:
2 mi × 4 mi = 8 mi2
The area of the tract is 8 miles squared.
Converting Feet to Miles
Another very common measurement of linear dimensions or area in the United States is feet. If you receive your measurements in feet but need the result to be in square miles, you have two options: Either convert the measurements to miles before you multiply length × width, or do it after. Which formula you use depends on when you make the conversion.
Convert Feet to Miles Before Multiplying
Divide by 5,280 to convert your measurements in (linear) feet into miles. So if you are calculating the area of a lake that measures 5,280 feet by 10,560 feet, you'd divide each measurement by 5,280:
5280 ÷ 5280 = 1
10560 ÷ 5280 = 2
So the area measures 1 mile by 2 miles. Now that you have your measurements in miles, you can simply multiply them together to get the area:
1 mi × 2 mi = 2 mi2
Convert Square Feet to Square Miles
If you already have the space's area in square feet and simply need to convert the result to square miles, divide the result by 27,878,400 ft2/mi2.
For example, if you had multiplied 5,280 feet × 10,560 feet without first converting into miles, you'd have an area of 55,756,800 ft2. Divide that by 27,878,400 and you'll have:
55756800 ÷ 27878400 = 2
So the space's area is 2 mi2. Note that you get the same answer from both Step 1 and Step 2; as long as you use the correct conversion factor, it doesn't matter if you convert before or after multiplying to find the area.
Converting Acres to Square Miles
The other unit of area you're likely to encounter, especially when dealing with landforms, is acres. Once defined as the area of land that can be plowed in one day by one man working with one ox, the acre has since been standardized to 1/640th of a square mile. Or, to put it another way, there are 640 acres in one square mile. So to convert from acres into square miles, divide by 640.
Example: You're told that a tract of land measures 1,920 acres. How many square miles is it? Divide by 640 to find out:
1920 ÷ 640 = 3
So the tract of land measures 3 mi2.