Measure the length and width of the area you wish to calculate. The more precise way to do this is hiring a surveyor to create a detailed schematic for you. However, you also can use the GPS on your smart phone, a pedometer or your car's odometer to measure yourself.
Write out the tenths of a mile measurements in decimal form. The "tenths" place is the one immediately after the decimal point. As an example, seven tenths times three tenths would be written as "0.7 x 0.3."
Multiply the figures to calculate area. In the example, 0.7 x 0.3 = 0.21 square miles. An alternative way to calculate the area is multiplying the tenths figures and moving the decimal point two places to the left. In the example, 7 times 3 equals 21. Moving the decimal point gives you the same answer of 0.21 square miles.