Determine how many units of measurement are in one foot. One foot is the same as 12 inches, 30.48 cm, 0.3048 meters, 0.000189393 miles or 0.333 yards.
Convert the units of measurement into feet. Divide the dimension's measurement by the amount of that unit in one foot. For example, if the dimension's length is 108 inches, divide 108 inches by 12 inches, and you find that the 108 inch measurement is equivalent to 9 feet.
Use a calculator to check your calculation. Online Conversion and Google provide online calculators for unit conversions.
Determine the shape of the object or area. The equations used to calculate square footage are based on shape. For example, a 1-foot diameter circle and 1-foot-long rectangle will not have the same square footage.
Choose the appropriate equation for calculating area. The equation for calculating the area of a rectangle or square is the length of its base multiplied by its height. The area for a circle is the radius squared and multiplied by π, approximated at 3.14159. A triangle's area is one half its base length multiplied by its height.
Substitute the dimension measurements in feet into the appropriate equation and calculate the area. For example, if you have a rectangular living room that is 25 feet long and 15 feet wide, the area is 25 feet multiplied by 15 feet or 375 square feet.