Length of the object times the width.
http://wiki.answers.com/Q/What_is_the_formula_to_c...

To convert inches to square feet you would need to measure inches in width and length. Divide the number of inches you have for both measurements by 12 to find the width and length
http://answers.ask.com/Reference/Other/how_to_conv...

1. Measure the diameter in centimeters. For this example, let the diameter measure 10 cm. 2. Multiply the diameter's length to itself to square it  10 cm multiplied by 10 cm results
http://www.ehow.com/how_8592343_convertdiameters...

A circle with a diameter of 4 inches has an area of
http://www.chacha.com/question/whatisthesquare...
