Place the protractor on one of the corners of the four-sided figure. Measure the inside angle to confirm that it is actually 90 degrees. Perform this step on the other remaining four sides and confirm that the angles are also 90 degrees.
Place a ruler flat along one edge of the square shape. Make sure the zero mark lies completely on the corner. Failure to conform to this will make your reading have a zero error, which will eventually bring inaccuracies in the final calculation. A zero error is when you record a figure slightly less or more than the actual reading due to the position of the eye. When the eye position is to the left or right of the mark, inaccurate readings are bound to be recorded. Repeat this procedure on the remaining three edges of the square.
Observe the mark that conforms to the other corner on the same edge where the ruler passes along. Record the reading to the nearest decimal figure in case the ruler mark falls between two millimeter marks on a piece of paper, because this is the actual length of the side of the square.
Key-in the length in the calculator and multiply it by four. This is because all the four sides of the square are equal. Alternatively, key in the length of the side of the square and add it to itself three times. The observed figure is the perimeter, or length, of the square.