Draw a vertical line from the end of one base of the parallelogram to intersect the other base, and do the same for the other side. The shape should look like a rectangle with a right triangle on either side of it.
Count the number of squares from the vertical line to the end of the parallelogram. This gives you one side of the triangle. Counting the squares from bottom to top gives you the height of the parallelogram as another side.
Square the measures of the two known sides and add them. For example, if you have 3 and 4 as your measurements, add 9 and 16 to get 25. Take the square root to get 5. Multiply this by 2 for a result of 10.
Add the lengths of the two horizontal bases. If each horizontal base is 15, the result is 30. Add this to 10 to get 40. The parallelogram has a total perimeter of 40 centimeters.