Measure one side of the parallelogram with a ruler. In this example, let one side measure 3 inches. The base is 3 inches.
Divide the parallelogram's area by its height to determine the length of the base if its height and area are known. Since the height could also be considered the base, you can employ this method to find the "other" base. For example, if the area is 90 cm and the height is 10 cm, then dividing 90 by 10 results in a base of 9 cm.
Multiply the height measurement by 2, subtract that product from the parallelogram's perimeter, then divide the difference by 2 to calculate the length of the base if its height and perimeter are known. Again, this can be seen as finding the length of the "other" base. For example, if the perimeter is 40 cm and the height is 12 cm, then multiplying 12 by 2 equals 24, subtracting 24 from 40, or 40 - 24, equals 16, and dividing 16 by 2 equals 8. The length of the base is 8 cm.