Measure the width of a pier platform beam. Standard pier platform beams are 2 inches by 8 inches or 2 inches by 12 inches.
Identify the thickness of the footing by determining the width of the pier platform beam. For example, if the platform beam is 12 inches wide, the footing must be 12 inches thick.
Double the width of the platform beam to calculate the total width of the footing. For example, a platform beam measuring 12 inches wide requires a 24-inch by 24-inch footing. Therefore, a pier with 12-inch wide platform beams requires a footing on the bottom of each support post measuring 12 inches thick and 24 inches by 24 inches.