The deck footing calculator computes the volume of concrete needed to fill post holes. For circular holes: Volume = π × (radius)² × depth The radius is half the hole diameter. For example, a 12-inch diameter hole has a 6-inch (0.5-foot) radius. Concrete bags have standard yields: - 40 lb bag: yields 0.015 cubic feet - 60 lb bag: yields 0.045 cubic feet - 80 lb bag: yields 0.06 cubic feet Number of bags = Total Volume ÷ Bag Yield (rounded up) Note: Building codes typically require footings to be placed below the local frost line depth (2-4 feet in most climates) to prevent frost heave damage.