How this calculator works
The calculator estimates footing count from deck length divided by target post spacing, plus one end footing. It treats each footing as a round cylinder using diameter and depth, then multiplies by footing count and waste allowance. Concrete bags are estimated using about 0.6 cubic feet per 80-lb bag.
Example: a 20 ft deck with 6 ft target spacing needs 5 footings. A 12 inch diameter by 36 inch deep footing is about 2.36 cubic feet, so five footings with 10% waste need about 13.0 cubic feet of concrete, or 22 bags.