Key Takeaways
- Footing count is the beam count multiplied by the posts per beam
- Beams run along the deck length; posts support each beam along the width
- Footing size and depth depend on soil, load and frost depth, not a formula
- Each footing is modelled as a cylinder for the concrete volume
A deck’s footings sit at every point where a support post meets a beam, so counting footings means working out the beam layout first, then the posts under each beam.
The beam and post grid
Beams run along the deck’s length, spaced at an interval you choose, commonly 8 feet. The beam count follows the same plus-one logic as joist count: ceil(deck length / beam spacing) + 1. Each beam is then supported by posts spaced along the deck’s width, again plus one: ceil(deck width / post spacing) + 1. Multiplying beam count by posts per beam gives the total footing count, since every beam-post intersection needs its own footing.
A 16 by 12 foot deck at 8 foot beam and post spacing has 3 beams and 3 posts per beam, for 9 footings total.
Concrete per footing
Each footing is modelled as a cylinder, a poured concrete pier or a sonotube form, the common shape for a deck footing: pi times the radius squared times the depth. A 12 inch diameter, 36 inch deep footing takes about 2.36 cubic feet of concrete. Multiplying by the footing count and adding a waste allowance gives the total concrete for the job.
Footing size is a site condition
Unlike beam and post count, footing diameter and depth are not something this calculator derives from the deck’s dimensions. Soil bearing capacity, the load the deck carries, and local frost depth all determine what a footing actually needs to be. This is why the calculator asks for those dimensions directly, rather than assuming a value: check with your local building department or a structural professional for the required footing size before pouring, especially for anything beyond a small, low deck.
Use the deck footing calculator to work out footing count and concrete for your own deck’s beam and post layout. For the joists that span across those beams, see the deck joist calculator, and for a full deck material estimate including decking boards, the deck calculator covers the whole project on one page.