Deck Footing Calculator
Work out how many footings a deck needs and how much concrete each takes.
What you'll need to order
- concrete
- sonotube forms
- post bases
- rebar
- gravel
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
footings = (deck length ÷ post spacing) + 1
This assumes a ledger-attached deck with one beam line — the house carries the other half of the load. A freestanding deck needs two beam lines and twice the footings.
Worked example
With deck length 16 ft, deck width 12 ft, post spacing along the beam 6 ft, footing diameter 12 in, this calculator returns 3.67 .
Frequently asked questions
How many footings does a deck need?
Divide the beam length by the post spacing and add one. A 16 ft ledger-attached deck with posts every 6 ft needs four footings along its single beam line.
How deep do deck footings go?
Below the frost line, which varies from 12 inches in the far south to 48 inches or more up north. The footing must bear on undisturbed soil, never on backfill.
What diameter should a deck footing be?
Twelve inches handles most residential decks on average soil. Calculate properly from the tributary area, the design load of 50 psf, and your soil bearing capacity — poor soil needs a much wider base.
Does a freestanding deck need more footings?
Yes, roughly double. A ledger-attached deck lets the house carry half the load; a freestanding one needs two beam lines with their own posts and footings.