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.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ICC International Residential Code 2021, R507.3 — deck footing size and depth against tributary load
Cited to a specific edition on purpose — table numbers and values move between cycles. Confirm against whichever edition is enforced locally.
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 .
Footings for decks of common size
Each row is a link that fills in the calculator above. Send one and the other end sees the same figures you do.
| Job | Size | Footings needed |
|---|---|---|
| Small landing | 8 ft × 6 ft × 6 ft × 10 in | 2.33 |
| Typical rear deck | 16 ft × 12 ft × 6 ft × 12 in | 3.67 |
| Wide deck | 24 ft × 14 ft × 8 ft × 16 in | 4.00 |
| Wrap-around section | 32 ft × 10 ft × 6 ft × 12 in | 6.33 |
| Hot tub pad edge Close spacing for concentrated load |
10 ft × 10 ft × 4 ft × 16 in | 3.50 |
This assumes a ledger-attached deck with one beam line. Freestanding decks need two, and twice the footings.
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.