Deck Footing Size Calculator (How Big Should Deck Footings Be?)

Quick answer: a typical attached deck with 12 ft joists and posts 8 ft apart puts about 2,400 lb on each post. On average soil (1,500 psf), that needs a footing of about 1.6 sq ft, which is a 18" round or 16" square footing. Enter your own numbers below.

Deck footing size calculator

Joist span, ledger to beam (ft)

Joist overhang past beam (ft)

Post spacing along beam (ft)

Design live / snow load 40 psf (standard deck) 50 psf snow 60 psf snow 70 psf snow

Soil type (bearing capacity) Clay, silt, sandy clay (1,500 psf) Sand, silty or clayey sand (2,000 psf) Sandy gravel, gravel (3,000 psf)

Post size 4x4 6x6

Assumes an attached deck with one beam, 10 psf dead load, and the soil values the IRC allows without a soil test (Table R401.4.1). If you are unsure of your soil, choose 1,500 psf. The bottom of the footing must sit below your local frost depth and at least 12" below grade.

Deck footing size chart (40 psf deck, 1,500 psf soil)

Minimum round footing diameter for each post, by joist span and post spacing.

How the calculation works

1. Area each post supports = (half the joist span + any overhang) × post spacing. The ledger on the house carries the other half of the joist span.
2. Load on the post = that area × (live load + 10 psf dead load).
3. Footing area needed = post load ÷ soil bearing capacity.
4. Round up to a standard tube or form size, then make the footing at least as thick as it sticks out past the post.

Common questions

What size footing do I need for a 6x6 deck post?

It depends on the load, not the post. Most residential decks need 16" to 24" round footings under 6x6 posts. Run the calculator with your spans to get the exact number.

How deep should deck footings be?

The bottom must be below your local frost line and at least 12" below undisturbed ground. In northern US states this can be 42" to 60" or more. Ask your building department for the local figure.

Is a 12" sonotube enough?

A 12" tube with no wider base gives only 0.79 sq ft of bearing area. That is enough only for small loads, for example short joist spans with posts close together. Many builders use a 12" tube on top of a wider footing or a flared footing base.

Do the corner posts need the same footing?

End posts carry about half the load of middle posts, so they can be smaller. Most builders use the same size everywhere to keep it simple.

Checked by: Engr. Abdul Latif, civil and structural engineer. Method follows the International Residential Code (Sections R401.4 and R507.3) and the AWC DCA 6 deck guide. Estimates for planning only. Your local code and inspector have the final say.