Deck Footing Calculator

What the deck footing calculator does

Each deck post carries the weight of the deck area around it, its tributary area, down to a footing. The footing spreads that load over enough soil to keep the bearing pressure within what the ground can take. The deck footing calculator works out the tributary area from the beam layout and post spacing, multiplies by the design load, divides by the soil capacity to get the bearing area, and rounds the diameter up to a standard size.

Deck footing size formula

For an attached deck with one beam, the beam carries half the joist span; with more beams, each interior beam carries one full span:

The count is beam rows × (length ÷ post spacing + 1). Concrete per footing is π ÷ 4 × diameter² × depth, and one 80 lb bag yields about 0.6 cubic feet.

Deck footing calculator example

A 16 by 12 ft deck attached to the house, one beam, posts every 8 ft, 50 psf, 1,500 psf soil, 36 in deep:

The deck needs 3 footings. Each takes 5.3 ft³ of concrete, about 9 bags of 80 lb, or 0.59 cubic yards for all three.

How to use the deck footing size calculator

Enter the deck length along the beam and the width in the joist direction. Choose attached if a ledger carries one side, or freestanding. Enter the number of beam rows and the post spacing along each beam. Keep 50 psf for a typical residential deck (40 live plus 10 dead) and 1,500 psf soil bearing unless you know your soil. Add the footing depth to get the concrete. Press Calculate.

Reading your deck footing size

The diameter is the minimum bearing size; a wider footing pad under a smaller post pier works the same. If the size comes out large, add a beam row or shorten the post spacing to cut the load per footing. The depth must reach below your local frost line, which ranges from a few inches in warm climates to over 4 feet in cold ones. Building departments publish footing tables that take precedence.

When to use a deck pier calculator

• Planning footings for a new deck
• Checking a contractor layout
• Ordering concrete and tubes
• Deciding between more posts or bigger footings
• Sizing footings for a hot tub area

Common deck footing calculator mistakes

• Footings above the frost line. Frost heave lifts the deck.
• Ignoring hot tubs. They add 100 psf or more locally.
• Assuming good soil. Use 1,500 psf unless tested.
• Counting end posts as fully loaded. Interior posts govern the size.

Tips for deck post footing size

• Call before you dig to locate buried lines.
• Pour footings on undisturbed soil.
• Use post bases to keep wood off the concrete.

Related deck tools

Footings, framing, and decking go together. The joist span calculator sets the beam spacing, the deck board calculator counts the decking, and the concrete bag calculator sizes the bag order.

Deck footing spacing and beam layout

Footing size, post spacing, and beam size trade off against each other. Wider post spacing means fewer footings but more load on each one and a deeper beam. Adding a second beam row halves the joist span and the load on the outer beam. Set the joist span first with the joist span calculator, since it decides how far the beams can sit from the ledger.

Concrete for deck footings and sonotubes

Most decks use cardboard form tubes on a wider footing pad, or a bell-shaped form that spreads the load at the bottom. Concrete volume grows with the square of the diameter, so an 18 in footing takes over twice the concrete of a 12 in one. For the bag count across all footings, the concrete bag calculator converts volume into 40, 60, or 80 lb bags. Once the frame is up, the deck board calculator counts the boards and screws.

Deck post footing size by tributary area (50 psf)

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