The cleanest way to think about deck-footing size is:

For a simple planning example, if one footing carries 4,000 lb and the allowable soil-bearing pressure is 1,500 psf, the required bearing area is about 2.67 sq. ft. A round footing with that area needs a theoretical diameter of about 22.1 inches, so a practical/code-compliant design must use a footing at least large enough to satisfy that area plus all applicable prescriptive requirements.

The post sitting on top does not, by itself, determine footing diameter. Two 6×6 posts can require very different footings if their reactions or soil conditions differ.

The chart below converts vertical footing reaction into the approximate theoretical diameter of an equivalent round bearing area. It is a planning aid for understanding the relationship—not a replacement for the applicable IRC footing table, approved plans, frost requirements, footing thickness, or local soil information.

Deck footings support the entire structural load of the deck, including:

• framing lumber
• decking boards
• railings
• furniture
• people using the deck
• snow loads in colder climates

If the soil beneath the footing cannot safely support the load, the footing may settle or shift.

Larger footings spread weight across more soil surface area, reducing pressure on the ground.

Deck framing follows a structural load path:

Every structural component transfers weight downward through the framing system.

Understanding this load path explains why footing size affects the stability of the entire deck structure.

Related: Deck Beam Span Chart, Deck Joist Span Chart, and Deck Post Spacing Chart.

See How the Load Reaches the Footing

Now hold the footing reaction constant at 4,000 lb and change only the allowable soil-bearing pressure.

Several structural variables determine how large deck footings must be.

Posts that are spaced farther apart carry more structural load from the beams above them.

Wider post spacing increases the tributary area supported by each footing.

As tributary load increases:

• beam loads increase
• post loads increase
• footings usually must become larger

Related: Deck Post Spacing Chart.

Longer beam spans and longer joist spans both increase structural load on the footing system.

Long joists transfer more load into the beam, and larger beam spans transfer more load into the posts and footings below.

General structural relationships:

• longer joists → heavier beam loads
• larger beam spans → larger footing loads
• wider post spacing → larger footing requirements

Related: Deck Joist Span Chart and Deck Joist Spacing.

Tributary load refers to the portion of the deck supported by a particular structural component.

Each footing supports part of the deck surface area through the posts and beams above it.

Larger tributary areas create larger structural loads.

Related: Deck Tributary Area Guide.

Allowable soil-bearing pressure tells you how much vertical load can be distributed over each square foot of footing area. For the same footing reaction, lower allowable soil pressure requires more bearing area.

The American Wood Council notes that the IRC allows 1,500 psf to be used unless local conditions are known otherwise. Do not assign a higher value just because the soil visually appears to be gravel, clay, or sand; use the value permitted by the adopted code, local building department, or project-specific geotechnical information.

Disturbed or undocumented fill, organic soils, expansive soils, steep slopes, groundwater, and other unusual site conditions can take a project outside a simple prescriptive assumption.

Footing diameter and footing depth solve different problems. Diameter/bearing area distributes vertical load into the soil. Foundation depth and frost protection help prevent seasonal soil movement from lifting or shifting the support.

Do not use a national “warm/moderate/cold climate” depth chart. Required frost protection is local. The adopted code, building department, and site conditions determine whether the footing must extend below the local frost line or may use another approved frost-protected foundation method.

A basic gravity bearing-area check starts with:

For the IRC/DCA 6 prescriptive condition used throughout this framing cluster, the baseline gravity loading is:

• 40 psf live load
• 10 psf dead load

Together, this equals roughly:

Imagine a footing supporting approximately 80 square feet of deck area.

Step 1 — Calculate total load:

Step 2 — Estimate soil bearing capacity:

Many residential planning assumptions use approximately:

Step 3 — Calculate required footing area:

Step 4 — Convert to round footing diameter:

For a circular bearing area, 2.67 square feet corresponds to a theoretical diameter of about:

This example shows why footing size follows the reaction at that support, not a generic deck-size or post-size rule. The final footing must also satisfy the applicable prescriptive table/minimums and local requirements.

Many simplified online footing charts show footing size based only on post spacing or post size.

In reality, footing size depends on:

• tributary load
• beam span
• joist span
• soil bearing capacity
• snow load
• deck configuration

Some deck features dramatically increase structural loading and may require engineered footing design.

Examples include:

• hot tubs
• outdoor kitchens
• masonry fireplaces
• multi-level decks
• rooftop decks
• large snow loads

These features can increase tributary loads significantly beyond standard residential assumptions.

Concrete quantity depends on the actual foundation geometry. A cylindrical pier, a widened footing beneath a pier, and a formed pad do not use the same volume even if the visible post support looks similar.

For a simple cylinder only:

For example, a 12-inch-diameter × 36-inch-high cylinder contains about 2.36 cubic feet of concrete. But that does not mean a 12-inch-diameter cylinder provides enough bottom bearing area for the deck load. If the required footing is wider than the pier/tube, calculate the widened base separately.

Use the Deck Footing Calculator for project quantities after the foundation geometry is established.

Most residential decks use poured concrete footings installed below frost depth.

Typical footing installation includes:

• excavation
• cylindrical footing forms
• concrete placement
• post anchors

Alternative systems may include:

• helical screw piles
• precast footing systems
• engineered footing pads

Helical piles are often used where:

• excavation is difficult
• soil conditions are poor
• access is limited

#ad

As an Amazon Associate, The Backyard Standard earns from qualifying purchases, at no additional cost to you.

These verified BYS database picks are useful for laying out supports and working through the calculations discussed in this guide.

• footings that are too small
• footings installed above frost depth
• poor soil preparation
• posts not centered on footings
• ignoring tributary load increases
• using generic charts without code verification

• deck settlement
• sloping or uneven deck surfaces
• beam sagging
• movement near posts
• cracking concrete
• shifting after freeze-thaw cycles

Larger footings increase:

• excavation work
• concrete volume
• labor time

However, undersized footings can create expensive structural repairs later.

How big should deck footings be?

There is no single correct diameter range. Determine the footing reaction and allowable soil-bearing pressure, then provide the bearing area and other dimensions required by the applicable prescriptive table or project-specific design.

Do deck footings need to go below the frost line?

Frost protection is required where applicable, but the exact depth or approved frost-protection method is determined by the locally adopted code and site conditions.

How far apart should deck footings be?

There is no universal footing spacing. For a conventional post-and-beam deck, footing locations normally follow the posts, and post spacing is established by allowable beam span.

Can deck blocks replace concrete footings?

Only where the deck configuration and locally adopted code permit that foundation system. Do not assume a surface deck block is interchangeable with a frost-protected footing for an attached or elevated deck.

Does soil type affect footing size?

Yes. Weak soils require larger footings because they support less pressure safely.

Do hot tubs require larger footings?

Yes. Hot tubs create very large concentrated loads that often require engineered footing design.

Why are some footing charts inaccurate?

Many charts ignore tributary load, soil conditions, beam span, and snow load — all of which affect actual footing requirements.

Deck footing size is one of the most important structural decisions in deck construction because every load from the deck ultimately transfers into the footing system.

Actual footing bearing-area and foundation requirements depend on:

• tributary load
• beam span
• joist span
• post spacing
• soil strength
• frost depth
• local code requirements

Technical references reviewed: September 2026

• International Code Council — 2024 IRC Chapter 5
• International Code Council — 2021 IRC Chapter 5
• American Wood Council — Residential Deck Resources
• American Wood Council — DCA 6 (2015 IRC-Based)