What Size Deck Post Should You Use?

See Why Tributary Area Changes the Post You Can Use

Hold the load condition constant at 100 sq. ft. of tributary area under the Southern Pine 40 psf table. Now compare the allowable structural post heights:

Deck Post Height Chart — Southern Pine

The table below shows maximum post heights for Southern Pine, No. 2 grade, under the 40 psf live-load condition in IRC Table R507.4. The table assumes a 10 psf dead load and includes the wet-service factor.

Tributary area is the amount of deck surface whose load is ultimately carried by the post.

Deck Post Height Chart — Douglas Fir, Hem-Fir & SPF

The same post dimensions can have different allowable heights when the lumber species changes.

Deck Post Height Chart — Redwood & Western Cedar Group

Redwood, Western Cedars, Ponderosa Pine, and Red Pine have lower prescriptive limits in several higher-load post configurations.

Why Older Deck Post Height Charts Look Different

If you search for deck post height limits online, you may find a much simpler table that says:

• 4×4: 6′-9″
• 4×6: 8′-0″
• 6×6: 14′-0″
• 8×8: 14′-0″

Those values match the simpler 2018 IRC Table R507.4 approach. Later IRC editions expanded the table to account for tributary area, species, post size, and loading.

The newer table is more sophisticated because it accounts for:

• lumber species
• post size
• tributary area
• design load

However, your local jurisdiction may still enforce an older IRC edition or local amendment, so always verify the code actually adopted where the deck is being built.

How Is Deck Post Height Measured?

For IRC Table R507.4, post height is measured from the underside of the beam to the top of the footing or pier.

That means the relevant structural post height is not necessarily the same as:

• deck surface height above grade
• top of railing height
• total length of lumber purchased

Example

A deck surface might be 10 feet above grade while the beam sits below the joists. The structural post height to the underside of that beam can be somewhat shorter than the deck-surface elevation.

Why Tributary Area Controls Deck Post Size

Tributary area is the portion of deck surface whose gravity load is delivered to a specific post.

The larger the tributary area, the more load that post generally carries.

This explains why the Southern Pine 4×4 table changes from 14 feet at 20 square feet of tributary area to only 6′-2″ at 160 square feet.

The lumber dimensions did not change.

The load did.

How Do You Estimate the Tributary Area of a Deck Post?

For a simple deck, tributary area can be visualized as the portion of deck halfway to the neighboring supports in each direction.

An interior beam post often carries more tributary area than an end post because it receives load from beam spans on both sides.

Simplified Example

Suppose posts are spaced 8 feet apart along a beam and the beam receives load from approximately 6 feet of deck width.

A simplified interior tributary area might be:

The actual tributary geometry depends on the complete joist, beam, cantilever, and support layout.

This is why post sizing should follow the framing design rather than being chosen before it.

See our Deck Tributary Area Guide and Deck Post Spacing Chart.

Can You Use 4×4 Posts for a Deck?

Yes, in qualifying prescriptive configurations.

Current residential deck provisions do not universally prohibit 4×4 support posts.

However, allowable height can decrease quickly as post load increases.

For example, under the Southern Pine 40 psf table:

• 40 sq. ft. tributary area → 13′-8″
• 80 sq. ft. → 9′-5″
• 120 sq. ft. → 7′-5″
• 160 sq. ft. → 6′-2″

When Would You Use a 4×6 Deck Post?

A 4×6 provides greater cross-sectional capacity than a 4×4 while using less material than a 6×6.

Under the Southern Pine 40 psf table, a 4×6 can remain substantially taller than a 4×4 at higher tributary areas.

Example

At 120 square feet of tributary area:

• 4×4 Southern Pine: 7′-5″
• 4×6 Southern Pine: 9′-8″
• 6×6 Southern Pine: 14′-0″

In practice, 6×6 is often more common because of simpler beam connections, symmetry, availability, and familiarity.

How Tall Can a 6×6 Deck Post Be?

Under the current 40 psf residential deck-post table, Southern Pine and the Douglas Fir/Hem-Fir/SPF group can use 6×6 posts up to 14 feet high throughout the listed tributary-area range from 20 to 160 square feet.

The Redwood/Western Cedar group begins to reduce below 14 feet at the highest tributary areas.

When Do You Need 8×8 Deck Posts?

Most ordinary residential decks do not require 8×8 support posts simply because the deck is elevated.

An 8×8 may be considered when:

• project-specific loads are unusually high
• large beams or engineered framing require a larger support
• architectural appearance calls for larger posts
• an engineer specifies them
• special loading falls outside common prescriptive deck design

4×4 vs 6×6 Deck Posts

How Should a Deck Beam Connect to a Post?

Post size is only useful if the beam has an adequate load path into the post.

Modern prescriptive deck framing requires the beam or girder to have structural bearing at its support rather than simply being attached to the side of a post with through-bolts and relying on those bolts to carry the gravity load.

Common solutions include:

• beam bearing on top of the post with an approved post cap
• a properly detailed notched-post connection where permitted
• another approved engineered connection

See our Deck Post-to-Beam Connection Guide, Deck Beam Size Chart, and Deck Beam Span Chart.

How Should a Deck Post Connect to the Footing?

The post-to-footing connection needs to transfer load while restraining the bottom of the post from unwanted lateral movement.

Common systems include approved manufactured post bases anchored to concrete or other approved foundation details.

Continue with our Deck Footing Size Chart and Deck Footing Spacing Guide .

Should Deck Posts Be Buried in the Ground?

Many modern deck foundations use concrete footings or piers with manufactured post-base connectors rather than burying the structural post directly in soil.

The appropriate foundation detail depends on:

• local code
• frost depth
• soil conditions
• wood treatment rating
• foundation system
• lateral-restraint requirements

Do Tall Deck Posts Need Bracing?

Tall decks deserve special attention to lateral stability because long posts can make the structure more flexible.

Bracing should not be improvised.

In AWC’s DCA 6 approach, diagonal knee braces are shown at corner posts rather than interior posts.

This matters because bracing changes how lateral loads move through the support system.

Can You Splice a Deck Support Post?

Do not assume two shorter posts can simply be stacked or joined together to create one taller structural post.

Prescriptive deck layouts generally rely on continuous posts between the foundation and the supported beam unless an approved structural detail specifically provides otherwise.

How Snow Load Changes Deck Post Height

The 40 psf live-load chart above is not appropriate for every location.

Current deck-post tables also include higher ground-snow-load conditions.

As required load increases, allowable post height can decrease, particularly for:

• 4×4 posts
• 4×6 posts
• weaker species groups
• posts carrying large tributary areas

Can You Use This Chart for a Roofed Deck?

Not automatically.

Adding a roof, canopy, pergola with structural roof loading, or other supported structure changes the load path.

The deck posts may then carry:

• deck floor loads
• roof dead load
• roof snow load
• wind loads
• additional lateral loads

How Post Spacing Changes Post Size

Wider deck-post spacing can reduce the number of posts but increases the tributary area carried by the remaining supports.

That creates an important structural tradeoff:

This is why post spacing, beam sizing, post sizing, and footing sizing should be designed together.

Continue with our Deck Post Spacing Chart .

How Post Size Affects Footing Design

The footing supports the load delivered through the post.

A larger post does not reduce the gravity load coming from the deck above.

Footing size still depends on:

• tributary area
• design load
• soil bearing capacity
• post reaction
• local foundation requirements

Example: Choosing a Deck Post Size

Suppose an interior support post carries approximately 100 square feet of tributary deck area.

Under the Southern Pine 40 psf table:

If the required structural post height is 10 feet:

• the 4×4 would exceed its tabulated height
• the 4×6 would fit under this specific table condition
• the 6×6 would also fit

How to Determine Deck Post Size Step by Step

1. Determine the deck dimensions.
2. Lay out the joists.
3. Determine the joist span and cantilever.
4. Size the beam.
5. Determine allowable post spacing from the beam span.
6. Calculate or estimate the tributary area at each post.
7. Identify the lumber species.
8. Determine the required structural post height.
9. Select a post size that satisfies the applicable table.
10. Size the footing for the actual post reaction and soil.
11. Design beam-to-post and post-to-footing connections.
12. Check lateral stability and bracing.
13. Verify the locally adopted code and design loads.

Common Deck Post Sizing Mistakes

1. Assuming Every Deck Requires 6×6 Posts

Six-by-six is common, but smaller posts can still be prescriptively permitted in qualifying conditions.

2. Assuming Every 4×4 Is Limited to 6′-9″

That shortcut reflects an older code table and does not represent the current tributary-area-based approach.

3. Ignoring Tributary Area

Two posts of the same size can have very different allowable heights because they carry different loads.

4. Ignoring Lumber Species

Species changes the tabulated height.

5. Using Deck Height Instead of Structural Post Height

The applicable table dimension is measured to the supported beam, not simply to the walking surface.

6. Treating a Larger Post as a Substitute for a Larger Footing

Footing size depends on the load and soil capacity.

7. Side-Bolting the Beam Without Proper Bearing

The beam needs an approved gravity-load path into the post.

8. Adding Knee Braces Everywhere

Bracing changes structural load behavior and should follow an approved design.

9. Splicing Posts Without Engineering

Structural support-post splices should not be improvised.

10. Using a 40 psf Table in a Higher-Load Location

Snow and other required loads can reduce allowable heights.

Signs an Existing Deck Post System Needs Attention

• leaning posts
• significant checking or splitting
• decay near the base
• posts no longer centered on footings
• movement at beam-to-post connections
• beam bearing that appears inadequate
• corroded post bases or caps
• settled or heaved footings
• improvised post splices
• tall supports with obvious lateral movement

Use our Deck Inspection Checklist .

Frequently Asked Questions

What size posts should I use for a deck?

Post size depends on post height, lumber species, tributary area, and design load. Six-by-six posts are very common, but 4×4 and 4×6 posts remain permitted in qualifying prescriptive configurations.

How tall can a 4×4 deck post be?

There is no single modern limit. Under the Southern Pine 40 psf table, a 4×4 ranges from 14′-0″ at 20 square feet of tributary area to 6′-2″ at 160 square feet.

How tall can a 6×6 deck post be?

Under the 40 psf table shown here, Southern Pine and the Douglas Fir/Hem-Fir/SPF group allow 6×6 posts up to 14 feet throughout the listed 20–160-square-foot tributary-area range.

Are 6×6 posts required for decks?

No. Six-by-six is very common and often practical, but current prescriptive deck provisions still allow smaller post sizes where their height and load satisfy the applicable table.

Can I use 4×4 posts for a 10-foot-high deck?

Sometimes, but not based on height alone. The answer depends on species, tributary area, load, and the locally adopted code. For example, a Southern Pine 4×4 under the 40 psf table can reach 11 feet at 60 square feet of tributary area but only 9′-5″ at 80 square feet.

Is a 4×6 stronger than a 4×4 deck post?

Generally yes, and the prescriptive height tables allow a 4×6 to remain taller under many higher-tributary-area conditions.

Do taller deck posts need bigger footings?

Not simply because they are taller. Footing size is primarily driven by the gravity load delivered through the post and the allowable soil-bearing capacity. Tall posts do create additional stability and lateral-design considerations.

How is deck post height measured?

IRC deck-post height is measured from the underside of the supported beam to the top of the footing or pier—not simply to the walking surface.

Can deck posts be spliced?

Do not assume support posts can be field-spliced simply by joining two shorter pieces. A structural post splice requires an approved design and connection.

Should deck beams sit on top of posts?

The beam needs proper structural bearing at its support. Common solutions include bearing on top of the post with an approved post cap or an approved notched-post detail.

Do deck posts need knee braces?

Bracing depends on the deck design and applicable prescriptive or engineered requirements. AWC DCA 6 uses diagonal bracing at corner posts rather than interior posts in its applicable configuration.

Can this chart be used for a deck with a roof?

Not automatically. Roof loads add gravity, wind, and potentially snow forces that are outside an ordinary single-level deck-post calculation.

The Backyard Standard Final Answer

There is no universal answer to:

The correct post depends on:

• post height
• tributary area
• lumber species
• design load
• post spacing
• beam configuration
• connection details
• lateral stability

For many modern residential decks, 6×6 is the most practical starting point because it fits a broad range of prescriptive conditions and provides substantial connection area at the beam and footing.

But 4×4 and 4×6 posts are not automatically prohibited. Current prescriptive tables allow them when their actual load and height fall within the listed limits.

Continue with our Deck Post Spacing Chart , Deck Beam Size Chart , and Deck Footing Size Chart .

Sources & Technical References

Technical references reviewed: September 2026

• International Code Council — 2024 IRC Chapter 5
• International Code Council — 2021 IRC Chapter 5 / Table R507.4
• American Wood Council — Residential Deck Resources
• American Wood Council — DCA 6 (2015 IRC-Based)