Many conventional attached residential decks have one exterior beam supported by approximately 3–5 posts, but there is no universal post count for a given deck size.

A basic 12×12 attached deck might use three posts beneath its exterior beam, while a longer deck may need four or more. A freestanding deck typically needs additional posts because it normally requires another support beam instead of relying on a house ledger for vertical support.

Simple Post Count Formula

For example, if a beam must be divided into three supported spans:

3 beam spans + 1 = 4 posts

This formula works only after you determine an acceptable beam span for the actual framing conditions.

Two decks with identical dimensions can require different numbers of posts.

Consider two 12×16 decks.

One might use a larger beam capable of creating two acceptable spans across the 16-foot beam run. That layout could use three posts.

Another design might use a smaller beam requiring three shorter spans. That layout would require four posts.

The deck dimensions did not change. The beam design and support spacing changed.

The easiest way to understand deck post count is to stop thinking about posts first and think about beam spans.

Each pair of neighboring support posts creates a beam span.

Therefore:

• 2 posts create 1 beam span
• 3 posts create 2 beam spans
• 4 posts create 3 beam spans
• 5 posts create 4 beam spans

Use the Deck Beam Span Chart to understand how beam size, joist span, lumber species, and load affect allowable span.

Eight-foot post spacing is common enough that it is sometimes repeated as though it were a universal deck-building rule.

It is not.

Post spacing is really the span of the beam between supports, and allowable beam span changes with:

• beam size
• number of beam plies
• lumber species
• joist span
• structural loading

Eight feet may be appropriate for one deck and unnecessarily conservative—or structurally excessive—for another.

For a more detailed explanation, see the Deck Post Spacing Chart.

Step 1: Draw the deck footprint

Start with the overall width and projection of the deck.

Step 2: Determine whether the deck is attached or freestanding

An attached deck may use a properly designed ledger as one side of the vertical support system. A freestanding deck normally needs structural support independent of the house.

Step 3: Establish joist direction

Joists usually run perpendicular to the house on a conventional attached deck.

Step 4: Determine joist span

Measure the supported joist distance—not simply the overall deck projection when a cantilever is involved.

Step 5: Select the beam configuration

Beam size, number of plies, species, grade, and joist span all affect allowable beam span.

Step 6: Determine allowable beam span

Use an applicable code table or approved design method rather than a generic post-spacing rule.

Step 7: Divide the beam into acceptable spans

Suppose a 20-foot supported beam run can be divided into spans no greater than approximately 8 feet under the selected design.

Two 10-foot spans would be too long, so the beam would need at least three spans.

Step 8: Add one support

Three spans require four support points.

Step 9: Check beam cantilevers

An allowable beam overhang beyond the end posts can sometimes change the required spacing and post locations.

Step 10: Verify footing requirements

Every support post transfers load into a footing. Post count therefore affects footing count, but footing size still depends on the load carried and soil bearing capacity.

Consider an attached deck that projects 12 feet from the house and runs 16 feet along the house.

A conventional layout might use:

• a ledger at the house
• joists running away from the house
• one exterior beam running parallel to the house
• support posts beneath that beam

If the selected beam design permits the 16-foot beam run to be divided into two acceptable spans, the beam could use:

If the selected beam can only accommodate shorter spans, the same deck may need four posts instead.

That is why “How many posts for a 12×16 deck?” does not have one universally correct answer.

The ends of a deck beam may be permitted to extend beyond the outer support posts.

This is called a beam cantilever.

A properly designed beam cantilever can move the end posts inward while still allowing the beam to reach the full deck width.

However, cantilevers are limited by the applicable beam design requirements and should not be used as an arbitrary way to eliminate supports.

See the Deck Cantilever Guide for joist and beam overhang requirements.

Deck posts are only one part of the structural system.

Each post receives load from the beam and transfers that load into the footing below.

Increasing post spacing can increase the tributary load carried by each post. That may also increase the required footing size.

In other words:

• fewer posts do not automatically mean a better design
• fewer posts may require larger beams
• fewer posts may increase individual footing loads
• more posts can reduce individual beam spans

Use the Deck Footing Size Chart to continue the load path into the foundation system.

For conventional post-supported deck framing, each structural post generally requires a properly designed footing beneath it.

That means a layout with four support posts will commonly have four corresponding post footings along that support line.

However, footing count and footing size are different questions.

Footing size depends on factors including:

• tributary load
• beam span
• joist span
• soil bearing capacity
• local frost requirements

See How Many Footings Do I Need for a Deck? for the complete footing-count calculation.

Post count should not be confused with post size.

The American Wood Council’s DCA 6 prescriptive deck guide uses 6×6 nominal posts as its minimum post size within the scope of that guide.

Post height and allowable loading also matter. Tall decks and unusual structural conditions may require additional analysis.

Correct post count does not help if the beam-to-post connection cannot safely transfer the load.

Modern prescriptive deck construction generally relies on the beam having proper bearing at its supports rather than simply fastening beam plies to the sides of a post and expecting bolts to carry the gravity load.

Common compliant approaches include:

• beam bearing on top of the post with an approved connector
• properly notched post configurations where permitted
• approved manufactured beam-to-post hardware

The connection must also restrain the beam against lateral displacement.

The post-count method in this guide is intended for preliminary planning of conventional residential decks.

Additional structural design may be required when the deck supports:

• hot tubs
• outdoor kitchens with concentrated loads
• roofs or pergolas transferring significant structural load
• large masonry features
• unusually high snow loads
• very tall posts
• complex multi-level framing
• unusual soil conditions

1. Assuming every deck uses 8-foot post spacing

Post spacing must come from the allowable beam span for the actual framing system.

2. Calculating post count from deck square footage

A 200-square-foot deck does not automatically require a particular number of posts. Deck shape and framing direction matter.

3. Ignoring joist span

Longer joists increase the load delivered to the beam and can reduce allowable beam span.

4. Forgetting that freestanding decks need additional support

Removing the ledger generally means replacing that support with another structural beam or approved support system.

5. Reducing post count without resizing the beam

Removing a post creates a longer beam span. The existing beam may not be capable of carrying it.

6. Ignoring footing capacity

Wider post spacing can place more load on each remaining support and footing.

7. Treating an online example as a code-approved design

Example layouts are useful for planning, but final framing should be checked against applicable span tables, loads, product requirements, and local code.

Accurate post layout is much easier when beam lines, footing locations, dimensions, and elevations are established before digging begins.

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How many posts do I need for a 12×12 deck?

A conventional attached 12×12 deck may use three posts beneath a single exterior beam, but the actual number depends on beam size, joist span, lumber species, loads, cantilevers, and local code requirements.

How many posts do I need for a 12×16 deck?

Many conventional attached 12×16 layouts may use three or four posts beneath the exterior beam. The correct number is determined by the allowable beam spans rather than deck size alone.

How many posts do I need for a 16×20 deck?

There is no universal number. A 16×20 deck may require several posts depending on which dimension the beam spans, joist length, beam size, support configuration, and whether the deck is attached or freestanding.

How far apart should deck posts be?

Many residential layouts place posts somewhere around 6–10 feet apart, but that range is not a structural rule. Allowable spacing is determined by beam span and loading.

Can deck posts be 10 feet apart?

They can be in some designs if the beam, joist span, lumber species, loads, connections, and footing system permit that beam span. Do not assume 10 feet is acceptable without checking the applicable design table.

Can deck posts be 12 feet apart?

Some beam configurations can span approximately 12 feet or more under certain conditions, but many common residential beams cannot. Verify the exact beam configuration rather than using 12 feet as a general rule.

Can I remove a deck post?

Removing a post increases the beam span and changes the loads carried by the remaining supports and footings. The beam and foundation system should be evaluated before a structural post is removed.

Do I need a footing under every deck post?

Structural deck posts generally transfer their loads to approved foundations or footings. The required footing design depends on the post load, soil conditions, frost requirements, and local code.

Do freestanding decks need more posts?

Usually. A freestanding deck commonly needs another beam/support line in place of the vertical support otherwise provided at the house side of a conventional attached deck.

The number of deck posts you need is ultimately determined by the framing layout—not simply the square footage of the deck.

The most reliable planning sequence is:

1. determine the deck layout
2. establish joist span
3. select the beam configuration
4. determine allowable beam span
5. divide the beam into acceptable spans
6. add one support to the number of spans
7. verify cantilevers, post capacity, connections, and footing design

Once you understand that relationship, calculating post count becomes much easier—and the rest of the deck load path begins to make sense.

Last reviewed: August 2026

• American Wood Council — Prescriptive Residential Wood Deck Construction Guide (DCA 6)
• American Wood Council
• International Code Council — International Residential Code
• North American Deck and Railing Association