Low deck, small tributary area
4x4
Legal when the table says so — 13'-8" at 40 ft² in southern pine — and paired with a 2-ply beam.
4x4 vs 6x6 deck posts: the post size chart
Our pick for most decks
6x6
Full 14-foot height across the live-load table in pine and fir, and wide enough to seat a 3-ply beam with every ply bearing.
Tall deck or heavy snow
8x8
The only size that holds 14 feet in every cell of the table, snow loads and the cedar group included.
Side by side
Our pick — 6x6
| Criterion | 4x4 | 6x6 | 8x8 |
|---|---|---|---|
| Wood in the cross-section | 12.25 in² | 30.25 in² | 56.25 in² |
| Pine, 40 psf live, 40 ft² | 13'-8" | 14'-0" | 14'-0" |
| Pine, 40 psf live, 120 ft² | 7'-5" | 14'-0" | 14'-0" |
| Cedar group, 40 psf live, 120 ft² | NP | 14'-0" | 14'-0" |
| Pine, 70 psf snow, 160 ft² | 3'-4" | 8'-7" | 14'-0" |
| Seats a 3-ply 2× beam fully | |||
| Allowed as a deck post at all |
4x4
The code's minimum wood column. Fine under short, lightly loaded beams; the table cuts its height quickly as tributary area grows.
Dressed size3½ × 3½ in
Max height, pine, 80 ft²9'-5"
Max height, pine, 160 ft²6'-2"
3-ply beam, full bearingno — 4½ in beam
Beam cap pairing (Simpson BCS)double 2×
6x6
The size most deck plans default to, for good reason: height is rarely the limit, and the top is wide enough for a 3-ply beam.
Dressed size5½ × 5½ in
Max height, pine, 80 ft²14'-0"
Max height, pine, 160 ft²14'-0"
3-ply beam, full bearingyes
Beam cap pairing (Simpson BCS)triple 2×
8x8
Rarely needed on a single-level deck. It earns its place on tall decks in heavy-snow country, or with cedar-group posts on big tributary areas.
Dressed size7½ × 7½ in
Max height, pine, 80 ft²14'-0"
Max height, pine, 160 ft²14'-0"
3-ply beam, full bearingyes
Heaviest to setby a distance
The deck post size chart, from the code
Post size is not a matter of taste. For single-level decks, Section IRC 2021 R507.4 sizes wood posts by Table R507.4. The table gives the tallest post allowed for each size, species group and tributary area, which is the square feet of deck that one post and its footing hold up. Pick your load and species, and every row below is the 2021 table as printed:
| Post | 20 ft² | 40 ft² | 60 ft² | 80 ft² | 100 ft² | 120 ft² | 140 ft² | 160 ft² |
|---|---|---|---|---|---|---|---|---|
| 4×4 | 14'-0" | 13'-8" | 11'-0" | 9'-5" | 8'-4" | 7'-5" | 6'-9" | 6'-2" |
| 4×6 | 14'-0" | 14'-0" | 13'-11" | 12'-0" | 10'-8" | 9'-8" | 8'-10" | 8'-2" |
| 6×6 | 14'-0" | 14'-0" | 14'-0" | 14'-0" | 14'-0" | 14'-0" | 14'-0" | 14'-0" |
| 8×8 | 14'-0" | 14'-0" | 14'-0" | 14'-0" | 14'-0" | 14'-0" | 14'-0" | 14'-0" |
Height is measured from the underside of the beam to the top of the footing or pier. NP = not permitted. 10 psf dead load, No. 2 grade, wet service factor included.
Check your post
4×4allowed up to 11'-0" at 60 ft²within table
4×6allowed up to 13'-11" at 60 ft²within table
6×6allowed up to 14'-0" at 60 ft²within table
8×8allowed up to 14'-0" at 60 ft²within table
Your area is rounded up to the next column, which is the conservative reading. The table allows interpolation; your building department reads the final answer.
Reading it honestly
Height is beam to footing, measured from the underside of the beam to the top of the footing or pier. It is not the deck height. A deck 10 feet off the ground with a 2×10 beam has a much shorter post than 10 feet.
The table stops at 14 feet and 160 square feet. Every cell tops out at 14'-0", and interpolation is permitted but extrapolation is not. A taller post, a bigger tributary area, or a deck with more than one level is outside the prescriptive table and goes to an engineered design.
The 4×4 is where the table bites. Under the 40 psf live load in southern pine, a 4×4 is allowed 13'-8" at 40 ft² but only 9'-5" at 80 ft² and 6'-2" at 160 ft². In the redwood and cedar group, under the same load, it is not permitted at all past 100 ft². The 6×6 holds 14'-0" across the whole live-load row for southern pine and for the fir, hem-fir and spruce group. The 8×8 holds it in every row of the table, snow loads included.
Why the 6×6 is the usual answer
Height is half the story. The other half is what sits on top. Section R507.5.1 says that where a multiple-span beam bears on intermediate posts, each ply must have full bearing on the post. A 3-ply 2× beam is 4½" wide and a 4×4 is 3½", so the arithmetic rules that out before the table does. At the beam ends the rule is a minimum of 1½" of bearing on wood.
Hardware says the same. Simpson Strong-Tie's BCS post cap connects double 2×s to a 4× post, or triple 2×s to a 6× post. And when a post is notched to seat the beam, the code's footnote is blunt: notched deck posts "shall be sized to accommodate beam size" under R507.5.2. Notching a 4×4 to take a beam leaves very little post.
When a 4×4 is the right post
A low deck on a short beam with posts close together can sit well inside the 4×4 row, and there is no prize for using more wood than the table asks. A 4×4 is also the floor: the 2021 IRC (R407.3) says wood columns shall be not less than 4 inches by 4 inches nominal. Use it with a 2-ply beam and a post cap made for that pairing. Bring the post spacing in with the beam span calculator until the tributary area sits comfortably in the table.
A different 4×4 rule people mix up with this one: where 4×4 posts carry guard loads, Section R507.10.2 says they shall not be notched at the connection to the supporting structure. That is about railing posts, not the posts under the beam. Guard heights are in deck railing height code.
What it changes below grade
The post bears on a footing through a post base. Section R507.4.1 wants lateral restraint at the foot from a manufactured connector, or 12 inches of embedment. Post bases are made to fit a post size, so the base follows the post you pick. The tributary area that sets the post size also sets the footing, through the footing size calculator. The pour is in the concrete calculator for deck footings, and the whole sequence is in how to build deck footings.
Code requirement
Post size by height and tributary area is IRC 2021 R507.4 for single-level decks. Your jurisdiction may amend the model code or adopt a different edition. Confirm with your building department before you cut.
Before you buy
- Work out the tributary area on your worst post first — the post with the longest beam spans either side.
- Measure post height from the top of the footing to the underside of the beam, not from the ground to the deck.
- Read your species group off the lumber stamp; the redwood and cedar group allows the least height.
- Use the load row for your area: 40 psf live load, or your ground snow load where that is greater.
- If the beam is 3-ply, plan 6x6 posts or a connector rated for that beam and post pairing.
- Above 14 feet, over 160 ft², or on a multi-level deck, stop and get an engineered design.
Sources
- 2021 IRC § R507.4 and Table R507.4, Deck Post Height (UpCodes) read 2026-10-18
- 2021 IRC § R507.5.1 Deck Beam Bearing and § R507.5.2 Deck Beam Connection to Supports (UpCodes) read 2026-10-18
- 2021 IRC § R507.4.1 Deck Post to Deck Footing Connection (UpCodes) read 2026-10-18
- 2021 IRC § R507.10.2 Wood Posts at Deck Guards (UpCodes) read 2026-10-18
- 2021 IRC § R407.3 Columns, Structural Requirements (UpCodes) read 2026-10-18
- Simpson Strong-Tie, Deck Connection and Fastening Guide (F-DECKCODE25), Beam-to-Post Connections, pp. 30–31 read 2026-10-18
Filed under Deck framing & structure — 19 tools, guides and comparisons in this pillar.
Next on the pile
Deck beam span calculator
Post spacing, which sets the tributary area this chart reads.
How to build deck footings
The footing and post base each of these posts stands on.
Deck footing size calculator
The same tributary area, sized into a footing.
Concrete calculator for deck footings
Bags and yards for the footings under the posts.
Deck joist span chart
The joist span that feeds the tributary area.