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Deck Post Calculator

Source: AWC DCA 6-2015 Table 4 · 4x4 heights from Table C4A

The post size question has a shorter answer than most people expect: DCA 6 requires a 6x6 minimum, and the 4x4 most homeowners picture is limited to about two feet.

DCA 6-2015. Assumes 40 psf live, 10 psf dead, LB/4 and LJ/4 overhangs, No. 2 grade and wet service. Incising assumed for Douglas fir-larch, hem-fir and SPF.

Worked example

A deck with beams spanning 12 ft between posts, carrying joists that span 14 ft, built in Douglas fir-larch, with the beam 9 ft above grade.

Change the species to ponderosa pine or SPF and the same row gives 5 ft, not 12. A 9 ft post in that species needs an 8x8. Species changes the answer by more than half here.

The formula

There is no formula to run. Post height is a table lookup, and the reason it is a lookup rather than a calculation is that the post is being checked for buckling under a load that depends on how much deck it carries:

This is why a longer joist span shortens the permitted post. The load rises with the area the post carries, and a taller column buckles at a lower load. Both move against you at once, which is why the table falls off so steeply in the bottom-right corner.

Why DCA 6 will not let you use a 4x4

The prescriptive text is unambiguous. DCA 6 states: "All deck post sizes shall be 6x6 (nominal) or larger, and the maximum height shall be in accordance with Table 4." There is no 4x4 option in the body of the document.

The Commentary explains the reasoning. "IRC section R407.3 specifies a minimum 4x4 (nominal) wood column size; however, it would often be overstressed in applications covered in this document. Requiring a minimum 6x6 post in DCA 6 provides adequate bearing for beams." The IRC minimum is a floor for columns generally, not a statement that a 4x4 suits a deck.

Table C4A exists for the approval case. Alternative Methods and Materials provisions let a building official accept another size. C4A is what those heights would be, and they are short: 2 ft in most rows, 4 ft at best. It is published here because knowing the number is more useful than being told no.

A 4x4 also cannot be notched. The Commentary is explicit that there is not enough cross sectional area in a 4x4 for the let-in notch detail, so a post cap is required, and connector hardware for a 4x4 is generally limited to 2-ply 2 inch nominal or 4 inch nominal beams.

Edition note. DCA 6-2015 is written to the 2015 IRC and its bracketed references use 2015 numbering. Deck posts are covered by IRC R507.4 in the 2018 and 2021 editions. Confirm which edition your jurisdiction has adopted, and confirm the section number against that edition rather than against this page.

Reproduced from AWC DCA 6 (2015). The 6x6 columns are Table 4, which is prescriptive. The 4x4 columns are Table C4A, which is in the Commentary and applies only as an Alternative Method approved by the building official. NP means not permitted.

Download this table as CSV. Same cells as printed above, nothing added.

Deck post size calculator: what actually sets the limit

Three things move the number, and post size is only one of them. The beam span sets how much deck sits on each post along the beam. The joist span sets how far the deck reaches away from the house. Multiply half of each and you have the tributary area one post carries, which at 40 psf live and 10 psf dead becomes an axial load. Height then decides whether the column can take it.

Species is the variable people forget. In the 12 ft beam, 14 ft joist row, a 6x6 goes 13 ft in southern pine and 5 ft in SPF. That is not a rounding difference, it is the difference between a normal deck and one that needs a bigger post. The grade stamp on the lumber tells you which you have, and the table is not interpolated between species.

Where this table stops applying

Table 4 assumes 40 psf live load, 10 psf dead, overhangs no greater than a quarter of the beam and joist spans, No. 2 grade and wet service. A hot tub, a planter, a roof over the deck, a second storey or snow load beyond the assumption puts you outside it and into engineered design.

The table also stops at 18 ft of beam span and 18 ft of joist span, and at 14 ft of post height even for an 8x8. Rows marked NP are not permitted at any height in that species. If your deck is past any of those edges, DCA 6 does not answer the question and an engineer does.

Lateral load is a separate matter entirely. These heights are for gravity load. DCA 6 notes that if posts are intended to resist lateral loads, they need to be designed per the NDS, which is why the bracing rules exist and why bracing is prohibited on centre posts: a braced centre post picks up lateral load on top of the larger vertical load it already carries.

Frequently asked questions

Can I use 4x4 posts on a deck?

Not under DCA 6, which requires 6x6 (nominal) or larger. A 4x4 is only possible as an Alternative Method approved by your building official, and Table C4A limits it to 2 ft in most cases and 4 ft at best. It also cannot be notched for the beam, so it needs a post cap.

How tall can a 6x6 deck post be?

Up to 14 ft in the lightest rows of DCA 6 Table 4, but as little as 2 ft in the heaviest. A 12 ft beam span carrying 14 ft joists allows 13 ft in southern pine and 5 ft in SPF.

Where is the post height measured from?

From grade, or the top of the foundation if that is higher, to the underside of the beam. Not to the deck surface, and not to the top of the guard.

Do I need diagonal bracing on deck posts?

DCA 6 requires diagonal bracing parallel to the beam at every corner post over 2 ft, fastened with 1/2 in diameter lag screws. It is prohibited on centre posts, because bracing there adds lateral load to the post already carrying the most vertical load.

Can I bolt the beam to the side of the post?

No. DCA 6 prohibits attaching the beam to the side of the post with fasteners only. Notch the post so the beam bears on wood, or use an approved post cap. All 3-ply beams require a post cap regardless.

Can I use an 8x8 instead?

Yes. DCA 6 allows an 8x8 to be substituted anywhere in Table 4, up to a maximum height of 14 ft. It is the usual answer when the 6x6 column runs out before your deck height does.

Do cut ends of posts need treating?

Yes. DCA 6 requires cut ends and notches to be field treated with an approved preservative such as copper naphthenate. The factory treatment does not reach the fresh cut.

Check these numbers yourself

• AWC DCA 6: Prescriptive Residential Wood Deck Construction GuideAmerican Wood Council, free PDF. The 2015 edition. POST REQUIREMENTS on page 10, Table 4 on page 12, and Table C4A in Commentary C7 are the text reproduced here.
• ICC Digital Codes: the IRC itselfInternational Code Council. Free to read online with registration. Check which edition your jurisdiction has adopted before relying on a section number.

Figures on this page last checked against the source documents on 2026-09-06. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Estimates only. These heights come from a prescriptive table with fixed assumptions: 40 psf live load, 10 psf dead load, overhangs no more than a quarter of the beam and joist spans, No. 2 grade and wet service. They are gravity load only and do not cover posts resisting lateral load, which need design per the NDS. A 4x4 is not permitted by DCA 6 and requires building official approval. Confirm the adopted code edition and every dimension with your building department before you build.