What size footing does a deck post need?

It depends on the beam span and the joist span the post carries, not on the post. From DCA 6 Table 4: a 12 ft beam span carrying 14 ft joists needs a 30 in round or 26x26 in square footing, 13 in thick.

How deep do deck footings have to be?

At least 12 in below undisturbed ground surface, or below the frost line, whichever is deeper. Your building department sets the frost line for your area.

How much concrete is in a deck footing?

A 30 in footing 36 in deep holds 14.73 cu ft - about 25 eighty-pound bags. Volume rises with the square of the diameter, so a small increase in footing size is a large increase in concrete.

What soil bearing capacity does DCA 6 assume?

1,500 psf. If the building official thinks the soil is weaker, a soils investigation is required to establish the actual capacity, and the table sizes no longer apply.

Can corner footings be smaller?

Footnote 2 allows the value to be multiplied by 0.9 for corner posts. That applies to bearing area, so the diameter reduces by about 5%, not 10%.

How tall can a 6x6 deck post be?

From Table 4, between 2 and 14 ft depending on species, beam span and joist span - and some combinations are marked NP, not permitted. A Ponderosa pine or SPF post at an 18 ft beam span with 14 ft joists is not permitted at any height.

Does a footing near the house need anything special?

Yes. Within 5 ft of an exterior house foundation wall, the deck footing must bear at the same elevation as the house foundation footing.

Home › Decking › Deck Footing Calculator

Deck Footing Calculator

Source: AWC DCA 6 Table 4 · footing depth per IRC R403

Size a deck footing from the beam span and joist span it carries, and get the concrete to pour it. The table below is AWC DCA 6 Table 4 in full — including the assumption that quietly governs every number in it.

Footing size & concrete

Beam span — post to post

Joist span carried — full span

Post species — sets the max post height

Number of footings

Footing depth — 12 in min, or below frost

Corner post reduction?

Concrete bag yield — 60 lb is 0.45, 80 lb is 0.60

cu ft

DCA 6 Table 4. Assumes 40 psf live, 10 psf dead, 1,500 psf soil bearing, 150 pcf concrete, 2,500 psi mix, No. 2 grade, wet service.

Worked example

A deck with beams spanning 12 ft between posts, carrying 14 ft joists, on Douglas fir-larch 6×6 posts. Four footings, 36 in deep for frost, 80 lb bags.

Find the row. DCA 6 Table 4, beam span 12 ft, joist span ≤14 ft → round footing 30 in diameter, square 26×26, minimum thickness 13 in.
Check the post. Same row, Douglas fir-larch column → maximum 6×6 post height is 12 ft. Taller than that and the table stops applying.
Concrete per footing. A 30 in circle 36 in deep is π × 15² × 36 = 25,447 in³, or 14.73 cu ft.
Bags. At 0.6 cu ft per 80 lb bag that is 25 bags per footing — 100 bags for four.
Which is the real answer. 100 bags is 8,000 lb of concrete to mix by hand. At 2.18 cubic yards, order ready-mix.

Change the joist span to 18 ft and the footing goes to 34 in, the post height drops to 9 ft, and the concrete rises to nearly 19 cu ft each. The joist span the beam carries moves the footing more than most people expect, because it is carrying tributary area, not length.

The formula

The footing size is a lookup; the concrete is geometry. The part worth writing out is the concrete, because the volume goes up with the square of the diameter:

V = π (d/2)² · h / 1728 bags = ⌈ V / yield ⌉ d_corner = d √0.9

V: concrete per footing, cu ft
d: footing diameter, in — from Table 4
h: footing depth, in — 12 in minimum or below frost, whichever is deeper
1728: cubic inches per cubic foot
yield: 0.45 cu ft for a 60 lb bag, 0.60 for an 80 lb bag

Going from a 24 in to a 30 in footing sounds like a 25% increase. It is 56% more concrete, because area scales with the square of diameter. This is why the jump from a 10 ft to a 14 ft joist span costs so much more concrete than it looks like it should.

The assumption that governs every number in this table

Footnote 2 of Table 4 assumes 1,500 psf allowable soil bearing capacity. Every footing size in the table falls out of that figure, and it is the weakest link in the whole calculation. DCA 6 is explicit: where the building official determines that in-place soils with a bearing capacity below 1,500 psf are likely, the capacity must be established by a soils investigation. Soft clay, fill, and organic soils can be well under it.

Depth. Footings must bear on undisturbed soil at least 12 in below the undisturbed ground surface, or below the frost line, whichever is deeper. The frost line is set by your jurisdiction — DCA 6 says to contact the authority having jurisdiction, and so do we. This page will not print a frost-depth table, because those figures are locally adopted and a copied approximation is exactly how wrong numbers spread.

The 5-foot rule. Any deck footing closer than 5 ft to an exterior house foundation wall must bear at the same elevation as the house foundation footing. On a ledger-attached deck the posts are often near the house, and this rule is missed routinely.

What is not in the table. Table 4 covers 40 psf live and 10 psf dead load, with LB/4 and LJ/4 overhangs. A hot tub, a roof over the deck, planters, or an outdoor kitchen are outside it entirely. Post heights are for 6×6; footnote 5 allows nominal 8×8 anywhere in the table up to 14 ft.

Bearing must be verified in the field. DCA 6 requires the building official to verify bearing conditions before concrete is placed. That is an inspection, not a formality — and it is the step that catches soil the table's assumption does not describe.

AWC DCA 6 Table 4 — Post height for 6x6 posts and footing sizes for all posts
Beam span	Joist span	Round dia.	Square	Thickness	S. pine	DF-L	HF / W. cedar	Redwood	PP / RP / SPF
6	10	18"	16x16	7"	14	14	14	14	14
6	14	21"	18x18	8"	14	14	14	14	14
6	18	24"	21x21	10"	14	14	12	14	11
8	10	20"	18x18	8"	14	14	14	14	14
8	14	24"	21x21	10"	14	14	14	14	11
8	18	27"	24x24	11"	14	13	11	12	8
10	10	23"	20x20	9"	14	14	14	14	12
10	14	27"	24x24	11"	14	13	11	13	8
10	18	31"	27x27	13"	12	11	8	11	2
12	10	25"	22x22	10"	14	14	12	14	10
12	14	30"	26x26	13"	13	12	9	11	5
12	18	34"	30x30	15"	11	9	6	9	2
14	10	27"	24x24	11"	14	13	11	13	8
14	14	32"	29x29	14"	11	10	7	10	2
14	18	37"	33x33	16"	9	8	2	8	NP
16	10	29"	26x26	12"	13	12	10	12	6
16	14	35"	31x31	15"	10	9	5	9	2
16	18	40"	35x35	18"	7	5	2	7	NP
18	10	31"	27x27	13"	12	11	8	11	2
18	14	37"	33x33	16"	9	8	2	8	NP
18	18	42"	37x37	19"	5	2	2	6	NP

Reproduced from AWC DCA 6 (2015) Table 4. Post heights are for 6x6; footing sizes apply to all posts. NP = not permitted. Assumes 1,500 psf soil bearing; verify yours.

Before you dig

DCA 6 states plainly: do not construct footings over septic systems or leach fields, utility lines, or enclosed meters. Call 811 before digging. It is free, it is a legal requirement in most states, and striking a gas or electrical line while augering a 36 in hole is the worst outcome available on a deck project.

Pre-manufactured post anchors must be galvanized or stainless steel. That is the same corrosion logic as the fastener requirements — treated lumber against bare steel in wet ground is the fastest corrosion environment on the whole deck.

Bagged concrete or ready-mix

The crossover is around a cubic yard. Below that, bags are simpler. Above it, bagged concrete becomes a serious amount of physical work — the worked example above is 100 bags and four tons of material — and ready-mix is usually cheaper per yard as well. Most suppliers have a one-yard minimum and a short-load fee below it, so price both before deciding.

One practical caution: all footings on a deck should be poured in a single session where possible. A footing poured a week later cures under different conditions, and mixed bag counts across footings is a common source of one post settling differently from the rest.

Frequently asked questions

What size footing does a deck post need?: It depends on the beam span and the joist span the post carries, not on the post. From DCA 6 Table 4: a 12 ft beam span carrying 14 ft joists needs a 30 in round or 26×26 in square footing, 13 in thick.
How deep do deck footings have to be?: At least 12 in below undisturbed ground surface, or below the frost line, whichever is deeper. Your building department sets the frost line for your area.
How much concrete is in a deck footing?: A 30 in footing 36 in deep holds 14.73 cu ft — about 25 eighty-pound bags. Volume rises with the square of the diameter, so a small increase in footing size is a large increase in concrete.
What soil bearing capacity does DCA 6 assume?: 1,500 psf. If the building official thinks the soil is weaker, a soils investigation is required to establish the actual capacity, and the table sizes no longer apply.
Can corner footings be smaller?: Footnote 2 allows the value to be multiplied by 0.9 for corner posts. That applies to bearing area, so the diameter reduces by about 5%, not 10%.
How tall can a 6x6 deck post be?: From Table 4, between 2 and 14 ft depending on species, beam span and joist span — and some combinations are marked NP, not permitted. A Ponderosa pine or SPF post at an 18 ft beam span with 14 ft joists is not permitted at any height.
Does a footing near the house need anything special?: Yes. Within 5 ft of an exterior house foundation wall, the deck footing must bear at the same elevation as the house foundation footing.

Check these numbers yourself

AWC DCA 6 — Prescriptive Residential Wood Deck Construction GuideAmerican Wood Council, free PDF. Table 4 on page 12 is the table reproduced here.
IRC R403 — FootingsUpCodes. Depth and frost-line requirements.
ICC Digital Codes — the IRC itselfInternational Code Council. Free to read online; registration required.

Figures on this page last checked against the source documents on 2026-09-05. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Estimates only. Footing sizes come from a prescriptive table that assumes 1,500 psf soil bearing capacity. If your soil is weaker — fill, soft clay, organic material — these sizes do not apply and a soils investigation is required. Bearing conditions must be verified in the field by the building official before concrete is placed. Confirm frost depth and every dimension with your building department before you dig.