DCA6-2015 Table 3A + Table 4 · IRC R507
Deck beam, post and footing calculator
Your deck
Along the beam line.
Away from the house — the joists run this way.
Pick a member; the tool returns how far apart the posts can go.
Grade to the underside of the beam.
Your local frost depth. Leave blank until you have it.
Within the prescriptive tables
(2)2x10 beam · 4 posts · 24" footings
DCA6-2015 Table 3A, p.6 · Table 4, p.12 (LB ≤ 8' / LJ ≤ 14')
Beam
(2)2x10
1 line(s)
Max post spacing
7'-1"
carrying ≤ 12'-0" of 2x10 joist
Posts
4
6x6 minimum per DCA6
Footing ⌀
24"
or 21" square
Footing thickness
10"
1,500 psf soil assumed
Max 6x6 post
14'
at this LB / LJ
- Note
DCA6-2015-T4-footingFootings auto-sized: 4 × 24" dia × 10" thick spread pad (DCA6-2015 Table 4 at 1,500 psf assumed soil bearing), with the design's round pier form above it. Verify soil class at permit.
- Verify
IRC-R403.1.4Footing depth must reach below the local frost line — verify with your AHJ (IRC R403.1.4). The default depth is not assumed code-adequate.
Footing depth — your input, not ours
Table 4 sizes the footing across, never down. Depth is set by your local frost line under IRC R403.1.4 and varies from nothing in the south to six feet in northern states — so it is a required input here, and there is no default. Call your building department, then enter it above for a concrete quantity.
Table 4 assumes 1,500 psf soil bearing and 150 pcf concrete (fn.2) at 2,500 psi (fn.4). Both get verified at permit — soft, filled or expansive soil changes the footing.
Why beam, post and footing are one page
They are three answers to one question. The beam size sets how far apart the posts can go. The post spacing sets how much deck each post carries. That load, together with the joist span, picks the footing out of Table 4. Split them into three calculators and each one has to assume the answer to the other two — which is exactly how a deck ends up with a correctly sized beam sitting on undersized piers.
One thing worth reading carefully in the table below: the columns are the joist span the beam carries, not the width of the deck and not the beam's own length. A 20-foot-wide deck that is only 10 feet deep loads its beam far less than a 12-foot-wide deck that is 16 feet deep.
Deck beam span table (DCA6-2015 Table 3A, p.6)
Southern Pine
| Beam | ≤ 6 ft | ≤ 8 ft | ≤ 10 ft | ≤ 12 ft | ≤ 14 ft | ≤ 16 ft | ≤ 18 ft |
|---|---|---|---|---|---|---|---|
| (2)2x6 | 6'-8" | 5'-8" | 5'-1" | 4'-7" | 4'-3" | 4'-0" | 3'-9" |
| (2)2x8 | 8'-6" | 7'-4" | 6'-6" | 5'-11" | 5'-6" | 5'-1" | 4'-9" |
| (2)2x10 | 10'-1" | 8'-9" | 7'-9" | 7'-1" | 6'-6" | 6'-1" | 5'-9" |
| (2)2x12 | 11'-11" | 10'-4" | 9'-2" | 8'-4" | 7'-9" | 7'-3" | 6'-9" |
| (3)2x6 | 7'-11" | 7'-2" | 6'-5" | 5'-10" | 5'-5" | 5'-0" | 4'-9" |
| (3)2x8 | 10'-7" | 9'-3" | 8'-3" | 7'-6" | 6'-11" | 6'-5" | 6'-1" |
| (3)2x10 | 12'-9" | 11'-0" | 9'-9" | 8'-9" | 8'-3" | 7'-8" | 7'-3" |
| (3)2x12 | 15'-0" | 13'-0" | 11'-7" | 10'-6" | 9'-9" | 9'-1" | 8'-7" |
Scroll the table sideways to see every column.
Columns are the JOIST SPAN the beam carries, not the deck depth. (2) and (3) are built-up beams — two or three 2x plies nailed together. 3x and 4x are solid timbers, which DCA6-2015 tabulates only in the Douglas Fir-Larch group.
Douglas Fir-Larch / Hem-Fir / SPF
| Beam | ≤ 6 ft | ≤ 8 ft | ≤ 10 ft | ≤ 12 ft | ≤ 14 ft | ≤ 16 ft | ≤ 18 ft |
|---|---|---|---|---|---|---|---|
| (2)2x6 | 5'-2" | 4'-5" | 3'-11" | 3'-7" | 3'-3" | 2'-10" | 2'-6" |
| 3x6 | 5'-2" | 4'-5" | 3'-11" | 3'-7" | 3'-3" | 2'-10" | 2'-6" |
| (2)2x8 | 6'-7" | 5'-8" | 5'-1" | 4'-7" | 4'-3" | 3'-10" | 3'-5" |
| 3x8 | 6'-7" | 5'-8" | 5'-1" | 4'-7" | 4'-3" | 3'-10" | 3'-5" |
| (2)2x10 | 8'-1" | 7'-0" | 6'-3" | 5'-8" | 5'-3" | 4'-10" | 4'-5" |
| 3x10 | 8'-1" | 7'-0" | 6'-3" | 5'-8" | 5'-3" | 4'-10" | 4'-5" |
| (2)2x12 | 9'-5" | 8'-2" | 7'-3" | 6'-7" | 6'-1" | 5'-8" | 5'-4" |
| 3x12 | 9'-5" | 8'-2" | 7'-3" | 6'-7" | 6'-1" | 5'-8" | 5'-4" |
| 4x6 | 6'-2" | 5'-3" | 4'-8" | 4'-3" | 3'-11" | 3'-8" | 3'-5" |
| 4x8 | 8'-2" | 7'-0" | 6'-3" | 5'-8" | 5'-3" | 4'-11" | 4'-7" |
| 4x10 | 9'-8" | 8'-4" | 7'-5" | 6'-9" | 6'-3" | 5'-10" | 5'-5" |
| 4x12 | 11'-2" | 9'-8" | 8'-7" | 7'-10" | 7'-3" | 6'-9" | 6'-4" |
| (3)2x6 | 7'-1" | 6'-5" | 5'-9" | 5'-3" | 4'-10" | 4'-6" | 4'-3" |
| (3)2x8 | 9'-5" | 8'-3" | 7'-4" | 6'-8" | 6'-2" | 5'-9" | 5'-5" |
| (3)2x10 | 11'-9" | 10'-2" | 9'-1" | 8'-3" | 7'-7" | 7'-1" | 6'-8" |
| (3)2x12 | 13'-8" | 11'-10" | 10'-6" | 9'-7" | 8'-10" | 8'-3" | 7'-10" |
Scroll the table sideways to see every column.
Columns are the JOIST SPAN the beam carries, not the deck depth. (2) and (3) are built-up beams — two or three 2x plies nailed together. 3x and 4x are solid timbers, which DCA6-2015 tabulates only in the Douglas Fir-Larch group.
Load basis: 40 psf live load + 10 psf dead load, No. 2 grade lumber, wet service conditions, L/360 deflection. Table 3A footnote 4 also requires the beam depth to be at least the joist depth wherever joist hangers are used.
Post height and footing size (DCA6-2015 Table 4, p.12)
The same table does two jobs: it caps how tall a 6x6 post may be for a given beam span and joist span, and it gives the footing that post needs. Find the row for your beam span and joist span together, and take the post height and the footing from that one row.
| Beam span × joist span | 6x6 max ht — SP | 6x6 max ht — DF-L | 6x6 max ht — HF | 6x6 max ht — SPF | Footing ⌀ | Square | Thickness |
|---|---|---|---|---|---|---|---|
| ≤ 6' × ≤ 10' | 14' | 14' | 14' | 14' | 18" | 16" | 7" |
| ≤ 6' × ≤ 14' | 14' | 14' | 14' | 14' | 21" | 18" | 8" |
| ≤ 6' × ≤ 18' | 14' | 14' | 12' | 11' | 24" | 21" | 10" |
| ≤ 8' × ≤ 10' | 14' | 14' | 14' | 14' | 20" | 18" | 8" |
| ≤ 8' × ≤ 14' | 14' | 14' | 14' | 11' | 24" | 21" | 10" |
| ≤ 8' × ≤ 18' | 14' | 13' | 11' | 8' | 27" | 24" | 11" |
| ≤ 10' × ≤ 10' | 14' | 14' | 14' | 12' | 23" | 20" | 9" |
| ≤ 10' × ≤ 14' | 14' | 13' | 11' | 8' | 27" | 24" | 11" |
| ≤ 10' × ≤ 18' | 12' | 11' | 8' | 2' | 31" | 27" | 13" |
| ≤ 12' × ≤ 10' | 14' | 14' | 12' | 10' | 25" | 22" | 10" |
| ≤ 12' × ≤ 14' | 13' | 12' | 9' | 5' | 30" | 26" | 13" |
| ≤ 12' × ≤ 18' | 11' | 9' | 6' | 2' | 34" | 30" | 15" |
| ≤ 14' × ≤ 10' | 14' | 13' | 11' | 8' | 27" | 24" | 11" |
| ≤ 14' × ≤ 14' | 11' | 10' | 7' | 2' | 32" | 29" | 14" |
| ≤ 14' × ≤ 18' | 9' | 8' | 2' | Not permitted | 37" | 33" | 16" |
| ≤ 16' × ≤ 10' | 13' | 12' | 10' | 6' | 29" | 26" | 12" |
| ≤ 16' × ≤ 14' | 10' | 9' | 5' | 2' | 35" | 31" | 15" |
| ≤ 16' × ≤ 18' | 7' | 5' | 2' | Not permitted | 40" | 35" | 18" |
| ≤ 18' × ≤ 10' | 12' | 11' | 8' | 2' | 31" | 27" | 13" |
| ≤ 18' × ≤ 14' | 9' | 8' | 2' | Not permitted | 37" | 33" | 16" |
| ≤ 18' × ≤ 18' | 5' | 2' | 2' | Not permitted | 42" | 37" | 19" |
Scroll the table sideways to see every column.
Footing sizes assume 1,500 psf soil bearing and 150 pcf concrete (Table 4 fn.2) at 2,500 psi compressive strength (fn.4). Table 4 fn.5 permits an 8x8 nominal post anywhere to a maximum height of 14'. Footing DEPTH is not in this table — it is set by your local frost line.
Where this calculator stops answering
A beam calculator that returns a number for every input is telling you something it cannot know once the input leaves the tables. Here is where this one refuses instead, and why.
A bay no joist in Table 2 can span
DCA6-2015-T2 · T2-fn7The beam carries whatever joist span the bay puts on it, so the joist is checked first, against the bay the beam lines actually leave. When the joist you picked cannot span that bay, the tool steps up to the smallest Table 2 size that can and says so — the same auto-size the designer runs. When even a 2x12 at your spacing falls short, or the joist would pass the 18'-0" length that Table 2 footnote 7 sets for footing design, there is no joist span left for Table 3A or Table 4 to be read at. The tool refuses rather than sizing the beam and footing off a span shorter than the one you would build.
A post spacing no beam in the table reaches
DCA6-2015-T3ASize the beam by your post spacing and the tool picks the smallest Table 3A member that reaches it at the joist span carried. When not even the largest member in the column reaches it, there is no cell to read: the tool says so and asks for another post rather than extrapolating the last row.
An interior beam carries joists from both sides
DCA6-2015-T3A-multispanTable 3A is titled "Supporting a Single Span of Joists", and every DCA 6 beam, post and footing table assumes joists framed from one side. An interior beam line carries two bays. The guide's Commentary gives the one prescriptive way through: for equal joist spans from both sides, read Table 3A at twice the joist span, and do the same for the posts and footings (DCA 6-2015 Commentary, p.C5). The tool does exactly that, and adds a beam line rather than read past the table's 18-foot column. Unequal spans either side of a beam are a design professional's call.
A 6x6 post taller than its cell allows
DCA6-2015-T4Post height is not a free variable. At an 18-foot beam span carrying an 18-foot joist span, Table 4 caps a Southern Pine 6x6 at 5' and a Douglas Fir-Larch 6x6 at 2' — a couple of feet, not a storey. Deep decks on wide post spacing hit this constantly, and the error is real: the post buckles, not the beam.
Cells where a 6x6 is not permitted at any height
DCA6-2015-T4-NP4 combinations in Table 4 are marked Not Permitted outright, all of them in the Spruce-Pine-Fir / Ponderosa column — a 6x6 post is not allowed there at any height. Pick Spruce-Pine-Fir in the calculator above and drive the beam span and joist span into one of those cells, and the tool refuses with this code instead of reading the taller Douglas Fir-Larch or Hem-Fir columns next door. Below the table's 14-foot height cap the SPF column reads lower — or Not Permitted — in every cell, not just at the NP ones, which is exactly why it is a separate species choice rather than a word in the grouped label.
What Table 4 assumes, and what it does not cover
The footing sizes assume 1,500 psf soil bearing capacity and 150 pcf concrete (footnote 2), at 2,500 psi compressive strength (footnote 4). If your soil is soft, filled, or expansive clay, the footing changes and the table stops applying — which is what the soil test at permit is for. Footnote 2 also permits the value to be multiplied by 0.9 for corner posts; this engine does not take that credit, so its corner footings are slightly conservative.
What Table 4 never gives you is depth. A footing is sized across by load and down by frost. Frost depth is jurisdiction-specific under IRC R403.1.4 and varies from effectively nothing on the Gulf coast to six feet in the northern plains, so there is no honest default to publish — call your building department, and the calculator will take your number and return a concrete quantity.
Common questions
How far can a double 2x10 deck beam span?
A built-up (2)2x10 Southern Pine beam spans 8'-9" between posts carrying an 8-foot joist span, and 7'-1" carrying 12 feet, at 40 psf live plus 10 psf dead, No. 2 grade, wet service, from AWC DCA 6-2015 Table 3A, p.6. In the Douglas Fir-Larch / Hem-Fir / SPF column the same beam spans 7'-0" at an 8-foot joist span on the same basis. Table 3A is keyed to the joist span the beam carries, not to the size of the deck.
What size beam do I need for a 12 foot span?
Carrying an 8-foot joist span, the only Southern Pine member in AWC DCA 6-2015 Table 3A that reaches a 12-foot post spacing at 40 psf live plus 10 psf dead, No. 2 grade, wet service is a (3)2x12. Reduce the joist span the beam carries to 6 feet and (3)2x10 or (3)2x12 will do it on the same basis. If none of those work for the deck you are building, the cheaper answer is almost always one more post rather than a bigger beam.
What size footing does my deck need?
AWC DCA 6-2015 Table 4 sizes the footing from the beam span and the joist span together, because those two set the load the post carries. It runs from a 18-inch round pier 7 inches thick at the light end, up to 42 inches across and 19 inches thick at an 18-foot beam span carrying an 18-foot joist span — every one of those cells at 40 psf live plus 10 psf dead, No. 2 grade, wet service, on 1,500 psf assumed soil bearing (Table 4 fn.2).
How many footings does my deck need?
One per post, and the post count comes from the beam, not from a rule of thumb. Each beam line needs a post at each end plus enough intermediate posts to keep every span inside the Table 3A allowable — so a wide deck on a short-spanning beam gets more posts, and a deeper deck may need a second beam line entirely.
Can I use 4x4 posts under a deck?
Not from Table 4, which is written for 6x6 posts — the minimum deck post on the main prescriptive path, with footnote 5 permitting an 8x8 to be substituted anywhere up to a 14-foot post height. The guide does publish a 4x4 alternative: Appendix B, Tables B1 and B2, keys post heights to the tributary area each post carries. This site does not compute that path, so the recommendation here stays 6x6 — sized and ordered from Table 4.
How deep do deck footings have to be?
Deeper than your local frost line, and no table on this page can tell you what that is. Frost depth is set by the local jurisdiction and ranges from essentially nothing along the Gulf to six feet in the northern plains. Table 4 sizes the footing across; depth comes from your building department, and it is a required input here rather than a default.
Working back up the structure? Size the joists this beam carries with the joist span calculator, or lay out the steps down to grade with the deck stair calculator.
Keep going
Joist span calculator
How far a 2x6, 2x8, 2x10 or 2x12 deck joist spans at 12", 16" or 24" on center — plus the overhang the same cell allows.
DCA6-2015 Table 2, p.4
Deck stair calculator
Riser height, tread depth, stringer count and cut layout, screened against IRC R311.7 — including the 3/8" riser uniformity rule.
IRC R311.7 · R312.1
Ledger fastener spacing
On-center spacing for ½" lags and through-bolts in a deck ledger, by the joist span it carries and the band joist behind it.
DCA6-2015 Table 5, p.15
Deck material calculator
Every board, joist, hanger, clip, concrete bag and rail section for one deck — counted by the engine that builds real order sheets, not square-foot math.
DCA6-2015 sizing · the in-app takeoff engine
Dock flotation calculator
Required buoyancy and float-drum count for a rectangular-drum floating dock — the criteria the dock engine sizes against, the one that governed, and the source behind each number.
Published guidance & documented practice · no national dock code
Now do the whole deck
The calculator sizes one member. The free designer runs the same tables across a real footprint — every joist, beam, post and footing sized together, with the counts and a preview of the material list underneath. No signup.
Source & limits
AWC DCA 6-2015, Prescriptive Residential Wood Deck Construction Guide (based on the 2015 IRC), © 2018 American Wood Council. The guide is published free by AWC.
Estimate only — not an engineered design. Spans and sizes are screened against the AWC DCA 6-2015 / IRC prescriptive baseline at 40 psf live + 10 psf dead load; quantities are takeoff arithmetic and trade practice, which those documents do not publish. Local codes and amendments vary; your Authority Having Jurisdiction (AHJ) and a licensed design professional govern. Verify everything with a permit and a licensed pro before ordering or building.