DCA6-2015 Table 3A + Table 4 · IRC R507
Deck beam 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 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" round pier × 10" thick (DCA6-2015 Table 4 at 1,500 psf assumed soil bearing). 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" |
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" |
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. Read the row for your beam span, the column for your joist span, and take both numbers from the same cell.
| Beam span (LB) | Joist span (LJ) | 6x6 max ht — SP | 6x6 max ht — DF-L / HF | 6x6 max ht — SPF | Footing ⌀ | Square | Thickness |
|---|---|---|---|---|---|---|---|
| ≤ 6' | ≤ 10' | 14' | 14' | 14' | 18" | 16" | 7" |
| ≤ 6' | ≤ 14' | 14' | 14' | 14' | 21" | 18" | 8" |
| ≤ 6' | ≤ 18' | 14' | 14' | 11' | 24" | 21" | 10" |
| ≤ 8' | ≤ 10' | 14' | 14' | 14' | 20" | 18" | 8" |
| ≤ 8' | ≤ 14' | 14' | 14' | 11' | 24" | 21" | 10" |
| ≤ 8' | ≤ 18' | 14' | 13' | 8' | 27" | 24" | 11" |
| ≤ 10' | ≤ 10' | 14' | 14' | 12' | 23" | 20" | 9" |
| ≤ 10' | ≤ 14' | 14' | 13' | 8' | 27" | 24" | 11" |
| ≤ 10' | ≤ 18' | 12' | 11' | 2' | 31" | 27" | 13" |
| ≤ 12' | ≤ 10' | 14' | 14' | 10' | 25" | 22" | 10" |
| ≤ 12' | ≤ 14' | 13' | 12' | 5' | 30" | 26" | 13" |
| ≤ 12' | ≤ 18' | 11' | 9' | 2' | 34" | 30" | 15" |
| ≤ 14' | ≤ 10' | 14' | 13' | 8' | 27" | 24" | 11" |
| ≤ 14' | ≤ 14' | 11' | 10' | 2' | 32" | 29" | 14" |
| ≤ 14' | ≤ 18' | 9' | 8' | Not permitted | 37" | 33" | 16" |
| ≤ 16' | ≤ 10' | 13' | 12' | 6' | 29" | 26" | 12" |
| ≤ 16' | ≤ 14' | 10' | 9' | 2' | 35" | 31" | 15" |
| ≤ 16' | ≤ 18' | 7' | 5' | Not permitted | 40" | 35" | 18" |
| ≤ 18' | ≤ 10' | 12' | 11' | 2' | 31" | 27" | 13" |
| ≤ 18' | ≤ 14' | 9' | 8' | Not permitted | 37" | 33" | 16" |
| ≤ 18' | ≤ 18' | 5' | 2' | Not permitted | 42" | 37" | 19" |
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.
Tributary past the table's 18-foot ceiling
DCA6-2015-T3ATable 3A stops at an 18-foot carried joist span. Past that there is no cell to read, so the tool stops and says engineering is required rather than extrapolating the last column.
An interior beam leaves the table's scope entirely
DCA6-2015-T3A-multispanTable 3A is titled "Supporting a Single Span of Joists". The moment a deck is deep enough to need a second, interior beam line, that interior beam is carrying two bays and is outside what the table tabulates. DCA 6 simply does not publish values for it. The tool keeps sizing — but it says plainly that the interior line is non-prescriptive and wants a professional, because a number from the wrong table is worse than no number.
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 Table 4, p.124 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. Being straight about it: this calculator's species selector reads the Southern Pine and Douglas Fir-Larch / Hem-Fir columns, neither of which contains an NP cell, so this is a limit of the printed table rather than something the tool will ever raise for you. If your posts are SPF, read the SPF column in the table above yourself — it is materially lower everywhere, not just at the NP cells.
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 when it carries an 8-foot joist span, and 7'-1" when it carries 12 feet. In the Douglas Fir-Larch / Hem-Fir / SPF column the same beam spans 7'-0" at an 8-foot joist span. Beam spans in AWC DCA 6-2015 Table 3A are 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 Table 3A that reaches a 12-foot post spacing is a (3)2x12. Reduce the joist span the beam carries to 6 feet and (3)2x10 or (3)2x12 will do it. 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?
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. All of it assumes 1,500 psf soil bearing.
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 under the DCA 6 prescriptive method. Table 4 is titled for 6x6 posts, and 6x6 is the minimum deck post the guide recognises — footnote 5 permits an 8x8 to be substituted anywhere up to 14 feet. A 4x4 is outside the prescriptive path, which is why this engine always sizes and orders a 6x6.
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-centre 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
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 full 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, sizes, and quantities are screened against the AWC DCA 6-2015 / IRC prescriptive baseline at 40 psf live + 10 psf dead load. 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.