Deck calculator
Enter the deck's size and height, the framing lumber and the joist spacing. You get the joist size that spans, the beam, how many posts and how far apart, the footings and their concrete, the ledger fasteners, the decking and a shopping list, each read from the American Wood Council's DCA 6 deck tables.
More: overhangs, ledger, decking, concrete
Joists
13 × 2x8
16 in on centre, span 10 ft of 11 ft 10 in allowed, 2 ft past the beam
- Beam
- 3-2x10
- Posts
- 3 × 6x6
- Post spacing
- 6 ft 4 13/16 in
- Post height
- 4 ft 6 in
- Footings
- 20 in round
- Concrete
- 8 bags
- Decking
- 26 × 2x6
- Deck screws
- 676
One source: AWC DCA 6, the American Wood Council’s prescriptive deck guide to the IRC
Each footing 20 in round or 18 in square and 8 in thick (DCA 6 Table 4): 1.454 ft³ each, 4.36 ft³ in all, of Quikrete Concrete Mix, 80 lb. Posts may be 14 ft as 6x6 here; 8x8 to 14 ft.
Two screws or 8d threaded nails where each board crosses each joist (26 rows × 13 joists × 2), boards about 1/8 in apart (DCA 6).
- OK: Ledger 2x8: 12 fasteners, 18 in on centre, staggered (Table 5).
- Attention: Over 30 in high: a guard at least 36 in tall, 8 posts no more than 6 ft apart, 81 2x2 balusters with gaps under 4 in (3.88 in).
- Attention: Corner posts over 2 ft need diagonal bracing parallel to the beam (DCA 6 Figure 10).
Stairs down 6 ft: work them out in the stair calculator.
DCA 6 covers a single-level deck attached to the house, no deeper than it is wide, for 40 psf live load and 10 psf dead load. It does not cover snow loads above 40 psf, hot tubs or other heavy point loads, or soil weaker than 1,500 psf. Frost depth is local: footings go at least 12 in below undisturbed ground or below your frost line, whichever is deeper. Check your local building code.
Why: 2x8 joists at 16 in, a 3-2x10 beam on 3 posts (6x6), 3 footings 20 in round.
Framing plan
Seen from above, the house and ledger along the top: joists 16 in apart, the beam in the accent colour 2 ft in from the outer rim, posts 6 ft 4 13/16 in apart, 19 3/16 in from each end.
Other joists and beams
| Joist | Spans | This deck |
|---|---|---|
| 2x6 | 9 ft | ✗ too short |
| 2x8 | 11 ft 10 in | ✓ spans |
| 2x10 | 14 ft | ✓ spans |
| 2x12 | 16 ft 6 in | ✓ spans |
| Beam | Spans | Posts a beam |
|---|---|---|
| 2-2x6 | 5 ft 1 in | 4 |
| 2-2x8 | 6 ft 6 in | 3 |
| 2-2x10 | 7 ft 9 in | 3 |
| 2-2x12 | 9 ft 2 in | 3 |
| 3-2x6 | 6 ft 5 in | 3 |
| 3-2x8 | 8 ft 3 in | 3 |
| 3-2x10 (chosen) | 9 ft 9 in | 3 |
| 3-2x12 | 11 ft 7 in | 2 |
Shopping list
- 1 × 2x8 × 16 ft (ledger)
- 1 × 2x8 × 16 ft (rim joist)
- 13 × 2x8 × 12 ft (joists)
- 3 × 2x10 × 16 ft (beam)
- 3 × 6x6 × 8 ft (posts)
- 26 × 2x6 × 16 ft (decking)
- 8 × 4x4 × 4 ft (guard posts)
- 4 × 2x4 × 12 ft (guard rails, sides)
- 2 × 2x4 × 16 ft (guard rails, front)
- 2 × 2x6 × 12 ft (guard cap, sides)
- 1 × 2x6 × 16 ft (guard cap, front)
- 81 × 2x2 × 3 ft (balusters)
- 13 joist hangers for 2x8 (at the ledger)
- 12 × 1/2 in lag screws with washers (ledger, 18 in on centre)
- 8 bags of Quikrete Concrete Mix, 80 lb (3 footings)
- 676 deck screws or 8d threaded nails (2 per joist)
| For | Pieces | Size | Board feet |
|---|---|---|---|
| ledger | 1 | 2x8 × 16 ft | 21.33 |
| rim joist | 1 | 2x8 × 16 ft | 21.33 |
| joists | 13 | 2x8 × 12 ft | 208 |
| beam | 3 | 2x10 × 16 ft | 80 |
| posts | 3 | 6x6 × 8 ft | 72 |
| decking | 26 | 2x6 × 16 ft | 416 |
| guard posts | 8 | 4x4 × 4 ft | 42.67 |
| guard rails, sides | 4 | 2x4 × 12 ft | 32 |
| guard rails, front | 2 | 2x4 × 16 ft | 21.33 |
| guard cap, sides | 2 | 2x6 × 12 ft | 24 |
| guard cap, front | 1 | 2x6 × 16 ft | 16 |
| balusters | 81 | 2x2 × 3 ft | 81 |
| Total | 1,035.67 | ||
Board feet at nominal size on the stock length; runs over 20 ft are bought in equal lengths and joined over a post. No waste is added to the framing. Joist hangers are at least 60% of the ledger’s depth (DCA 6).
Why these numbers?
The situation
Southern Pine No. 2, joists 16 in on centre, 12 ft out from the house.
From your input: the deck and its lumber
What that means
DCA 6 Table 2: a Southern Pine 2x8 at 16 in spans 11 ft 10 in and may overhang its beam by the lesser of its printed overhang and a quarter of the span.
One source: American Wood Council, the American Wood Council’s prescriptive deck guide to the IRC , sources [1]
So
Overhang 2 ft past the beam leaves a joist span of 10 ft, within 11 ft 10 in.
Arithmetic: exact by definition
What that means
DCA 6 Table 3A: a 3-2x10 carrying joists up to 10 ft spans 9 ft 9 in between posts, and may run past its end posts by a quarter span.
One source: American Wood Council, the American Wood Council’s prescriptive deck guide to the IRC , sources [1]
So
3 posts 6.4 ft apart per beam, the beam running 19 3/16 in past each end post.
Arithmetic: exact by definition
What that means
DCA 6 Table 4 (beam span ≤ 8 ft, joist span ≤ 10 ft): 6x6 posts up to 14 ft, footings 20 in round or 18 in square, 8 in thick; 8x8 posts up to 14 ft.
One source: American Wood Council, the American Wood Council’s prescriptive deck guide to the IRC , sources [1]
So
Each post is about 4 ft 6 in (deck height less decking, joist and beam), so 6x6. Concrete = π × 10² × 8 ÷ 1,728 = 1.454 ft³ a footing.
Arithmetic: exact by definition
The number
DCA 6 Table 5: 1/2 in fasteners 18 in on centre along the ledger for this joist span, staggered top and bottom (Figure 19).
One source: American Wood Council, the American Wood Council’s prescriptive deck guide to the IRC , sources [1]
The number
Above 30 in a guard is required: at least 36 in high, posts no more than 6 ft apart, no opening that passes a 4 in sphere (Figure 24).
One source: American Wood Council, the American Wood Council’s prescriptive deck guide to the IRC , sources [1]
The number
DCA 6 covers a single-level deck attached to the house, no deeper than it is wide, for 40 psf live load and 10 psf dead load. It does not cover snow loads above 40 psf, hot tubs or other heavy point loads, or soil weaker than 1,500 psf. Frost depth is local: footings go at least 12 in below undisturbed ground or below your frost line, whichever is deeper. Check your local building code.
From your input: where you build
Sources
How the framing is worked out
The guide is DCA 6-15, the Prescriptive Residential Wood Deck Construction Guide, which the American Wood Council publishes free to show one way of building a deck that meets the International Residential Code. This is the 2015 IRC edition (reprinted 2018), the latest AWC publishes. The 2021 and 2024 IRC print their own deck tables in section R507, and they have not been checked against DCA 6 here, so where your town has adopted those editions its tables govern.
Joists come from DCA 6 Table 2, by species group, size and spacing. A joist may run past its beam by the lesser of the table's overhang and a quarter of its span, so the calculator puts the beam as far in as that allows unless you type an overhang. The span is counted to the outside of the deck rather than the face of the beam, which errs on the long side. Decks over 30 inches high need a guard, and the joists and rims a guard post bolts to must be 2x8 or larger, so a guarded deck never gets 2x6 joists. The ledger is always at least a 2x8.
Beams come from Table 3A, read in the first column at or above the joist span. The beam may run past its end posts by a quarter of a span, which gives the fewest posts; untick that to put posts at the corners. Posts are 6x6 or larger, and Table 4 limits their height (from the ground to the underside of the beam) by species, beam span and joist span. Where a 6x6 is too tall, an 8x8 may be used up to 14 feet. The same row of Table 4 gives the footing: a round or square pad and its thickness, sized for 1,500 psf soil. The concrete is that pad's volume, divided by the yield the maker prints on the bag.
Ledger fasteners come from Table 5: half-inch lag screws or bolts, spaced by the joist span and by what the ledger bolts to, staggered top and bottom. Decking runs across the joists about 1/8 inch apart, with two screws or 8d threaded nails at every joist. Hidden clip counts are each maker's own figure for its own clip: Trex prints one only for joists 16 inches on centre, so it gets no count at other spacings, and TimberTech prints its box coverage without a spacing.
What the calculator will not do
DCA 6 covers a single-level deck attached to the house, no deeper than it is wide, for a 40 psf live load and 10 psf dead load. It does not cover snow loads above 40 psf, hot tubs or other heavy point loads, multi-level decks, cantilevered house floors, brick veneer or soil weaker than 1,500 psf; those need a design. Frost depth is set locally: footings go at least 12 inches below undisturbed ground or below the frost line, whichever is deeper. Glued laminated beams (Table 3B) and joist hanger models are not worked out here. Check your local building code before you build.
For the steps down, the stair calculator lays out risers, treads and the stringer, and the stair code requirements list the IRC limits. The board foot calculator prices the lumber list, and the 2x10 actual size page shows why a 2x10 joist is 9 1/4 inches deep.
Sources
- American Wood Council, DCA 6-15, Prescriptive Residential Wood Deck Construction Guide (based on the 2015 IRC, © 2018): Tables 2, 3A, 4 and 5; Figures 19 and 24, read 2026-11-08
- Trex, Hidden Fasteners box: 90 universal clips cover 50 ft² with boards on joists 16 in on centre, read 2026-11-08
- Fiberon, Hidden deck fastener FAQ: Phantom clips needed per 100 ft² at 12, 16 and 24 in joist spacing, read 2026-11-08
- TimberTech, TimberTech fasteners sell sheet (2024): CONCEALoc 100 ft² box, 175 clips, read 2026-11-08
- Quikrete, Quikrete Concrete Mix (No. 1101) data sheet: yield per bag; method for setting fence posts, read 2026-11-07
- Quikrete, Quikrete Fast-Setting Concrete Mix (No. 1004-50) data sheet: yield per bag; method for setting posts, read 2026-11-07
- Sakrete, Sakrete Fast Setting Concrete Mix technical data sheet (rev. 04/16): yield, post setting, read 2026-11-07
- NIST, PS 20-25 American Softwood Lumber Standard, Table 3: minimum dressed sizes, read 2026-11-07