Deck calculator: framing & materials
Lay out joists, beams and posts, count deck boards with the right gap, and get screws, hangers, balusters and a printable materials list. The span check uses a cited IRC-based table — always confirm with your local code.
Deck size & framing
Decking
Railing
The IRC requires that a 4″ (102 mm) sphere cannot pass through a guard; a 3 1/2″ target leaves room for shrinkage. Other countries use different limits — check yours.
Span check (reference table)
- Joist span (2×10 at 16″ o.c.): 12′ vs. table maximum 14′ — within the table
- Beam span between posts (2-ply 2×10): 7′ vs. table maximum 7′ 4″ — within the table
Source: City of Alexandria, VA — Typical Deck Details, Based on International Residential Code (Tables 1 and 2). Basis: Southern pine No. 2, 40 psf live load + 10 psf dead load, wet service, deflection L/360. Other species and grades span less; snow loads, hot tubs and planters need engineering. Spans are measured between bearing centrelines.
Lumber to buy
| Item | Size | Length | Net | Buy |
|---|---|---|---|---|
| Joists | 2×10 | 12′ | 13 | 14 |
| Rim joists | 2×10 | 16′ | 1 | 1 |
| Ledger | 2×10 | 16′ | 1 | 1 |
| Beam plies | 2×10 | 16′ | 2 | 2 |
| Blocking | 2×10 | 8′ | 2 | 2 |
| Posts | 6×6 | 8′ | 1 | 1 |
| Deck boards | deck board | 16′ | 26 | 29 |
| Guard posts | 4×4 | 8′ | 4 | 4 |
| Top & bottom rails | 2×4 | 20′ | 5 | 6 |
| Rail cap | 2×6 | 20′ | 3 | 3 |
Hardware & fasteners
| Joist hangers (at ledger) | 13 |
| Hurricane ties / joist-to-beam connectors | 13 |
| Post bases (one per post) | 3 |
| Footings | 3 |
| Deck screws (2 per board per joist) | 710 |
| or hidden fastener clips (1 per board per joist) | 355 |
| Balusters (2×2) | 88 |
Screw count: 2 per board at every joist plus 2 at each butt joint, +5%. Ledger fastening, footing size and post bases come from your local code. Posts: 16 1/2″ above the footing to the underside of the beam (deck height minus decking, joist and beam depth).
Deck boards by length
| Board length | Boards | Total length |
|---|---|---|
| 8′ | 52 | 416 ft |
| 10′ | 52 | 520 ft |
| 12′ | 52 | 624 ft |
| 14′ | 52 | 728 ft |
| 16′ · least material | 26 | 416 ft |
| 18′ | 26 | 468 ft |
| 20′ | 26 | 520 ft |
26 rows of 5/4×6 wood (5 1/2″) with a 3/16″ gap cover 12′. Without waste. Stagger butt joints and land every joint on a joist.
Railing
40 ft of guard in 7 sections, 8 posts, 88 balusters — widest gap 3 1/2″ between 1 1/2″ balusters.
Planning estimate only. Joist, beam, post, footing, ledger and guard requirements depend on loads, species, grade and your local building code. Verify with your building department or a licensed professional before you buy or build.
Planning estimates only. Stud counts and lumber lists are for budgeting and ordering. Structural sizes — headers, joists, beams, posts, footings, ledgers and guards — must follow your local building code. Verify with your building department or a licensed professional before you build.
What a 16′ × 12′ deck needs
The default example — attached with a ledger, 2×10 joists 16″ on center and a two-ply 2×10 beam — uses 13 joists, 26 rows of deck boards (26 boards of 16′), about 710 deck screws and 3 posts and footings. Change any input and the list updates.
How to use the deck calculator
- Enter the deck size. Width along the house and depth out from it, attached with a ledger or freestanding, and the deck height.
- Choose the framing. Joist size and spacing, any overhang, beam size and maximum post spacing. The span check compares them with a cited IRC-based table.
- Choose decking and railing. Board type, gap and length (or let the calculator pick the length with least waste), guarded sides, stair opening and baluster gap.
- Print the materials list. Joists, rims, ledger, beams, posts, blocking, deck boards, guard parts, hangers, ties and screws — with waste — on one printable page.
Joist and beam span reference
The calculator checks your joist span and post spacing against these tables. They are for one species and grade only; other lumber spans less.
| Joist | 12″ o.c. | 16″ o.c. | 24″ o.c. |
|---|---|---|---|
| 2×6 | 9′ 11″ / 6′ 8″ | 9′ / 6′ 8″ | 7′ 7″ / 6′ 8″ |
| 2×8 | 13′ 1″ / 10′ 1″ | 11′ 10″ / 10′ 1″ | 9′ 8″ / 9′ 8″ |
| 2×10 | 16′ 2″ / 14′ 6″ | 14′ / 14′ | 11′ 5″ / 11′ 5″ |
| 2×12 | 18′ / 18′ | 16′ 6″ / 16′ 6″ | 13′ 6″ / 13′ 6″ |
| Joist span ≤ | (2) 2×6 | (2) 2×8 | (2) 2×10 | (2) 2×12 | (3) 2×6 | (3) 2×8 | (3) 2×10 | (3) 2×12 |
|---|---|---|---|---|---|---|---|---|
| 6′ | 6′ 11″ | 8′ 9″ | 10′ 4″ | 12′ 2″ | 8′ 2″ | 10′ 10″ | 13′ | 15′ 3″ |
| 8′ | 5′ 11″ | 7′ 7″ | 9′ | 10′ 7″ | 7′ 5″ | 9′ 6″ | 11′ 3″ | 13′ 3″ |
| 10′ | 5′ 4″ | 6′ 9″ | 8′ | 9′ 5″ | 6′ 8″ | 8′ 6″ | 10′ | 11′ 10″ |
| 12′ | 4′ 10″ | 6′ 2″ | 7′ 4″ | 8′ 7″ | 6′ 1″ | 7′ 9″ | 9′ 2″ | 10′ 9″ |
| 14′ | 4′ 6″ | 5′ 9″ | 6′ 9″ | 8′ | 5′ 8″ | 7′ 2″ | 8′ 6″ | 10′ |
| 16′ | 4′ 3″ | 5′ 4″ | 6′ 4″ | 7′ 6″ | 5′ 3″ | 6′ 8″ | 7′ 11″ | 9′ 4″ |
| 18′ | 4′ | 5′ | 6′ | 7′ | 5′ | 6′ 4″ | 7′ 6″ | 8′ 10″ |
Read more in deck joist spacing and span. For just the boards, use the deck board calculator; for guards, the baluster spacing calculator.
Frequently asked questions
How many deck boards do I need?
Rows = (deck depth + gap) ÷ (board width + gap), rounded up. A 12 ft deep deck with 5 1/2″ boards and a 3/16″ gap needs (144 + 0.1875) ÷ 5.6875 = 25.4 → 26 rows. Multiply by the boards per row for your board length and add about 10% waste.
How many screws per deck board?
Two screws at every joist the board crosses, plus two more at each butt joint. At 16″ on center with 5 1/2″ boards that is about 3.2 screws per square foot, or roughly 320 per 100 sq ft.
What joist spacing should I use?
16″ on center is the most common. Composite and PVC boards usually require 16″ (12″ when laid diagonally) — follow the product's installation guide. Closer spacing lets joists span farther.
How far can deck joists span?
It depends on species, grade, size, spacing and load. For southern pine No. 2 at 16″ on center, a common IRC-based reference allows 2×8 joists 11′ 10″, 2×10 14′ 0″ and 2×12 16′ 6″ (40 psf live, 10 psf dead). Check your local code before building.
How far apart should balusters be?
Guards must not let a 4″ sphere pass through any opening under the IRC. Aim for about 3 1/2″ between 2×2 balusters so the gap stays under 4″ after the wood shrinks.