Deck board calculator
Enter the deck size, board width and gap to get the number of rows, the board length with the least waste, the boards to buy and the screws or hidden clips to fasten them.
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): 5′ 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′ | 10 | 11 |
| Rim joists | 2×10 | 12′ | 1 | 1 |
| Ledger | 2×10 | 12′ | 1 | 1 |
| Beam plies | 2×10 | 12′ | 2 | 2 |
| Blocking | 2×10 | 8′ | 2 | 2 |
| Posts | 6×6 | 8′ | 1 | 1 |
| Deck boards | deck board | 12′ | 26 | 29 |
Hardware & fasteners
| Joist hangers (at ledger) | 10 |
| Hurricane ties / joist-to-beam connectors | 10 |
| Post bases (one per post) | 3 |
| Footings | 3 |
| Deck screws (2 per board per joist) | 546 |
| or hidden fastener clips (1 per board per joist) | 273 |
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′ · least material | 26 | 312 ft |
| 14′ | 26 | 364 ft |
| 16′ | 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.
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.
The deck board formula
rows = (depth + gap) ÷ (board width + gap), rounded up. Then boards = rows × boards per row (or rows ÷ rows per board when the deck is shorter than half a board). A 12 × 12 ft deck with 5 1/2″ boards and 3/16″ gaps needs 26 rows, so 26 boards of 12 ft — or 52 of 8 ft.
Deck boards for common deck sizes
| Deck | Rows | Best board length | Boards | Buy | Screws |
|---|---|---|---|---|---|
| 8 × 8 ft | 17 | 8′ | 17 | 19 | 250 |
| 8 × 10 ft | 17 | 10′ | 17 | 19 | 322 |
| 10 × 10 ft | 22 | 20′ | 11 | 12 | 416 |
| 10 × 12 ft | 22 | 12′ | 22 | 24 | 462 |
| 12 × 12 ft | 26 | 12′ | 26 | 29 | 546 |
| 12 × 16 ft | 26 | 16′ | 26 | 29 | 710 |
| 16 × 16 ft | 34 | 16′ | 34 | 37 | 929 |
| 10 × 20 ft | 22 | 20′ | 22 | 24 | 740 |
Each size has its own page with every board length and 3 1/2″ boards too. The full method, gaps and screws are explained in how many deck boards do I need.
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.