The quick answer: a 12 × 12 ft deck needs 26 deck boards, each 12 ft long, using standard 5.5 inch wide boards with a 1/8 inch gap. Buy 28 boards to allow for a couple of bad boards and cutting mistakes.
Below is the method, a table for other common deck sizes, and a quick guide to joists and screws. For any size or board width, use the deck board calculator, which also works in metres.
How the 26 boards are worked out
- Find the width each board covers. A board is 5.5 in wide, plus a 1/8 in (0.125 in) gap = 5.625 in.
- Convert the deck width to inches. 12 ft × 12 = 144 in.
- Divide. 144 ÷ 5.625 = 25.6, so you need 26 rows of boards.
- Match board length to the deck. On a 12 ft deck, one 12 ft board covers each row, so 26 boards.
- Add 5–10%. 26 × 1.08 ≈ 28 boards.
The boards run at right angles to the joists. The last row is usually ripped down to fit, which is why you round up.
Deck boards for other sizes
These figures use 5.5 in boards with a 1/8 in gap, laid straight, before adding any spares.
| Deck size | Board length | Rows | Boards needed |
|---|---|---|---|
| 8 × 8 ft | 8 ft | 18 | 18 |
| 10 × 10 ft | 10 ft | 22 | 22 |
| 10 × 12 ft | 12 ft | 22 | 22 |
| 12 × 12 ft | 12 ft | 26 | 26 |
| 12 × 16 ft | 16 ft | 26 | 26 |
| 16 × 16 ft | 16 ft | 35 | 35 |
| 20 × 12 ft | 12 ft (running the short way) | 43 | 43 |
If you cannot get boards as long as the deck, stagger the joints so they do not all line up on the same joist. Each joint must land on a joist, with a screw in each board end.
Metric decks
In the UK and Australia, deck boards are commonly around 120–145 mm wide and sold in 2.4 m, 3.6 m and 4.8 m lengths. For a 3.6 × 3.6 m deck using 140 mm boards with a 5 mm gap:
- Cover width per board: 140 + 5 = 145 mm
- Rows: 3,600 ÷ 145 = 24.8, so 25 boards of 3.6 m
- With spares: about 27 boards
Joists for a 12×12 deck
With joists at 16 inch centres, a 12 ft wide frame needs 144 ÷ 16 = 9 spaces, which is 10 joists. Composite boards and diagonal layouts often need joists at 12 inch centres, which means 13 joists. Always check the board maker’s spacing and your local building rules.
Screws and fixings
Face screwing uses two screws at every joist on every board. For a 12×12 deck with 26 boards and 10 joists, that is 26 × 10 × 2 = 520 screws. A rough rule of thumb is 350 screws per 100 sq ft. Buy a pack of at least 600 to cover dropped and snapped screws. Hidden clip systems use one clip per board at each joist instead.
Board gaps: wood vs composite
- Kiln-dried or seasoned timber: gap of about 1/8 in (3–5 mm).
- Wet pressure-treated timber: lay tight or with a very small gap. The boards shrink as they dry.
- Composite: follow the maker’s guide, typically 3/16 in (about 5 mm), with extra end gaps in hot weather.
A bigger gap means each board covers more width, so you need slightly fewer boards. The calculator lets you set your own gap.
Frequently asked questions
How many 16 ft deck boards for a 12×16 deck?
If the boards run the 16 ft length, you need 26 boards of 16 ft, or 28 with spares.
Can I use 2×6 boards instead of 5/4 x 6?
Yes. Both are 5.5 in wide, so the board count is the same. 2×6 boards are stiffer and can span joists up to 24 in apart; 5/4 boards normally need 16 in spacing.
Do I need extra boards for a picture-frame border?
Yes. Add the perimeter of the deck in boards, plus extra blocking so the border boards are supported. For a 12×12 deck that is four extra 12 ft boards.
How many square feet is a 12×12 deck?
144 square feet, or about 13.4 m².
Related: the DIY Project Budget & Materials Planner turns your measurements into quantities, costs and a shopping list.
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