Guide · Decks
How Many Deck Boards Do I Need for a 12×12 Deck?
26 boards — assuming standard 5/4×6 decking (5½ inches wide) with a ¼-inch gap, laid across the 12-foot width in full 12-foot lengths. Add 10% for cuts and culls and order 29. The math: each board covers 5.75 inches of width, and 144 ÷ 5.75 = 25.04, which rounds up to 26.
Below: the step-by-step calculation, a chart for other deck sizes, how gap size and board length change the count, and the joists and screws to go with them. For the full material list — boards, joists, posts, and screws — use our free deck material calculator.
Boards, joists, posts, beam lines, and screw counts for your exact deck size.
Open the deck calculator →12-ft 5/4×6 boards at a ¼-in gap. Plus 520 screws (2 per joist, 16-in joist spacing).
The math, step by step
Boards = 144″ ÷ 5.75″ = 25.04 → 26
- A 5/4×6 deck board is actually 5.5 inches wide. (Nominal vs. actual strikes again.)
- Add the gap. 5.5 + 0.25 = 5.75 inches of deck width covered per board.
- Divide the deck width. 12 ft = 144 inches: 144 ÷ 5.75 = 25.04, round up to 26 boards.
- Board length. Each board runs the full 12-ft length — no butt joints needed.
- Add waste. 26 × 1.10 = 28.6 — order 29.
Deck boards for common deck sizes
Boards run along the deck’s length; the count comes from the width. Assumes 5/4×6 boards with a ¼-in gap.
| Deck size | Boards (no waste) | Board length | Order (10% waste) |
|---|---|---|---|
| 10×10 | 21 | 10 ft | 24 |
| 12×12 | 26 | 12 ft | 29 |
| 12×16 | 26 | 16 ft | 29 |
| 14×14 | 30 | 14 ft | 33 |
| 16×16 | 34 | 16 ft | 38 |
| 16×20 | 34 | 20 ft | 38 |
Long boards may need to be special-ordered — confirm availability before you finalize the design.
How the gap changes the count
A ⅛-inch gap gives an effective width of 5.625 inches: 144 ÷ 5.625 = 25.6 — still 26 boards for a 12×12 deck. Gap choice rarely changes the board count by more than one board either way, but it matters for drainage and for how the last board lands at the rim. Composite decking uses the same math; check the manufacturer’s gapping specs, which can differ from wood.
What if boards don’t span the full length?
If 12-footers aren’t available, you’ll butt-joint shorter boards over joists. Every joint needs a joist beneath it, joints must be staggered (never two in a row on the same joist), and each cut end is a potential cull — bump waste to 12–15%. The piece count climbs fast: a 12×12 deck built from 8-ft boards needs about 39 boards before waste (26 courses × 12 ft ÷ 8 ft), or roughly 44 with waste — nearly 50% more pieces than 12-footers, plus a lot more joints to keep straight.
Joists, screws, and posts
Boards are only the surface. At 16 inches on center, a 12-ft-wide deck needs 10 joists (144 ÷ 16 + 1), each 12 feet long. Face-fastening takes 2 screws per joist per board: 26 × 10 × 2 = 520 screws before waste. Posts, beams, and footings are structural — size them from span tables and local code, never from rules of thumb — and set footings below your frost line (our concrete calculator sizes the footing concrete). For board-foot pricing on the framing lumber, the board-foot lumber calculator converts your cut list to cost. Our deck material calculator bundles boards, joists, posts, and screws into one list.
Which way should the boards run?
Deck boards run perpendicular to the joists — the joist direction decides the board direction, not the other way around. On a square 12×12 deck it makes no numerical difference, but on a rectangular deck, run boards along the long dimension: a 12×16 deck needs 26 sixteen-footers instead of 34 twelve-footers, which means fewer boards to buy, fewer ends to finish, and no butt joints. Aesthetically, boards usually run parallel to the house or out toward the view — pick the direction first, then let the joists follow.
Pressure-treated wood vs. composite: what changes?
The board-count math doesn’t care what the board is made of — a 5½-inch-wide composite board covers exactly the same 5.75 inches per course as a 5/4×6 pressure-treated board. What changes: check the actual width on the product spec, because some composite and PVC boards run slightly narrower or wider than 5.5 inches. Composite also demands stricter joist spacing (many brands require 12 inches on center for diagonal patterns, and some for all installs — check the spec), and grooved-edge boards for hidden fasteners replace screws with clips, so the fastener count comes from the manufacturer’s clip spacing, not the 2-per-joist rule. Gapping specs differ too: composites move more with temperature, so follow the brand’s end-gap chart rather than the ¼-inch wood default.
What this count doesn’t cover: railing and stairs
Boards, joists, and screws are the deck surface and frame — a finished deck usually needs more. Railing: posts roughly every 6 feet, top and bottom rails, and balusters (typically spaced under 4 inches apart to meet code — that’s a lot of balusters on a 12×12). Stairs: stringers, treads, and risers sized from your total rise — our stair layout calculator works out the riser count, tread depth, and stringer length. And the whole structure sits on footings and posts sized from span tables, set below your frost line. Take off railing and stairs as separate mini-projects after the main deck.
Picture-frame borders
A popular upgrade is running a border course around the deck perimeter: it hides end grain and looks sharp. It changes the field count, though — the field shrinks by the border width on each side, so calculate the border boards from the perimeter first, then the field from the remaining width. (Our deck calculator assumes a plain field — adjust manually if you’re framing the picture.)
How we calculate
Boards = deck width ÷ (board width + gap), rounded up; board length = deck length. Assumes 5.5-in.-wide 5/4×6 boards with a ¼-in gap and full-length runs (no butt joints). The 10% waste allowance covers cuts, culls, and end checks. Joists = width ÷ spacing + 1; screws = boards × joists × 2. Structural members (posts, beams, footings) must follow span tables and local code.
Reviewed September 2026 · TrueMeasure
Frequently asked questions
Does 26 boards include waste?
No — 26 is the exact count for a 12×12 deck with 5/4×6 boards and a ¼-in gap. Order 29 with a 10% cutting allowance.
What gap should I leave between deck boards?
¼ inch is the common default for wood decking; ⅛–¼ inch depending on material and manufacturer specs. The gap drains water and lets boards move with the seasons.
How far apart should deck joists be?
16 inches on center is standard for 5/4 decking; 12 inches for diagonal layouts and some composites. Span tables and local code govern — joist spacing is structural, not a matter of taste.
How many screws do I need per deck board?
Two per joist crossing for face-fastened boards — that’s 20 screws per 12-ft board on 16-in joists. Hidden-fastener systems use the manufacturer’s clips instead of screws.
Can I use 8-foot boards with butt joints?
Yes — stagger the joints over joists and never align two joints on the same joist. Expect about 39 boards before waste for a 12×12 deck (≈44 with waste), roughly 50% more pieces than using 12-footers.