VolunaroShort answer: count the rows of boards across the deck, then work out how many boards you need to fill each row given the board length you buy. For a 16 × 12 ft deck laid with 16 ft boards, that is 26 rows and 26 boards; with a 10 percent allowance for spares you order 29. The same deck built with 12 ft boards needs 43, because every row now needs a joint and cutting leaves offcuts. So the length you buy matters as much as the deck size.
This guide covers material quantities only: boards, joists as a count, and fasteners. It does not size joists, beams, posts or footings. Use the deck calculator to run your own measurements, and read on for how it reaches its numbers.
Step 1: rows of boards
Boards run across the deck's width, and each row takes up the board's actual width plus the gap.
- Use the actual width. A nominal 5/4 × 6 board is 5-1/2 in wide, not 6 in. Western Forest Products' data sheet for 5/4 × 6 cedar decking lists an actual width of 5-1/2 in, and the actual thickness of a 5/4 board is about 1 in, though it varies by product. Other materials, such as composites and tropical hardwoods, have their own widths, so check the board you are buying.
- Add the gap. Gap depends on the decking and the conditions. Follow your manufacturer's gapping guide. The calculator uses 3/16 in as an editable default.
- Rows = (deck width + gap) ÷ (board width + gap), rounded up.
For a 12 ft (144 in) wide deck: (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.35, so 26 rows.
Step 2: boards per row
If the board is as long as the deck, one board fills a row. If not, a row needs several pieces, and the end joints (butt joints) must land on a joist. The calculator staggers joints between neighbouring rows and allows for the saw cut, because a cut removes a little material every time.
Here is the same 16 × 12 ft deck with different board lengths (10 percent spares, 3/16 in gap, joists 16 in on center):
| Board length | Rows | Butt joints | Boards used by the cut plan | Boards to order (+10%) | Screws (2 per joist) |
|---|---|---|---|---|---|
| 16 ft | 26 | 0 | 26 | 29 | 676 |
| 12 ft | 26 | 26 | 39 | 43 | 728 |
| 8 ft | 26 | 39 | 65 | 72 | 754 |
How the table comes about: a butt joint has to land on a joist, and two neighbouring rows should not share a joist, so the calculator alternates two row patterns. With 8 ft boards and joists 16 in on center, one row is two 8 ft pieces (1 joint) and the next is 4 + 8 + 4 ft (2 joints). Over 26 rows that is 39 joints and 65 pieces, and since two 4 ft pieces plus a saw cut come to more than 8 ft, each short piece takes a board of its own: 65 boards. With 12 ft boards the rows alternate 12 + 4 ft and 5-1/3 + 10-2/3 ft, so 26 joints and 52 pieces, but the 26 short pieces pair up on shared boards and the plan uses 39.
Shorter boards mean more joints, more cutting and more leftover pieces. Compare the price of what your supplier stocks against the count rather than assuming the cheapest board is the cheapest deck.
Why dividing by area is unreliable
The tempting shortcut is deck area ÷ area of one board. For 192 ft² and a 12 ft board that covers 5.5 ft², it gives 34.9, so 35. The cut plan needs 39 before spares, because the area method ignores the gaps between boards and the pieces left over after cutting. With 8 ft boards the shortcut gives 53 against 65. It is not always low: with 16 ft boards it gives 27, one more than the 26 the layout needs, because it ignores that each row takes exactly one board. Use the area method only as a rough sanity check, not as the order.
Step 3: joists
Joists run perpendicular to the boards. The count is the deck length ÷ the spacing on center, plus one for the last joist.
For 16 ft (192 in) at 16 in on center: 192 ÷ 16 + 1 = 13 joists. Each is 12 ft long here, so 156 ft of joist in total.
Spacing is limited by the decking. Simpson Strong-Tie documents joist spacing of 16 in on center for decking laid perpendicular to the joists and 12 in for diagonal decking, and our calculator warns above 16 in. That is a typical limit, not a universal one: some decking, including some composite boards, calls for closer spacing. The decking manufacturer's instructions and your local code take precedence, and this guide does not set the spacing.
Step 4: fasteners
Fasteners = rows × joists × fasteners per board at each joist, plus the extra ones at butt joints.
For the 16 ft board layout, which has no butt joints: 26 rows × 13 joists × 2 = 676 screws. Each butt joint adds one more crossing, so the 12 ft layout (26 joints) needs (338 + 26) × 2 = 728 and the 8 ft layout (39 joints) needs 754. Trex's guide to installing its Trex Plug screws says to use two screws per joist for that system. Some hidden-clip systems use one fastener per crossing, which would halve the count (338 in the first layout), but follow the installation instructions of the system you are buying and change the setting in the calculator.
Common mistakes
- Using 6 in instead of 5-1/2 in. Rows come out about 8 percent short.
- Leaving out the gap. The row count is wrong: without the gap the example above gives 27 rows instead of 26.
- Ignoring the saw cut. With short boards, the width lost to each cut can push the count up by a board.
- Joints that do not land on a joist. A butt joint needs support underneath, and manufacturers may ask for doubled joists or blocking there.
- Using the same fastener count for every system. Face screws and hidden clips differ.
- Treating this as a structural plan. It is not.
What this guide cannot tell you
- How strong the deck must be. Joist, beam, post, footing and ledger design depend on the deck's height, loads, soil and local code.
- Permits and inspections. Check with your local building department.
- Layouts with diagonal boards, picture frames or curves. They need more material and closer joists than this estimate assumes.
- Prices. They depend on material and region, and we do not publish them.
Assumptions are listed on the methodology page.
Frequently asked questions
How many deck boards do I need for a 16 × 12 ft deck?
With 16 ft boards running the full length, 26 rows of 5/4 × 6 boards with a 3/16 in gap need 26 boards, or 29 with a 10 percent allowance for spares. With 12 ft boards the count rises to 43, because rows need butt joints and cuts.
Why not just divide the deck area by the area of one board?
That ignores the gaps between boards, the cuts and the offcuts that cannot be reused. With 12 ft boards it gives 35 while the cut plan needs 39 before spares, and with 8 ft boards it gives 53 against 65. With boards as long as the deck it can even come out one too high (27 against 26).
Is a 5/4 × 6 board really 6 inches wide?
No. The actual width is 5-1/2 in, and the actual thickness of a 5/4 board is about 1 in.
Can I use this to size joists, beams or footings?
No. This is a material estimate only. Structure depends on your design, loads, local code and the decking manufacturer's instructions.