Deck boards are bought by the piece, in fixed lengths, and their real width is less than the label suggests. This calculator works out how many rows of boards fit across the deck, how many boards each row takes, and how many joists the frame needs at your chosen spacing — then adds a waste allowance and an optional cost.
How to use the deck calculator
- Enter the deck length in the direction the boards run, and the width across them. Boards usually run parallel to the house.
- Enter the actual board width. A “5/4×6” or “2×6” deck board is 5.5 in wide.
- Enter the gap recommended by the board manufacturer.
- Choose a board length that suits the deck — matching it to the deck length avoids butt joints.
- Set the joist spacing. 16 in on center is standard; 12 in for diagonal decking or some composites.
Deck formulas
Boards per row = ⌈ deck length ÷ board length ⌉
Boards = ⌈ rows × boards per row × (1 + waste% ÷ 100) ⌉
Joists = ⌊ deck length ÷ joist spacing ⌋ + 1
Joists run perpendicular to the boards, so they are spaced along the deck's length. The +1 accounts for the joist at the far end.
Worked example: a 16 × 12 ft deck
- Rows: 144 in ÷ (5.5 + 0.1875) = 25.3 → 26 rows
- Boards per row with 16 ft boards: 16 ÷ 16 = 1
- Boards with 10% waste: 26 × 1.10 = 28.6 → 29 boards
- Joists at 16 in: 192 in ÷ 16 = 12, plus one = 13 joists
Deck boards by deck size
5.5 in boards, 3/16 in gap, boards as long as the deck, 10% waste.
| Deck size | Area | Rows | Boards | Joists at 16 in |
|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 22 | 25 | 8 |
| 12 × 12 ft | 144 sq ft | 26 | 29 | 10 |
| 16 × 12 ft | 192 sq ft | 26 | 29 | 13 |
| 16 × 16 ft | 256 sq ft | 34 | 38 | 13 |
| 20 × 12 ft | 240 sq ft | 26 | 29 | 16 |
Before you build
- Most decks need a permit. Attached decks in particular are inspected for ledger attachment, footings and railings.
- Railings are usually required on decks above a certain height — check your local code.
- Use ground-contact rated lumber for joists and posts close to the ground.
- Stagger butt joints if your boards are shorter than the deck, and land every joint on a joist.
Setting footings? The Concrete Calculator handles round holes. Adding steps? Use the Stair Calculator. Fencing the yard? See the Fence Calculator.
Frequently asked questions
How many deck boards do I need for a 12×16 deck?
With boards running the 16 ft direction, the 12 ft (144 in) width is covered by rows of 5.5 in boards plus a 3/16 in gap: 144 ÷ 5.6875 = 25.3, so 26 rows. Using 16 ft boards that is 26 boards, or 29 with 10% waste.
What joist spacing should I use?
16 in on center is standard for most wood decking laid perpendicular to the joists. Many composite boards require 16 in or less, and diagonal decking usually needs 12 in. Always follow the decking manufacturer's span rating and local code.
How wide is a deck board really?
Lumber is sold by nominal size. A 2×6 or 5/4×6 deck board is actually 5.5 in wide, and a 2×4 is 3.5 in. Composite boards vary, so check the product sheet.
How big a gap should deck boards have?
It depends on the material and its moisture content. Composite makers specify gaps in their installation guides. Wet pressure-treated boards are often installed tight and shrink as they dry. Follow your product's instructions.
Does this include the deck frame?
It counts deck boards and joists only. Beams, posts, footings, rim joists, ledger board, hardware and railings need to be designed for your site — often with a permit and inspection.