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This deck calculator works out how many deck boards you need for a new deck or a re-deck, together with the number of joists and deck screws. It accounts for the board width, the gap between boards, the length of boards you buy and your joist spacing.
How to Use the Deck Calculator
- Enter the deck length in the direction the boards will run, and the deck width.
- Enter the actual board width. A nominal 6 in board (5/4×6 or 2×6) is 5.5 in wide; many metric boards are 140–145 mm.
- Enter the gap between boards. Around 1/8–3/16 in (3–5 mm) is typical; check the manufacturer’s instructions for composite boards.
- Choose the board length you plan to buy and your joist spacing, usually 16 in (400 mm) on center.
- Add a waste allowance of about 10% for cuts and defects.
Deck Board Formulas
Rows of boards = Deck width ÷ (Board width + Gap), rounded up
Boards = Rows × Deck length × (1 + Waste) ÷ Board length, rounded up
Joists = Deck length ÷ Joist spacing + 1
Screws = Rows × Joists × 2
Worked Example: 16 × 12 ft Deck
- Rows: 144 in ÷ (5.5 + 0.125) in = 25.6, so 26 rows
- Board length: 26 × 16 ft = 416 linear ft
- With 10% waste and 16 ft boards: 416 × 1.1 ÷ 16 = 28.6, so 29 boards
- Joists at 16 in on center: 192 ÷ 16 + 1 = 13 joists of 12 ft
- Screws: 26 × 13 × 2 = 676 screws
Metric example: a 4.8 × 3.6 m deck with 140 mm boards and 5 mm gaps needs 25 rows, 28 boards of 4.8 m with 10% waste, 13 joists at 400 mm centers and about 650 screws.
Typical Joist Spacing
| Decking | Typical maximum joist spacing |
|---|---|
| 5/4 (1 in) wood boards, straight | 16 in (400 mm) |
| 2× (1.5 in) wood boards | 16–24 in (400–600 mm) |
| Composite or PVC boards | Usually 16 in (400 mm); 12 in (300 mm) for diagonal layouts |
| Diagonal board layouts | 12 in (300 mm) is common |
Always follow the board manufacturer’s span rating and your local building code. Joist size, beam size, footing size and post spacing must be designed for the deck’s height and loads.
Choosing Board Lengths
- Boards that span the full deck length avoid butt joints and look cleaner.
- If the deck is longer than your boards, stagger the joints and make sure every joint lands on a joist.
- Buying a mix of lengths can reduce waste; plan the layout before ordering.
Deck Building Tips
- Most areas require a permit for decks attached to a house or raised above a certain height.
- Use corrosion-resistant fasteners rated for treated lumber, or hidden fasteners for composite boards.
- Leave a gap at the house wall and follow flashing details for ledger boards.
- Need footings? Use our concrete calculator, and our fence calculator for a railing or privacy screen.
Frequently Asked Questions
How many deck boards do I need for a 12×16 deck?
With 5.5 in boards, 1/8 in gaps and boards running along the 16 ft side, you need 26 rows of 16 ft boards, or 29 boards with 10% waste.
How many screws do I need for a deck?
Plan on two screws where each board crosses each joist. A 16 × 12 ft deck with 13 joists and 26 rows of boards needs about 676 screws, so buy a box of 750 or more.
What gap should I leave between deck boards?
A gap of about 1/8 to 3/16 in (3–5 mm) is typical for dry wood and most composites. Wet pressure-treated boards are often butted tight because they shrink as they dry.
What is the standard joist spacing for a deck?
16 inches (400 mm) on center is the most common spacing. Some composite boards and all diagonal layouts need closer spacing, often 12 inches (300 mm).
Does this calculator include the deck frame?
It counts the joists and their length. Beams, posts, footings, ledger boards, rim joists, stairs and railings are separate and depend on your design.
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