Decking Calculator
Work out exactly how many deck boards, joists and screws you need, based on your deck size, board dimensions and joist spacing. Add your own prices for a full cost estimate.
How this calculator works
The calculator first works out how many rows of boards are needed to cover the deck's width, and how many individual boards each row needs to span the deck's length:
rows = ceil( deck width ÷ (board width + gap) ) boards per row = ceil( deck length ÷ board length ) total boards = ceil( rows × boards per row × (1 + waste%) )
Joists run perpendicular to the decking and support it at regular intervals along the length the boards span:
joists = ceil( deck length ÷ joist spacing ) + 1 joist crossings per board = ceil( board length ÷ joist spacing ) + 1 screws = total boards × joist crossings per board × 2
Worked example
A 16 ft × 12 ft deck, 12 ft boards, 5.5 in wide with a ¼ in gap, 16 in joist spacing:
- Rows across the 12 ft width: 12 × 12 in ÷ (5.5 + 0.25) = 144 ÷ 5.75 ≈ 25 → 26 rows.
- Boards per row to span 16 ft: 16 ÷ 12 ≈ 1.33 → 2 boards per row.
- Total boards: 26 × 2 = 52, plus 5% waste ≈ 54.6 → buy 55 boards.
- Joists: 16 ft ÷ (16 in ÷ 12) = 16 ÷ 1.33 = 12 → 13 joists (adding one for the starting edge).
- Each 12 ft board crosses 12 ÷ 1.33 = 9, plus one → 10 joist crossings; screws: 55 × 10 × 2 = 1,100 screws.
Joist spacing by decking type
| Decking type | Typical spacing | Why |
|---|---|---|
| Solid wood (standard) | 16" | Wood has good stiffness between supports |
| Composite | 12–16" | Some composites sag more; check the span rating |
| Diagonal or angled layout | 12" | Any layout needs tighter support at an angle |
Always check the specific manufacturer's span table for composite or PVC decking — spacing requirements vary by brand and board profile, and installing outside spec can void the warranty.
Fasteners and finishing
Hidden fastener clips generally replace face screws one-for-one at each joist crossing, so the fastener count here is a reasonable stand-in even if you're planning to use clips instead of screws — just price them per clip rather than per screw box. Pressure-treated or untreated wood decking typically benefits from a sealant or stain, reapplied every one to three years; composite and PVC decking is usually pre-finished and skips this step.
Frequently asked questions
What joist spacing do composite boards need vs wood?
Wood decking is commonly installed over joists spaced 16 inches apart. Composite decking often needs closer spacing, frequently 12 inches, especially on angled or diagonal layouts, since composite boards can sag more than wood between supports. Check the manufacturer's span rating for your specific product.
How many screws does a deck board need?
A typical rule is two screws at every point where a board crosses a joist. A 12 ft board over joists spaced 16 inches apart crosses about 10 joists, which works out to roughly 20 screws for that one board. Wider boards or picture-frame borders may need additional fasteners at the edges.
Do hidden fasteners change how much material I need?
Hidden fastener clips generally replace face screws one-for-one at each joist crossing, so the fastener count is similar either way, though hidden fastener systems are usually priced per clip rather than by the box. Some systems also require a grooved board edge, so check compatibility with your chosen decking first.
Do I need to seal a deck after building it?
Pressure-treated and untreated wood decking generally benefits from a sealant or stain, both for appearance and to slow moisture damage and UV fading, typically reapplied every one to three years. Composite and PVC decking is usually pre-finished and doesn't need sealing, though guidance varies by product.