Deck Calculator
Estimate the materials for a timber or composite deck: how many deck boards (allowing for expansion gaps), joists, and screws, from the deck's dimensions and your board width.
How to use this calculator
- Enter the deck length (board direction) and width (across the boards).
- Enter your board width and the gap between boards (3–6 mm).
- Set joist spacing — 400 mm for timber, 300 mm for composite.
Formula used
The gap between boards is added to each board's width when counting rows — it allows for drainage and thermal/moisture movement. Joists follow the fencepost rule: one more than the number of spaces. Two screws fix each board at each joist crossing.
Example calculation
A 5 m × 4 m deck with 145 mm boards and 5 mm gaps: 4000 ÷ 150 = 27 board rows, each 5 m long = 135 linear m of decking.
Joists at 400 mm: 5000 ÷ 400 + 1 = 14 joists; screws: 27 × 14 × 2 = 756 (buy ~830 with spares).
Gaps, joists and movement
Decking design is governed by movement: timber swells and shrinks with moisture, composite expands and contracts with heat, and both need gaps between boards (3–6 mm) so they don't buckle or trap water. Those gaps also enter the board count — each board effectively occupies its width plus one gap.
Joist spacing is the structural half of the equation and differs by material: timber decks commonly use 400 mm centres, but composite boards are less stiff and usually need 300 mm to avoid sagging underfoot. Getting spacing wrong means a bouncy or, over time, a failing deck — so it's worth confirming the board manufacturer's required spacing before ordering the frame.
Why use this calculator?
- Counts board rows including the essential expansion gaps.
- Estimates joists and screws, not just the deck surface.
- Reminds you composite needs tighter joist spacing than timber.
Frequently asked questions
How many deck boards do I need?
Divide the deck width by (board width + gap) to get the number of rows, then multiply by the deck length for total linear metres. A 5×4 m deck with 145 mm boards and 5 mm gaps needs about 27 rows / 135 linear metres.
What gap should I leave between deck boards?
Typically 3–6 mm for drainage and to allow the boards to move with moisture and temperature. Composite manufacturers specify exact gaps (often with end gaps too) — follow their guidance, as composite expands more with heat.
How far apart should deck joists be?
400 mm centres is common for timber decking; composite usually needs 300 mm because it's less rigid and will sag on wider spacing. Angled (diagonal) board layouts also need closer joists. Always check the decking maker's spec.