Deck Board Calculator
Count the decking boards a deck needs from its size, the board width and gap, and the board length — with the linear length and screw count.
How many deck boards and screws.
How the deck board calculator works
Boards run across the deck width with a gap between them for drainage and movement: rows of boards = deck width ÷ (board width + gap), rounded up. Along the deck length, boards per row = deck length ÷ board length, rounded up. Total boards is the product; linear length is boards × board length. Screws: two per board at each joist, with joists at 450 mm centres.
Buying boards a little longer than the deck avoids joins; if joins are needed they should land on a joist and be staggered.
Formula: rows = ⌈width ÷ (board width + gap)⌉ · per row = ⌈length ÷ board length⌉
Worked examples
| Inputs | Boards to buy | Note |
|---|---|---|
| 6 × 4 m deck, 90 mm boards, 5 mm gap, 4.8 m lengths | 86 | 43 rows × 2 = 86 boards |
| Same deck with 6 m boards | 43 | no joins: 43 boards |
| 20 × 13 ft deck, 5.5 in boards, ¼ in gap, 16 ft boards | 56 | 28 rows × 2 = 56 boards |
FAQFrequently asked questions
What gap should I leave between deck boards?
3–6 mm for dry hardwood and composites; treated pine is often laid tight because it shrinks. Check the board manufacturer's guidance.
How many screws per board?
Two at every joist. With joists at 450 mm centres, a 4.8 m board needs about 24 screws.
Should I include waste?
The rounding up already leaves offcuts; add 5% for a diagonal layout or a deck with many corners.
What joist spacing do I need?
Typically 450 mm (16 in) for 19–22 mm boards; composites often require 400 mm or less. The board manufacturer and local code decide.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Cement Concrete & Aggregates Australia — concrete basics and yields — bag yields, density 2,400 kg/m³
- Australian Building Codes Board — the National Construction Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.