Trade & construction Australia
Deck Calculator Australia
A deck is a grid of posts, bearers and joists topped by boards. This calculator counts each from the deck's length and width, the board width and gap, and the joist spacing, so you can price the timber and fixings before you buy.
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The direction the boards run.
Common lengths are 2.4, 3.0, 3.6, 4.2, 4.8 and 5.4 m.
Set by the board thickness and timber species; follow your supplier's span table.
Decking boards needed
—Rows of boards across
—
Lineal metres of decking
—
Joists
—
Bearers
—
Posts or stumps (approx.)
—
Screws (2 per joist crossing)
—
The working
Allow for your own site. A materials estimate only. Deck structure, balustrades and bushfire requirements are set by the National Construction Code and Australian Standards, and many decks need approval. Use a builder or engineer for the design.
How many decking boards do I need?
Divide the deck's width by the board width plus the gap to get the number of boards across, and multiply by the board length needed to span the deck. A 5.0 × 3.6 m deck using 140 mm boards with a 5 mm gap needs 25 rows of boards, and 13 joists at 450 mm centres.
Design and approval
Many decks need a building permit or council approval, particularly if they are high above the ground, attached to a house or near a boundary. The National Construction Code and Australian Standards cover deck structure and balustrades, and decks more than a metre high usually need a balustrade.
A deck built wrongly can collapse, so decks over a certain height or attached to a house are best designed by a qualified builder or engineer.
Choosing boards
Hardwood species such as spotted gum and merbau last longest but cost more, treated pine is cheaper and needs regular sealing, and composite boards avoid maintenance but carry a higher price. Ask for the durability class and bushfire rating, which matters in bushfire-prone areas.
To work out the concrete for footings, use the concrete calculator.
How this is calculated
The rule, step by step, so you can check any figure by hand.
rows of boards = deck width ÷ (board width + gap), rounded uplineal metres = rows × deck length × (1 + wastage)boards to buy = lineal metres ÷ board length, rounded upjoists = deck length ÷ joist spacing + 1bearers = deck width ÷ bearer spacing + 1; posts under each bearer at about the joist span
Joist spacing is set by the board thickness and the species, not by preference: 19 mm boards typically need joists closer together than 32 mm boards. Always follow your supplier's span tables, and size bearers and joists for the load and the span. This calculator gives a count, not a structural design.
The gap is for drainage and for the boards to swell. A 5 mm gap is common, but dry timber should be spaced wider than green timber, which shrinks as it dries, and the amount varies by species.
A worked example
A 5.0 × 3.6 m deck with 140 mm boards, a 5 mm gap, joists at 450 mm and 2.4 m boards.
- Rows of boards, 3,600 ÷ 14525
- Lineal metres with 10% wastage137.5 m
- Boards of 2.4 m58
- Joists13
- Screws, 2 per crossing650
About 58 boards and 13 joists.
Order longer boards where possible to reduce joins and waste.
Fixings
Use stainless or galvanised screws suited to the timber, because the tannins in some hardwoods corrode ordinary steel. Pre-drill hardwood to avoid splitting. Allow two screws at every joist crossing, or use concealed clips if you prefer a clean surface.
What it leaves out
Footings, balustrade, stairs, fascia, flashing and finishing oil are not included. Bushfire-prone areas have restrictions on deck materials and construction, which a designer must follow.
What this calculator does not do
- A materials estimate, not a structural design
- Rectangular decks; add sections separately
- Joist and bearer sizes need an engineer or span table
- Approval and bushfire rules vary by site
Common questions
How many decking boards do I need?
Divide the deck width by board width plus gap for the rows, then multiply by the deck length. A 5 × 3.6 m deck needs about 25 rows of 140 mm boards.
How far apart should joists be?
It depends on board thickness and species. 450 mm is common for 19 mm boards and wider for thicker ones, but follow the supplier's span table.
Do I need approval to build a deck?
Often, if it is high, attached to a house or near a boundary. Check with your council.
What gap should I leave between boards?
About 3 to 6 mm, more for dry timber and less for green timber that will shrink.
What screws should I use?
Stainless or galvanised screws suited to the timber, and pre-drill hardwood.
Where these numbers come from
- National Construction Code (ABCB) The code that sets minimum building requirements in Australia
- Your Home — Australian Government guide to sustainable homes Independent guidance on insulation, materials and design
Method last reviewed 2026-10-28 This calculator uses fixed rules rather than rates that change, and its method is re-checked against the sources above at least once a year. Out of date? Tell us and we will fix it.
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