How we work it out
Deck width, divided by the board's face width plus the gap left between boards, gives the number of rows you need, a 90 mm treated pine board laid with a 6 mm gap is 96 mm all up, so a 3 m-wide deck needs 32 rows. Multiply that by the deck length for raw linear metres of board, then add 10% for end-cuts, rejects and the partial boards every real edge needs, the standard allowance for a simple rectangular deck.
Worked example: a 4 m × 3 m deck (12 m²) in treated pine: 32 rows × 4 m = 128 m raw, 141 m to buy once 10% wastage is added, which at $4.40 a linear metre is about $620 in boards. The frame underneath needs 9 joists at the standard 450 mm centres. Fully installed, subframe, boards, fixings and labour, treated pine typically runs $2,400 to $3,360 for that same 12 m².
Which material?
Treated pine is the budget option and the easiest to cut and fix with standard tools, but wants H4-rated timber wherever it sits close to the ground and a re-oil every year or two to stop it greying. Merbau and other hardwoods (spotted gum, ironbark) cost more but need almost no maintenance beyond an occasional oil, and weather gracefully even left bare. Composite decking costs the most up front, it's largely recycled plastic and timber fibre, but never splinters, cups or needs oiling, which is why it's the common pick around a pool or anywhere that gets bare feet. Pick your deck's use in the calculator above and it'll turn that into a straight steer instead of leaving you to weigh it up yourself.
Board gaps and joists
Timber decking wants a wider gap (5–8 mm) than composite (around 5 mm) because real timber swells and shrinks with moisture; skip the gap and a wet season can cup or buckle the boards. Joists sit underneath, running the other way to the boards, at 450 mm centres for timber or as tight as 300–400 mm for composite, always check the specific product's span table, since composite doesn't have timber's own stiffness.
The materials total doesn't include the borer
The "materials only" line above is boards and fixings, not the sub-frame's stump holes. Most decks sit on posts set into the ground, and boring those holes by hand with a spade is a full day's digging most people skip by hiring a one-person post-hole borer instead, priced at roughly $100 to $120 a day dry hire, per an Australian plant-hire rates guide. That's what the "Materials + tool hire" line above adds to the board cost, so it's a real DIY total, not just a shopping list for the timber yard.
Common questions
How many decking boards do I need?
Divide your deck's width by the board's face width plus the gap to get the number of rows, multiply by the deck length for raw linear metres, then add 10% for end-cuts and wastage. A 3 m-wide deck in 90 mm treated pine (96 mm with the gap) needs 32 rows.
What gap should I leave between decking boards?
5 to 8 mm for treated pine or hardwood, to allow for the timber swelling in wet weather. Composite decking needs less, typically around 5 mm, but always check the specific product's install guide.
How much does decking cost per square metre in Australia?
Treated pine runs roughly $200 to $280 per square metre fully installed, hardwood like merbau or spotted gum $280 to $420, and composite $350 to $550, including subframe, boards and labour.
Do I need to hire a tool to build my own deck?
Most decks sit on posts set into stump holes, and boring those by hand is a full day's work most people skip by hiring a one-person post-hole borer instead, roughly $100 to $120 a day dry hire. Worth adding to the board cost above rather than treating the boards as the whole DIY total.
Sources
Electricity rates: AER Default Market Offer (NSW, SA, south-east QLD), Victorian Default Offer, and each remaining state's regulated tariff, current for 2026–27. Appliance figures: the energyrating.gov.au registration database (July 2026 snapshot). Methods and assumptions are described on each page.