Composite Decking Calculator Australia — Boards & Cost in A$
Planning a new deck? This free composite decking calculator helps Australian DIYers and builders work out exactly how many boards, hidden fasteners, and starter clips a deck needs — plus a full cost estimate — in metres and Australian dollars. It's built around the 5.4m board length stocked by suppliers like Bunnings, the only length in the local composite lineup, so there's no board-length guessing game here. Results update live and include a full cost breakdown.
How to use this calculator
1. Measure your deck area
Measure your deck's length and width in metres and enter them directly — for a 3m × 4m deck, that's 12 sqm of raw area before any waste allowance. If your deck isn't a simple rectangle — an L-shape, a wraparound, or a stepped section — break it into rectangular sections, measure each one, and add the areas together before entering the total.
2. Choose your install pattern
Straight board runs carry a 10% waste allowance for end cuts. Diagonal cuts push that to 15%, herringbone needs 20%, and a picture-frame border adds 12%. Since Australian composite ships in one board length, your install pattern is really the only lever you have for controlling waste and cost — pick it carefully, and see the worked example below for exactly how much that choice is worth.
3. Choose a composite tier
Choose an economy, mid-range, or premium composite tier. Unlike the US, UK, or Canada, there's no board-length decision to make here — Bunnings and most Australian composite suppliers, including Ekodeck, stock a single 5.4m length, so tier is where your budget choice actually lives.
4. Review your boards, fasteners, and cost breakdown
Results update live. You'll see total boards needed (waste already included), linear metres, fastener clip and starter clip counts if hidden fasteners are switched on, and a full cost breakdown by line item, in metres and Australian dollars — everything you need before a Bunnings run.
The formula we use
Here's the exact math. Deck area is length × width. We add your pattern's waste allowance — straight ×1.10, diagonal ×1.15, herringbone ×1.20, picture-frame ×1.12 — to get total area with waste. Divide that by board coverage (5.4m board length × 0.14m effective width, so 0.756 sqm per board) and round up to get boards needed. The 0.14m figure is the effective width of a standard composite board once you account for the gap left between boards for drainage and expansion.
Unlike the US, UK, or Canadian versions of this calculator, there's no board-length tradeoff to show here — Australian composite decking, including Ekodeck at Bunnings, ships almost exclusively in 5.4m lengths. That makes install pattern the real lever on your total: straight-pattern waste (10%) versus herringbone (20%) is the difference that actually moves your board count and cost, not a board-length choice.
If you've switched on hidden fasteners, the calculator adds roughly 38 clips per square metre of deck area, plus one starter clip per 0.6m of perimeter — the same physical clip density as the imperial regions, just expressed in metric units. Confirm exact spacing against your fastener system's installation guide before ordering, since spacing recommendations vary slightly by manufacturer.
A worked example
Let's walk through a real 3m × 4m deck — 12 sqm — mid-tier composite, hidden fasteners on, comparing straight against herringbone since board length isn't a variable here.
Straight pattern: total area with waste is 12 × 1.10 = 13.2 sqm. At 0.756 sqm per 5.4m board, that's 13.2 ÷ 0.756 = 17.5, rounded up to 18 boards. At A$98 a board (Ekodeck-style mid-tier), that's A$1,764.
Herringbone pattern, same deck: total area with waste jumps to 12 × 1.20 = 14.4 sqm, needing 14.4 ÷ 0.756 = 19.05, rounded up to 20 boards — A$1,960, a real A$196 more than straight, just from the pattern choice.
Add hidden fasteners to the straight-pattern version: the calculator estimates 452 clips (12 sqm × 38) at A$0.55 each, plus 23 starter clips around the 14m perimeter, also at A$0.55. That's roughly A$261 in hardware — a bigger line item than face-fastening, so budget for it rather than treating clips as an afterthought.
Regional cost context
Australian composite decking prices sit well above UK or US equivalents once converted to a common unit — the tradeoff for a smaller, more consolidated local market.
Estimates are indicative only. As an Amazon Associate we earn from qualifying purchases. Retaining walls over 1 m and decks above certain thresholds may require council approval and/or an engineer (Form 15/16 in QLD). Verify with your local council before starting work.
| Board length | Economy | Mid | Premium |
|---|---|---|---|
| 5.4m | A$79 | A$98 | A$142 |
Prices from Bunnings, current as of our last check — see our Methodology page for sources and verification dates. Fastener clips run A$0.55 each; starter clips A$0.55 each.
Bunnings' economy-tier composite (Ekodeck-style) is the budget pick — a real step down from mid-tier at nearly half the price. Mid-tier is the standard choice for most Australian decks, balancing fade resistance against cost. Premium runs about 45% above mid-tier, for a thicker profile and longer warranty — a smaller jump than the roughly 2x step seen in the US market.
Fastener and starter clip pricing here (A$0.55 each) is an estimate rather than a directly sourced figure — treat it as reasonably confident but not retailer-verified. All board prices reflect our last direct Bunnings check (see our Methodology page for sources and dates); expect variation between stores and states.
Shop on Amazon: Trex Enhance composite decking
Do you need a permit or approval?
Deck approval in Australia is genuinely state-based, so the numbers below are a starting point, not a final answer for your address.
NSW is a useful reference: a deck can be exempt development — no council approval needed — if it's behind the building line, no more than 1m above ground at any point, at least 900mm from all side and rear boundaries, and no larger than 25 sqm (or 15% of your home's ground floor area, whichever is smaller). Miss any one of those conditions and you likely need a complying development certificate or a full development application instead.
Quick heads-up: other states set their own thresholds under their own building regulations, so don't assume NSW's numbers transfer to Victoria, Queensland, or elsewhere. A deck over about 1m, or one attached to the house in a way that affects structural loads, is worth a council or certifier check regardless of state. Confirm current rules with your local council before you start.
Common mistakes to avoid
- Ordering board count from raw area, no waste allowance. This almost always ends in a mid-project supply run.
- Assuming a longer board length is available to cut your board count. It usually isn't, in most of the local market — pattern choice is your real lever, as shown above.
- Forgetting joists and substructure aren't included. Budget 15-30% on top if you're building from scratch.
- Picking the wrong install pattern. Herringbone at straight's 10% waste rate will leave you well short.
- Building over 1m or inside the 900mm boundary setback without a council check. Both conditions matter, not just height.
- Assuming your board's real width matches the 140mm nominal figure exactly. Actual profiles vary slightly by brand — this calculator uses a standard effective width, so treat the board count as a solid planning estimate, then confirm exact coverage against your chosen product's spec sheet.
- Building a garden with terracing? See our retaining wall block calculator. Planning to stain or paint your deck? Use our paint coverage calculator.
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Frequently asked questions
How much decking do I need for my deck?
It depends on your deck's area and install pattern, not a flat number. For a 3m × 4m deck (12 sqm) laid straight, budget 13.2 sqm of effective coverage after a 10% waste allowance; herringbone needs 20% instead. Since Australian composite ships almost exclusively in 5.4m lengths, that works out to 18 boards for a straight layout. Use the calculator above with your exact dimensions and pattern for a precise board count.
How much does Ekodeck or ModWood cost at Bunnings?
Bunnings' composite decking runs roughly A$118-213 per square metre in 2026 depending on tier — about A$118/sqm for economy (Ekodeck-style), A$147/sqm for mid-tier, and around A$213/sqm for premium, based on a 3m × 4m deck. Add hidden fasteners and you're looking at roughly A$22 more per square metre. These figures reflect our last direct Bunnings check — always confirm current pricing in-store, since it varies by state and stock.
Do I need council approval for a deck in Australia?
In NSW, a deck can be exempt development — no council approval needed — if it's behind the building line, no more than 1m above ground at any point, at least 900mm from all boundaries, and no larger than 25 sqm or 15% of your home's ground floor area, whichever is smaller. Miss any one condition and you likely need approval. Other states set their own thresholds, so confirm current rules with your local council before you start.
What joist spacing for composite decking (450mm)?
450mm centres is the standard joist spacing recommendation for composite decking in Australia when boards are laid straight. Tighten that to roughly 300-350mm for diagonal or angled installations, since fewer joists cross each board along its length at an angle. This calculator doesn't size your substructure, so always check your specific board manufacturer's installation guide before framing — spacing requirements vary slightly between products like Ekodeck and ModWood.
Is composite decking better than Merbau or hardwood?
It depends what you're optimizing for. Composite typically costs more upfront than Merbau but needs far less upkeep — no annual oiling, no splintering, and a 25-30 year lifespan with minimal maintenance. Merbau needs regular oiling to hold its colour and can silver or crack without it, but many people prefer its natural timber look, and it's often cheaper to buy initially. Neither is objectively "better" — it's a real tradeoff between upfront cost, maintenance time, and appearance.
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Estimates are indicative only. As an Amazon Associate we earn from qualifying purchases. Retaining walls over 1 m and decks above certain thresholds may require council approval and/or an engineer (Form 15/16 in QLD). Verify with your local council before starting work.
Last updated: August 2026. Estimates are for planning purposes only. Verify with local suppliers before purchase, and check local codes and permit requirements before starting work.