Timber & Decking Calculator
Calculate decking boards, joists, and bearers for your deck build. Accounts for waste, spacing, and standard timber lengths. No sign-up, no ads.
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Estimate only. Actual quantities depend on deck shape, board availability, and site conditions. Bearer and joist sizes depend on span and load — consult an engineer for elevated or complex decks.
Standard is 5mm for drainage and expansion
10% is recommended. Increase for complex layouts or angled cuts
Boards typically overhang 20–30mm past the outer joist
Standard Australian Timber Lengths
Australian timber merchants stock boards in standard lengths. Planning your layout around these lengths will minimise waste and off-cuts, saving you money.
| Length | Best For | Availability |
|---|---|---|
| 2.4m | Small decks, balconies | Very common |
| 3.0m | Medium decks | Common |
| 3.6m | Most popular length | Very common |
| 4.8m | Large decks, fewer joins | Common |
| 5.4m | Long spans, minimal joins | Less common |
Deck Area
24.60 m²
Actual size: 6.05m × 4.05m (incl. overhang)
Decking Boards
Number of Boards
96
Lineal Metres
345.6 lm
Joists
Number of Joists
15
Lineal Metres
60.75 lm
Bearers
Number of Bearers
4
Lineal Metres
24.20 lm
Tip: Boards run along the deck length. Joists run across the width. Bearers run along the length supporting the joists. Always buy 10% extra to account for waste and off-cuts.
How It Works
Enter your deck dimensions and board preferences, and the calculator works out exactly what you need. Everything runs in your browser — no data leaves your device.
Enter Deck Dimensions
Enter your deck length and width, then choose your preferred board width and thickness.
Set Spacing & Waste
Choose joist spacing, board length from standard Australian sizes, and set your waste allowance.
Get Your Material List
Get a complete breakdown — decking boards, joists, and bearers with quantities and lineal metres.
Decking Best Practices
Use 450mm Joist Spacing
Required for 19mm boards. 32mm+ boards can use 600mm spacing. Always check your board manufacturer's maximum span recommendations before building.
Allow for Drainage Gaps
5mm between boards is standard. Boards will also shrink slightly as they dry, especially hardwood. Green timber may only need 2–3mm initially.
Buy Standard Lengths
2.4m, 3.6m, and 4.8m are the most readily available lengths. Plan your layout around these to minimise off-cuts, waste, and cost.
Frequently Asked Questions
What's the best timber for decks in Australia?
Merbau (kwila) is the most popular choice — it is durable, naturally resistant to rot and insects, and weathers to a silver-grey if left unoiled. Spotted gum and blackbutt are premium Australian hardwoods with excellent durability ratings (Class 1–2). Treated pine (H3) is the budget-friendly option at $3–6 per lineal metre, but it needs regular oiling every 1–2 years and has a shorter lifespan of 15–20 years. Composite decking is virtually maintenance-free but costs more upfront.
What joist spacing should I use for decking?
Use 450mm centres for 19mm boards — this is the maximum safe span for thinner boards. 32mm boards can go up to 450–600mm centres. 42mm boards can handle up to 600mm centres. Always check your board manufacturer's maximum span recommendations, as this varies by timber species, profile, and whether the board is laid flat or on edge. Closer joist spacing gives a firmer feel underfoot.
How do I calculate bearer size and spacing?
Bearer size depends on the span between supporting posts and the load they carry. For typical residential decks, 1.2–1.8m bearer spacing is common. For size: use 90×90mm bearers for short spans (under 1.5m between posts), 140×90mm for medium spans, and 190×90mm for longer spans. For elevated decks over 1 metre high, or decks with spas or heavy loads, always get an engineer's advice. This calculator uses 1.8m bearer spacing as a general guide.
Should I use treated pine or hardwood?
Treated pine: $3–6 per lineal metre, H3 rated for above-ground exterior use, needs oiling every 1–2 years, lifespan of 15–20 years. It is readily available, easy to work with, and the most affordable option. Hardwood (merbau, spotted gum, blackbutt): $8–15 per lineal metre, naturally durable (Class 1–2), oil every 1–2 years for appearance (it is fine left to grey), lifespan of 25–40 years. Hardwood looks significantly better and lasts longer, but costs 2–3 times more upfront.
What gap should I leave between deck boards?
5mm is standard for most kiln-dried timbers. This gap allows for drainage, air circulation, and minor expansion. Green (unseasoned) hardwood may only need 2–3mm because it will shrink as it dries — the gap will open up over time. Kiln-dried timber should have 4–5mm gaps. Some composite boards have specific gap requirements (often 5–6mm) — always check the product installation guide. Use a spacer tool or a 5mm drill bit as a quick gauge.
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Use CalculatorDisclaimer: This calculator provides estimates only and is intended as a general guide. Actual material quantities may vary based on deck shape, board availability, joist and bearer sizing requirements, and site conditions. Bearer and joist sizes depend on span and load — consult an engineer for elevated or complex decks. Always confirm quantities with your builder or supplier before ordering. Devalo does not accept liability for any loss or damage arising from the use of this calculator.
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