Building
Decking Calculator
Boards, lineal metres, joists and screws for your deck, with cost by material.
Quantities are a guide for planning. Structural design, spans and footings should follow AS 1684 and your local council requirements.
How this calculator works
The calculator works out how many rows of boards fit across your deck width, using the board width plus your gap. Rows multiplied by deck length gives lineal metres, and 10 percent is added for cuts, offcuts and the odd bowed board.
Joists are counted at 450mm centres, the standard spacing for 19mm to 22mm deck boards. Fixings are estimated at 14 screws or clips per lineal metre, which assumes two per board at each joist crossing.
Costs are board-only using typical July 2026 prices per lineal metre: treated pine $5 to $9, merbau $10 to $16 and composite $12 to $22. Subframe, fixings, stain and labour are extra.
| Material | Per lineal metre | Character |
|---|---|---|
| Treated pine | $5 to $9 | Budget friendly, needs regular coating |
| Merbau | $10 to $16 | Durable hardwood, rich colour, most popular |
| Composite | $12 to $22 | No oiling, consistent look, cooler brands cost more |
Board prices only. Allow for joists, bearers, fixings and finishing on top.
| Item | Standard | Notes |
|---|---|---|
| Board widths | 86mm or 136mm | 136mm covers faster but moves more |
| Board gap | 4mm to 5mm | Drainage and expansion |
| Joist spacing | 450mm centres | 300mm for diagonal or 136mm composite |
| Common lengths | 5.4m and 5.7m | Match your deck to cut waste |
Frequently asked questions
How many lineal metres of decking do I need per square metre?
With 86mm boards and a 4mm gap, about 11.1 lineal metres per square metre before waste. With 136mm boards, about 7.1 lineal metres. Add 10 percent for cuts.
Is merbau or composite decking better?
Merbau is cheaper upfront and looks like real timber because it is, but wants oiling every year or two. Composite costs more, needs almost no maintenance and suits fire-prone or coastal areas well. Over ten years the total costs are surprisingly close.
What joist spacing does decking need?
450mm centres is standard for 19mm hardwood and treated pine boards laid square. Tighten to 300mm for diagonal decking, some composites and 32mm+ traffic loads. Always check the board manufacturer specs.
How many screws does a deck need?
Two screws per board at every joist. That works out near 14 fixings per lineal metre of board at 450mm joist spacing. Stainless or coated screws are worth the extra in any Australian climate.
Do I need council approval for a deck?
Often yes: rules vary by state and council, and height above ground, size and proximity to boundaries all matter. Low decks under 400mm to 600mm are exempt in many areas, but always check before building.
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