A deck is one of the most satisfying home projects, and the maths is the easy part. This guide shows how to convert your deck area into lineal metres of board, get the joist spacing right, weigh timber against composite, and account for the fixings, footings and council rules that complete the job.
Every decking estimate begins with the area of the deck in square metres — length times width. A deck 6 metres by 4 metres is 24 square metres. That area drives the two things you need to buy: the decking boards that form the surface, and the framing that supports them.
Work out the area with our square metre calculator.
Decking boards are sold by length, so you need to convert your area into lineal metres of board. The trick is that each board covers its own width plus the small gap left between boards for drainage and expansion.
Lineal metres = Deck area ÷ (board width + gap)
For a common 90mm board with a 3mm gap, each board effectively covers 93mm, or 0.093 metres. A 24 square metre deck therefore needs about 24 ÷ 0.093 = 258 lineal metres of decking board, before wastage. Add around 10% for cuts and the occasional unusable board, so order closer to 285 lineal metres.
Under the boards sits the frame — joists running one way, supported by bearers running the other, on posts or footings. Joist spacing is critical and is set by the decking material: many timber and composite boards require joists at 450mm centres, some at 300mm for diagonal laying or thinner boards. Closer spacing means more joists and more cost, so check the board manufacturer's span requirements before estimating the frame.
The big material choice is timber versus composite. Hardwood timber decking looks superb and costs less upfront, but needs regular oiling and maintenance and weathers over time. Composite decking costs more to buy but is low-maintenance, consistent, and long-lived. Over a decade the total cost can be closer than the sticker prices suggest, once you factor in the oil, time and eventual board replacement that timber demands.
Beyond boards and frame, budget for the fixings — screws or hidden clips, which add up over hundreds of board-metres — plus post footings (usually concrete, estimated with our concrete calculator), joist protection tape, and any balustrade required by the building code for raised decks. These extras are easy to forget and can add a meaningful slice to the total.
Fix your deck area, choose your board type and joist spacing, then get quotes on the full package — boards, frame, footings, fixings and balustrade — rather than boards alone. Because timber prices and labour vary widely, a couple of local quotes against your exact quantities will beat any generic per-square-metre figure. A deck is one of the most satisfying home projects to get right, and the maths is the easy part.
How do I calculate how much decking I need?
Work out the deck area (length times width), then divide by the board coverage — the board width plus the gap between boards. A 90mm board with a 3mm gap covers 93mm, so a 24 square metre deck needs about 258 lineal metres of board before adding roughly 10 per cent wastage.
What joist spacing does decking need?
It depends on the board. Many timber and composite boards require joists at 450mm centres, some at 300mm for diagonal laying or thinner boards. Always follow the manufacturer's span table — wrong spacing causes bouncy, sagging decking that voids warranties.
Is timber or composite decking better?
Timber looks superb and costs less upfront but needs regular oiling and weathers over time. Composite costs more to buy but is low-maintenance and long-lived. Over a decade the total cost can be closer than the sticker prices suggest once maintenance is counted.
What else do I need besides boards?
Framing (joists and bearers), post footings in concrete, fixings (screws or hidden clips), joist protection tape, and a compliant balustrade for raised decks. These extras are easy to forget and add a meaningful amount to the total.
Do I need council approval for a deck?
A raised deck above a certain height generally requires council approval, engineered footings and a compliant balustrade. Thresholds vary by council. Low ground-level decks are usually simpler, but it is still worth checking before you build.