Decking Materials Calculator Guide: How Much Timber, Screws, and Posts Do You Need?

Why a Materials List Matters Before You Go to the Hardware Store

Walking into Bunnings with a rough idea of what you need is how decking projects end up 40% over budget. Timber costs have risen sharply in Australia since 2022, and merbau hardwood in particular is not cheap. Getting your materials list right before you buy saves real money.

Use our decking calculator to enter your deck dimensions and get a complete materials breakdown. This guide explains the logic behind the numbers.

Step 1: Calculate Your Deck Area

Start with the simple stuff: length × width. A 5m × 4m deck is 20 m². That's your reference number for everything that follows.

If your deck has an irregular shape, break it into rectangles and add them. Deduct any built-in planter boxes or feature cutouts from the total.

Step 2: How Many Decking Boards Do You Need?

This depends on your board width. Common Australian decking profiles:

The practical formula: divide your deck width by (board width + gap). For 140mm boards with 3mm gaps on a 4m wide deck: 4000 ÷ 143 = 27.97, round up to 28 boards. Then multiply by deck length to get lineal metres. 28 boards × 5m = 140 lineal metres of decking board.

Add 10–15% for waste from end cuts, bow in boards, and any damaged stock. So order around 154–161 lineal metres for this example.

Step 3: Framing Timber (Joists and Bearers)

The subframe is what holds the deck together. Standard residential deck framing uses:

Joists

Joists run perpendicular to the decking boards. Standard spacing is 450mm centres for most deck loads — reduce to 300mm if you're using thinner (19mm) boards or anticipate heavy loads like a hot tub.

Number of joists = (Deck length ÷ joist spacing) + 1. For a 5m span at 450mm: (5000 ÷ 450) + 1 = ~12 joists. Each joist runs the full 4m width. So 12 × 4m = 48 lineal metres of joist material.

Common joist sizes: 90×45mm or 140×45mm MGP10 pine for standard spans. Consult span tables in AS 1684 for anything over 2.4m unsupported span.

Bearers

Bearers run under the joists and transfer load to the posts. Spacing depends on your joist size — typically 1.2m to 1.8m centres. For a 4m wide deck with bearers at 1.5m centres: 4 bearers, each 5m long = 20 lineal metres of bearer.

Common bearer sizes: 140×45mm, 190×45mm, or 190×90mm LVL for longer spans. When in doubt, overspec — the cost difference is small and the structural benefit is significant.

Step 4: Posts and Footings

Post layout depends on your bearer and joist span design. For a freestanding ground-level deck:

• Posts at bearer intersections, typically 1.8m × 1.5m grid
• Common post sizes: 100×100mm or 150×150mm treated pine
• Post height = finished deck height + footing embedment depth (minimum 450mm for non-structural, 600mm+ for structural)

Each post needs a footing. Standard residential footing = 300mm diameter × 600mm deep concrete pier. Use our concrete calculator to work out the volume for all your post footings — it adds up faster than you'd expect on a large deck.

Step 5: Screws and Hardware

This is where DIY deckers consistently under-order. Budget based on these rough guides:

• Decking screws: 2 screws per joist crossing per board. For 28 boards crossing 12 joists = 672 screws. Buy in 1kg boxes (roughly 120–150 screws per kg for 50mm × 10g). You'll need 5–6 boxes.
• Joist hanger sets: One per joist end where they connect to a ledger or rim joist. For 12 joists, 24 hangers.
• Post caps and stirrups: One per post. If posts sit on concrete piers, use adjustable stirrups rather than embedding — it prevents moisture wicking into end grain.
• Stainless steel vs galvanised: For hardwood like merbau, use stainless steel screws. The tannins in hardwood will corrode galvanised fasteners within a few years.

A good impact driver with a screw depth setter attachment is worth every cent on a decking project — hand-driving 600+ screws is how tradespeople develop RSI, and a good impact driver makes the job dramatically faster and produces consistent countersink depth.

Timber Species Comparison

Composite decking has the lowest lifetime maintenance cost and is increasingly the choice for waterfront and high-humidity properties. Its higher upfront cost is typically recouped within 8–10 years compared to hardwood on maintenance savings alone.

Quick Materials Checklist for a 5m × 4m Deck

• Decking boards (140mm merbau): ~155 lineal metres
• Joists (90×45 MGP10): ~48 lineal metres
• Bearers (140×45 MGP10): ~20 lineal metres
• Posts (100×100 H4 pine): ~8–10 posts at appropriate height
• Concrete for footings: ~0.4–0.6 m³ (use the concrete calculator)
• Decking screws (stainless 50mm): 6 × 1kg boxes
• Joist hangers: 24
• Post stirrups: 10
• Decking oil or sealant (merbau): ~4–6 litres for first coat