Planning a new deck is exciting until you get to the part where you have to figure out exactly how much timber to order. Order too little and you're back at the store mid-job. Order too much and you've blown the budget on offcuts. Getting the quantity right from the start is one of those small things that makes a big difference to how smoothly your project runs.

The good news? The calculation is simpler than most people think. Here's exactly how to work it out.

Step 1: Measure Your Deck Area

Start with the basics. Measure the length and width of the area you want to deck (in metres), then multiply them together to get your square meterage.

Formula:

Deck Area (m²) = Length (m) × Width (m)

Example: A deck that's 6m long and 4m wide = 24m²

If your deck has an irregular shape an L-shape or a wraparound design break it into rectangles, calculate each section separately, then add them together.

Step 2: Know Your Board Width

This is where a lot of people get tripped up. The width printed on a decking board (like 90mm) is the nominal width the actual face coverage after the board is installed with a gap is slightly less.

Most decking is laid with a 3–5mm gap between boards to allow for drainage and timber movement. So a 90mm board with a 5mm gap gives you roughly 85mm of coverage per board.

Here are the most common decking sizes and their approximate coverage per board (with a 5mm gap):

Step 3: Calculate the Number of Boards

Once you know your deck area and the coverage per board, use this formula:

Formula:

Number of boards = Deck Area (m²) ÷ (Board Width in metres × Board Length in metres)

Example: 

• Deck area: 24m²

• Board size: 90x22mm, 3.6m long

• Coverage per board: 0.085m wide × 3.6m long = 0.306m² per board

• Boards needed: 24 ÷ 0.306 = 78.4 boards → round up to 79

Always round up, never down.

Step 4: Add a Waste Allowance

Timber is a natural material and real-world installs always involve cuts, joins, and the occasional board with a knot or bow that you'd rather not use. A standard waste factor covers this.

• Straight deck (boards running parallel): Add 10%

• Diagonal or herringbone pattern: Add 15–20%

• Deck with lots of cut-outs (posts, steps): Add 15%

Updated formula with waste:

Final board count = Number of boards × 1.10 (for 10% waste)

Using our example: 79 × 1.10 = 87 boards

Step 5: Choose the Right Board Length

Decking boards come in standard lengths — typically 1.8m, 2.4m, 3.0m, 3.6m, 4.2m, and 4.8m. Where possible, choose a board length that runs the full span of your deck without a join. Joins look messy and create potential weak points if not supported by a joist underneath.

If your deck is 4.5m wide, for example, a 4.8m board is a better choice than joining two shorter lengths — even though you'll have a small offcut at each end.

A Quick Real-World Example

Let's say you're building a 5m × 3m deck with 90x22 treated pine decking in 3.6m lengths, running straight:

• Deck area: 5 × 3 = 15m²

• Coverage per board: 0.085m × 3.6m = 0.306m²

• Base board count: 15 ÷ 0.306 = 49 boards

• Add 10% waste: 49 × 1.10 = 54 boards

So you'd order 54 boards of 90x22 treated pine decking in 3.6m lengths.

Don't Forget the Subframe

The decking boards are just the surface. You'll also need to calculate joists, bearers, and posts for the subframe. A quick rule of thumb: joists are typically spaced at 450mm centres for softwood decking and 600mm centres for hardwood. If you're not sure, our team can help you work out your full material list.

Use Our Decking Calculator

Rather than doing all this by hand, A Class Building Materials has a free Decking Calculator on our website. Plug in your dimensions and it'll estimate your board count automatically.

We stock a wide range of decking options including treated pine decking, Merbau decking, and composite decking — available for pickup from our stores in Ravenhall, Diggers Rest, and Geelong, or delivered Australia-wide.

If you're unsure about quantities or want advice on the best decking type for your project, give our team a call on 0450 612 719 or visit us in store.