How to Calculate Decking Boards
Building a timber or composite deck is a fantastic way to extend your living space outdoors. However, calculating the materials can be confusing because decking is sold in long linear lengths, while the space you want to cover is measured in square metres.
Our Decking Calculator bridges this gap. By entering your total deck area and the dimensions of your chosen boards, it instantly calculates the total linear metres required and breaks that down into the exact number of boards you need to buy.
The Importance of the Expansion Gap
When calculating how many boards will fit across a patio, you must include the expansion gap. Timber and composite boards swell when wet and shrink when dry. If laid tightly together, they will buckle and warp as soon as it rains.
A standard decking gap is 5mm to 8mm between every board. Our calculator automatically adds this gap to the width of the board during the calculation to ensure an incredibly accurate material count.
Accounting for Waste and Offcuts
Unlike patio slabs, you can often reuse a large decking offcut at the end of the next row. However, you will still lose material to saw cuts, squaring off end-grain, and working around obstacles like downpipes or soil stacks.
It is universally recommended by carpenters to add a 10% waste allowance to your total linear metreage before ordering. If your deck features an intricate diagonal herringbone design, increase this allowance to 15%.
Don't Forget the Sub-Frame
This calculator determines the finished surface boards. Remember that you will also need to calculate the timber for the structural joists (usually treated 4x2 or 6x2 timber) installed at 400mm centres below the deck surface.