Decking calculator

A deck is boards on top and a frame underneath, and both need working out before you order. This calculator gives you the deck boards and the joists from the deck size and your board and spacing choices, with wastage built in. Here's how it's worked out.

How to work out how many deck boards you need

Each board covers its own width plus the expansion gap, so the total length of board you need is the deck area divided by that effective width. Divide again by the length of one board to get the number of boards.

Board length needed = area ÷ (board width + gap), then boards = that ÷ board length.

A 4m by 3m deck is 12 square metres. With 144mm boards and a 5mm gap, each board covers 149mm, so you need about 80 linear metres of board, near 88 with 10% wastage, which is 25 boards of 3.6m. The calculator does all of that from your numbers.

The gap between boards

Leave a gap of around 5 to 8mm between boards, so the timber can expand and water drains through rather than sitting on top. The gap isn't just good practice, it changes the count, because each board effectively covers its width plus the gap, so it's built into the calculation.

Joists and spacing

The frame under the boards is joists, usually run across the width and spaced at 400mm centres for standard decking. Go closer, around 300mm, if you're laying boards diagonally or want a firmer feel underfoot, since the boards are then unsupported over a longer span. The number of joists is the deck length divided by the spacing, plus one for the far end, and the timber is that many joists times the deck width.

Board sizes and wastage

A common deck board is about 144mm wide and comes in lengths like 3.6m or 4.8m, but sizes vary, so set yours to match what you're buying. Allow around 10% wastage for cuts and offcuts, and more, nearer 15%, for a diagonal deck or a shape with lots of angled cuts, which waste more board you can't reuse.

Don't forget the rest of the build

Beyond boards and joists, a deck needs posts or a solid base to sit on, decking screws or hidden fasteners, and usually a weed-control membrane underneath. Ground-contact posts are typically set in concrete, and the whole frame wants a slight fall for drainage. Price those in so the quote covers the real job.

From materials to a priced job

Boards and joists are the start; the labour in the frame and the fixing is the bigger part. Price the whole job with how to quote a landscaping job fast, put the materials into a clean, itemised quote with the quote generator, and if you're setting posts in concrete, the paving calculator and concrete calculator cover the groundwork.

This tool is for general estimating and gives approximate quantities. Board sizes, spacings and layouts vary, so check your own measurements and follow the span guidance for your boards, and order a sensible amount over rather than exactly to the calculation.