The short answer
The number of decking boards per square metre depends on the board width and the gap left between boards for drainage. As a rough guide, a common 144mm-wide board laid with a small gap covers roughly 0.15m2 of width per metre of length, so you need around 6.5 to 7 linear metres of board per square metre of deck. To get a board count, divide the deck area by the area one board covers, then add a waste margin of about 10% for cutting, with more for complex shapes. The exact figure changes with the board you choose, so always work from the actual width and length of your boards rather than a generic number.
Counting boards is straightforward once you fix the board width and the drainage gap. Getting it right avoids running short mid-job or over-ordering, and it feeds directly into the material cost.
Decking boards per m2 at a glance
- Depends onBoard width and the gap between boards
- Common board widthAround 144mm
- Linear metres per m2Roughly 6.5–7 (144mm board, small gap)
- Waste marginAround 10%, more for complex shapes
- Always useYour actual board width and length
How to calculate boards per square metre
The calculation is a short sequence once you know your board's dimensions and the gap you will leave for drainage. The gap matters because each board effectively occupies its own width plus the gap beside it:
- Find the effective width: add the board width and the drainage gap. For a 144mm board with a 5mm gap, that is 149mm, or 0.149m.
- Boards across one metre of width: divide 1 metre by the effective width. One metre divided by 0.149m is about 6.7, so roughly seven board-widths span a metre.
- Linear metres per square metre: that same figure is the linear metres of board needed to cover one square metre — around 6.5 to 7 for a 144mm board.
- Total linear metres: multiply by your deck area in square metres.
- Number of boards: divide the total linear metres by your board length to get how many lengths to buy.
The table below shows how the linear metres per square metre shift with board width. Figures are indicative and assume a small drainage gap.
| Board width | Approx. effective width (with gap) | Linear metres of board per m2 |
|---|---|---|
| 120mm | Around 125mm | Around 8 |
| 144mm | Around 149mm | Around 6.7 |
| 150mm | Around 155mm | Around 6.5 |
Indicative figures for guidance only; use your actual board width and gap. Wider boards mean fewer linear metres per m2.
Why the gap and board width matter
Two details change the board count more than people expect: the width of the board and the gap between boards. Both are worth fixing before you order:
- Board width: wider boards cover more area each, so you need fewer linear metres per square metre. A 150mm board covers a deck with fewer boards than a 120mm one, which can mean fewer fixings and a little less labour, though wider boards can cost more each.
- The drainage gap: boards are laid with a small gap between them so water drains and the wood can move with moisture. A larger gap means each board effectively occupies more width, so you need slightly fewer boards but the look and feel of the deck changes. Composite systems often specify a particular gap for expansion, so follow the maker's guidance.
- Board length: boards come in set lengths. Matching the length to your deck reduces offcuts and waste, while an awkward fit leaves more wasted ends.
Because of this, a generic boards-per-metre figure is only a guide. The reliable count comes from your specific board width, the recommended gap and the board length, which is why merchants and calculators always ask for those before giving a quantity.
Adding waste and turning boards into cost
The bare board count is never the order quantity, because cutting and fitting always produce waste. Boards are trimmed to length, cut around obstacles and mitred at borders, and those offcuts cannot always be reused. A sensible approach is:
- Add a waste margin: a common rule of thumb is around 10% extra on top of the calculated boards. Add more — 15% or so — for decks with curves, multiple levels, diagonal laying or picture-frame borders, which generate more cuts.
- Round up to whole boards: you buy boards in whole lengths, so round the total up rather than down to avoid running short mid-job.
- Keep a few spares: a couple of extra boards are useful for future repairs, especially if the colour or batch may be hard to match later.
To turn the count into a cost, multiply the number of boards by the price per board, or the total linear metres by the price per linear metre, then add the subframe, fixings and labour if you are pricing a full deck rather than just the surface. Remember that the boards are only part of a fitted deck's cost — the subframe, groundworks and labour usually make up the larger share — so a board count tells you the material for the surface, not the price of a finished deck.
Frequently asked questions
How many linear metres of decking board are in a square metre?
For a common 144mm board laid with a small drainage gap, you need roughly 6.5 to 7 linear metres of board per square metre of deck. Wider boards need fewer linear metres and narrower boards need more, so always base the figure on your actual board width and the recommended gap.
How much extra should I order for waste?
A common rule of thumb is around 10% extra on top of the calculated boards, rising to roughly 15% for decks with curves, diagonals, multiple levels or picture-frame borders that produce more cuts. It is also worth keeping a couple of spare boards for future repairs, since matching colour or batch later can be difficult.
Does board width change how many boards I need?
Yes. Wider boards cover more area each, so you need fewer linear metres per square metre, fewer fixings and a little less labour, though they can cost more per board. The drainage gap also matters, since a larger gap means each board effectively occupies more width and slightly reduces the count.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific garden. They are guidance, not a quotation.