To work out how many deck boards per square metre you need, divide 1,000 mm by the board width plus the gap. A 144 mm board with a 5 mm gap gives about 6.7 linear metres of board per m², which is roughly 2.8 boards of 2.4 m, 1.9 boards of 3.6 m or 1.4 boards of 4.8 m for every square metre of deck.
That’s the quick figure. It’s good for budgeting, but it always runs a little short on a real deck, because every row needs a whole board and offcuts don’t always get used. Below are the per-m² numbers for common UK board widths, then a worked 4 × 3 m deck to show what you’d actually buy. For a quick answer with UK board sizes, lengths and prices in pounds already set, use the UK decking calculator.
The formula: rows per metre
Deck boards are sold by length, not by area, so the useful number is linear metres per square metre:
Linear metres per m² = 1,000 ÷ (board width + gap), with both in millimetres.
Here’s why it works. Across 1 m of deck, each board takes up its own width plus one gap. So 1,000 ÷ (width + gap) is the number of rows in a metre. Each of those rows is 1 m long over a 1 m × 1 m patch, so the number of rows is also the number of linear metres of board in that square metre.
Example: a 144 mm board with a 5 mm gap covers 149 mm per row. 1,000 ÷ 149 = 6.71 linear metres per m².
To turn that into boards, divide by the board length: 6.71 ÷ 3.6 = 1.86 boards of 3.6 m per m².
Use the actual width, not the nominal one. UK treated softwood sold as “32 × 150” is typically about 28 × 144 mm finished once it’s planed, so 144 mm is the number to use. The deck board sizes guide explains nominal vs actual sizes in more detail.
Deck boards per square metre: quick table
Linear metres per m², and boards per m² by length, with no waste allowance:
| Board width | Gap | Linear m per m² | 2.4 m boards | 3.6 m boards | 4.8 m boards |
|---|---|---|---|---|---|
| 120 mm | 5 mm | 8.00 | 3.33 | 2.22 | 1.67 |
| 120 mm | 6 mm | 7.94 | 3.31 | 2.20 | 1.65 |
| 125 mm | 5 mm | 7.69 | 3.21 | 2.14 | 1.60 |
| 125 mm | 6 mm | 7.63 | 3.18 | 2.12 | 1.59 |
| 144 mm | 5 mm | 6.71 | 2.80 | 1.86 | 1.40 |
| 144 mm | 6 mm | 6.67 | 2.78 | 1.85 | 1.39 |
| 145 mm | 5 mm | 6.67 | 2.78 | 1.85 | 1.39 |
| 145 mm | 6 mm | 6.62 | 2.76 | 1.84 | 1.38 |
| 150 mm | 5 mm | 6.45 | 2.69 | 1.79 | 1.34 |
| 150 mm | 6 mm | 6.41 | 2.67 | 1.78 | 1.34 |
Two things stand out. Going from a 5 mm to a 6 mm gap barely changes anything (under 1%). Board width matters much more: 120 mm boards need about 24% more length than 150 mm boards to cover the same area.
To estimate a whole deck, multiply the deck area by the linear-metre figure and add a waste allowance. A 10% allowance is a common starting point (it’s the default in our calculator), so a 144 mm board at 5 mm becomes about 7.4 linear metres per m².
Common UK board sizes, gaps and joist spacing
These are the sizes you’ll usually find at UK timber merchants and decking suppliers:
- Treated softwood, about 144 × 28 mm. JK Timber lists its 144 mm board in 2.4, 3.0, 3.6, 4.2, 4.8 and 5.4 m lengths.
- Treated softwood, 120 × 28 mm. Savoy Timber sells a 120 × 28 mm deck board in lengths from 1.8 m to 6 m.
- Composite, 126–150 mm and wider. Lawsons’ composite range runs from 126 mm to 200 mm wide, in 3.6 and 4.8 m lengths, and Ultra Decking’s board is 150 mm wide and 3.6 m long.
Gaps. The Timber Decking and Cladding Association (TDCA) says to leave a gap of 5 mm minimum and 8 mm maximum between timber boards, kept consistent across the deck. Composite boards often use clips that set the gap for you, so follow the manufacturer’s figure. Our deck board spacing guide covers gaps by material.
Joist spacing. In the same Code of Practice (Table 5), the TDCA gives a maximum support spacing of 400 mm centers for 120 × 27 mm and 145 × 27 mm boards in C16 softwood, rising to 450 mm for stronger grades. That’s why 400 mm is the usual UK joist spacing for standard softwood decking. Diagonal boards need closer joists.
Width limit. The TDCA also says timber deck boards shouldn’t be wider than 145 mm, to limit movement and help drainage. The 150 mm row in the table is mainly for composite boards, which follow their own manufacturer’s rules.
Worked UK example: a 4 × 3 m deck
Here’s a typical garden deck, run through the deck board calculator in metric mode:
- Size: 4 m × 3 m (12 m²)
- Boards: 144 mm treated softwood, 5 mm gap
- Joists: 400 mm centers
- Lengths available: 3.6 m and 4.8 m
- Waste allowance: 10%
The square-metre shortcut says 12 m² × 6.71 = 80.5 linear metres, or about 22.4 boards of 3.6 m. Here’s what the calculator actually recommends for each board direction:
| Boards run | Rows | Boards used | Buy (with 10%) | Linear metres bought | Last row |
|---|---|---|---|---|---|
| Across the 3 m width | 27 | 27 × 3.6 m | 30 × 3.6 m | 108.0 m | ripped to 126 mm |
| Along the 4 m length | 21 | 21 × 4.8 m | 24 × 4.8 m | 115.2 m | ripped to 20 mm |
The calculator picks boards running across the 3 m width. It buys about 6% less timber, and every row is a single 3.6 m board cut to 3 m with no butt joints. Running boards the long way needs 4.8 m boards (a 3.6 m board can’t span 4 m without a joint) and leaves a 20 mm sliver for the last row, which is hard to cut and fix.
Notice the gap between the shortcut and the real order: 80.5 m vs 108 m. Most of that is the 0.6 m offcut from every 3.6 m board, plus the 10% allowance. That’s why a per-m² figure is fine for a rough budget, but you should count rows before you order.
Tweaking the gap. At 3 m wide, 20 boards of 144 mm with 6 mm gaps fit almost exactly. If you run the boards the long way with a 6 mm gap, the calculator drops the sliver: 20 full-width rows, 22 × 4.8 m (105.6 m) with the allowance. That’s slightly less timber than the cross-width layout’s 30 × 3.6 m (108 m, unchanged at 6 mm), with no ripped row and no joints. A 1 mm change in gap can flip which layout is best. The deck board spacing calculator finds a gap that fits whole boards.
Framing and fixings for the recommended layout. With boards running across the 3 m width, the joists run the 4 m way, so the calculator counts 9 joists at 400 mm centers, spaced along the 3 m length of the boards. The TDCA Code of Practice calls for two screws at every joist crossing, placed 25% in from each edge of the board, which comes to 486 screws (before spares) for this deck.
Why the per-m² method runs short
The table gives the minimum length of board that physically covers each square metre. Real decks add to that in three ways:
- Offcuts. Unless your deck dimension matches a stock length, each row leaves an offcut. A 3 m row from 3.6 m boards wastes 0.6 m per row (17%). The choosing deck board lengths guide shows how to pick lengths that waste less.
- Whole rows. A row has to be bought even if you only need a strip of it. The last row is often ripped down but still costs a full board.
- Defects and mistakes. Splits, bowed boards and miscuts. A 10% allowance covers most of it on a simple rectangle.
Picture-frame borders, breaker boards and stairs add more on top.
How to work it out for your deck
- Measure the deck in metres and work out the area. The deck square footage calculator also works in square metres, including L-shaped decks.
- Check the board’s actual width and the gap you’ll use (5–8 mm for timber under TDCA guidance, or the clip gap for composite).
- Use the table for a quick linear-metre budget.
- Then count rows. Enter the deck in the deck board calculator, switch it to metric, and add the lengths your supplier stocks. It compares both board directions, mixes lengths, places any joints on joists, and tells you the exact number of each length to buy.
Frequently asked questions
How many deck boards do I need per square metre?
With 144 mm boards and a 5 mm gap you need about 6.7 linear metres per square metre. That’s about 2.8 boards of 2.4 m, 1.9 boards of 3.6 m or 1.4 boards of 4.8 m, before waste. Narrower 120 mm boards need about 8 linear metres per m².
How many linear metres of decking is 10 m²?
With 144 mm boards and a 5 mm gap, 10 m² × 6.71 = about 67 linear metres, or roughly 74 m with a 10% allowance. Offcuts can push the real figure higher, so check the layout in a calculator before ordering.
What size gap should I leave between deck boards in the UK?
The TDCA’s Code of Practice says to leave 5 mm to 8 mm between timber deck boards, kept consistent across the whole deck, and at least 5 mm where a board meets a wall or post. For composite boards, use the gap in the manufacturer’s installation guide.
What joist spacing do UK deck boards need?
400 mm centers is standard for 27–28 mm softwood boards. The TDCA’s table allows 400 mm for 120 × 27 mm and 145 × 27 mm boards in C16 timber. Thicker or stronger boards can span a bit further, and diagonal layouts need closer joists.
For an exact board count for your deck, use the deck board calculator in metric mode.