To calculate decking linear metres (LM), divide your deck area in square metres by the coverage width of one board in metres, where coverage width means the board face width plus the gap between boards. For example, 20 m2 of 86 x 19 mm boards laid with a 4 mm gap needs 20 / 0.090 = 222.2 LM before waste. Add a 10% waste allowance and you have your order quantity. This article walks through each step with worked numbers, then shows when it is quicker to let the STS timber calculator do it for you.
Why decking is sold by the linear metre
Sydney Timber Supply sells hardwood and Merbau decking per linear metre, supplied in random lengths (preferred lengths by quote). That means your job is to convert the area of your deck into a total run of board, in metres, and order that quantity. Once you know the LM figure you can price any board profile on the same basis and compare species fairly.
The formula
The calculation has three parts:
- Step 1: coverage width. Coverage width = board face width + gap between boards, converted to metres. An 86 mm board with a 4 mm gap covers 90 mm, or 0.090 m, per row.
- Step 2: linear metres. LM = deck area (m2) / coverage width (m).
- Step 3: waste. Multiply by 1.10 to add the recommended 10% waste allowance for docking, defects and off-cuts.
On gaps: WoodSolutions deck design guidance describes typical gaps of 3-6 mm between boards. The supplier installation guide attached to our product pages specifies a minimum 4.5 mm gap for 90 x 19 mm decking, so always check the supplier guide for the board you are laying.
Worked example: 20 m2 of 86 mm boards
Say your deck is 5 m x 4 m = 20 m2, and you are laying 86 x 19 mm boards with a 4 mm gap.
- Coverage width = 86 mm + 4 mm = 90 mm = 0.090 m
- LM = 20 / 0.090 = 222.2 LM
- With 10% waste: 222.2 x 1.10 = 244.4 LM
So you would order roughly 245 LM of 86 x 19 mm decking for a 20 m2 deck.
Coverage widths for STS board profiles
These are the coverage widths used on the STS site for the common decking profiles:
| Board face width | Coverage width used | Typical products |
|---|---|---|
| 86 mm | 90 mm (0.090 m) | 86 x 19 mm Australian hardwood decking |
| 135 mm | 139 mm (0.139 m) | 135 x 19 mm wide-board hardwood decking |
| 90 mm | 94 mm (0.094 m) | 90 x 19 mm Merbau decking |
The same 20 m2 deck in each profile
Here is the full calculation for a 20 m2 deck across all three coverage widths, including the 10% waste allowance:
| Profile | Coverage width | LM (20 m2) | LM + 10% waste |
|---|---|---|---|
| 86 mm board | 0.090 m | 222.2 | 244.4 |
| 135 mm board | 0.139 m | 143.9 | 158.3 |
| 90 mm Merbau | 0.094 m | 212.8 | 234.0 |
Notice the wider 135 mm board cuts the linear metres by more than a third, which is worth knowing when you compare per-LM prices between profiles.
Converting linear metres to board counts
Hardwood and Merbau decking from STS comes in random lengths, so most customers simply order by LM. If you are planning around a nominal board length, divide total LM by the board length:
Board count = total LM / board length.
Using 5.4 m as the example board length from our calculator: 244.4 LM / 5.4 m = 45.3, so allow 46 boards for the 20 m2 deck of 86 mm boards. Always round up.
When to just use the STS timber calculator
The maths above is simple, but it is easy to slip a decimal place or forget the gap. The timber calculator on our site does the whole chain for you: enter your deck area, pick the board profile, and it applies the correct coverage width, the 10% waste allowance and the board-count conversion. It is also built into the buy box on our decking product pages, so you can go straight from area to order quantity. Use the manual method when you have an irregular deck shape and want to sum several rectangles first, then feed the total area into the calculator.
FAQ
Do I really need the 10% waste allowance?
We recommend it. Docking board ends, cutting around posts and setting aside the odd defect all consume timber, and running short mid-job costs more time than the extra boards.
What gap should I use between boards?
WoodSolutions deck design guidance describes typical gaps of 3-6 mm. For 90 x 19 mm decking, the supplier installation guide specifies a minimum 4.5 mm gap. Check the supplier guide attached to your product page.
Can I order set lengths instead of random lengths?
Hardwood and Merbau decking is supplied in random lengths as standard. Preferred lengths are available by quote through our request a quote page.
Talk to us
If you would rather have a second set of eyes on your numbers, send your deck dimensions through our quote form and we will confirm quantities. Pickup is available from our West Gosford yard, and we deliver to Sydney Metro, the Central Coast, Newcastle and the Hunter, and Wollongong and the Illawarra. See delivery and pickup for details.
Sydney Timber Supply team
Technical details drawn from supplier specification sheets; reviewed 19 July 2026.