Quick answer: To work out how many decking boards you need, first decide which way the boards will run. The dimension across the boards determines how many rows you need; the dimension along the boards determines the length of timber required for each row.
For timber decking, the Timber Decking and Cladding Association (TDCA) recommends leaving a consistent 5–8mm gap between adjacent boards. That range gives you some flexibility to plan the finished deck neatly rather than blindly using one fixed spacer size.
Start with the finished board width, not the nominal size
Always calculate using the actual finished width of the decking board you are buying. Our 125 × 32mm treated timber decking boards finish at approximately 120mm wide after machining, so around 120mm is the useful figure for layout calculations.
View our treated timber decking boards and available lengths.
How to calculate the number of decking rows
For a straight rectangular deck, use the dimension running across the boards. A quick estimate is:
Deck width ÷ (finished board width + planned gap)
That gets you close, but it is not the best way to finalise the layout because there is one fewer internal gap than there are boards.
For an exact check, use:
Total deck width = (number of boards × board width) + ((number of boards − 1) × gap)
Worked example: making 120mm decking boards fit a 3m deck
Suppose you want the finished boarded area to be 3,000mm wide and your boards finish at approximately 120mm.
If you use 24 rows:
- 24 boards × 120mm = 2,880mm of timber
- 3,000mm − 2,880mm = 120mm left for the gaps
- 24 boards create 23 internal gaps
- 120mm ÷ 23 = approximately 5.2mm per gap
A consistent gap of about 5.2mm sits inside the TDCA's 5–8mm recommendation, so the boards can finish neatly across the 3m width without automatically ripping a narrow strip from the final board.
This is why we recommend planning the frame around the finished board layout where possible, rather than building the frame first and then trying to make the boards fit it.
For more detail on choosing and maintaining the gap, see What Gap Between Decking Boards?.
Board direction changes the calculation
Imagine a deck measuring 4m × 3m. If the boards run along the 4m direction, the 3m dimension determines the number of rows. If the boards run along the 3m direction, the 4m dimension determines the number of rows.
Decide the board direction before ordering. Turning the boards through 90 degrees can change the number of rows, the board lengths you need, the number of joints and the amount of offcut.
Rows are not always the same as the number of physical boards
If every row can be made from one full-length board, the row count and physical board count may be the same. If the deck is longer than the boards you are using, each row may need two or more pieces.
A useful way to separate the calculation is:
- Across the deck: calculate how many rows you need.
- Along the deck: calculate the timber length required for each row.
- Then: plan those lengths around the stock lengths available to reduce unnecessary joints and offcuts.
Our treated decking boards are currently available in 2.4m, 3.0m, 3.6m, 3.9m, 4.2m and 4.8m lengths.
Do not automatically add 10% waste
A flat waste percentage is convenient, but it can hide a poor layout. Before adding extra boards, work out where your cuts fall and whether offcuts from one row can be reused elsewhere.
A simple rectangular deck using a suitable board length may create very little cutting waste. Borders, angled boards, short returns and awkward shapes can create much more. Plan the cuts first, then add a sensible allowance for the actual job.
What about borders or picture-frame decking?
Calculate border boards separately. Work out how much space the border occupies, subtract that from the area being filled, then calculate the internal decking rows using the remaining dimension.
The border can also change where the joists or noggins need to sit, so decide on it before finalising the frame.
What about L-shaped or awkward decking?
Split the deck into simple sections and calculate each section separately. Keep the board direction consistent unless the design deliberately changes it, and take care not to count an overlapping section twice.
For diagonal or heavily patterned layouts, a simple width calculation is no longer enough to predict the physical board count accurately. Draw the board layout and plan the cuts so you can see where full boards and reusable offcuts will go.
Want the quick answer instead?
If you are building a normal rectangular deck and simply want the materials worked out for you, use our Decking Calculator. It can calculate decking boards, framing timber and screws together using the current products and prices.
This guide is more useful when you want to understand the maths yourself, alter the board spacing to land neatly on a finished size, or plan a layout the calculator does not cover.
Related decking guidance
- What Gap Between Decking Boards?
- Which Way Up Should Decking Boards Go?
- Decking Joist Calculator
- Shop Timber Decking
Technical spacing guidance referenced from the Timber Decking and Cladding Association. Always follow any product-specific installation instructions supplied by the manufacturer.
