Decking Boards Calculator
Calculate how many decking boards you need, based on your deck's dimensions, board width and standard board length.
How to use the decking boards calculator
- Enter the deck length — the direction the boards will run along.
- Enter the deck width — the direction across the boards.
- Enter the board width (including the gap between boards) and the standard length of the product you plan to use.
How the board count is calculated
The calculator first finds how many rows of boards are needed across the deck, then works out the total number of boards based on standard board length:
Number of boards = (Number of rows × Deck length) ÷ Standard board length
The board width should include the gap between boards (usually 4-5 mm), since this affects how many rows are needed.
Timber that moves after it is laid
Decking boards are rarely fully dry when bought. Pressure-treated softwood is often delivered damp straight from treatment, and shrinks across the grain as it dries out over the first season. Lay such boards with a generous gap and the gaps can become uncomfortably wide within a year. Laid tight, the drying process opens up a suitable spacing on its own.
Dry material behaves the other way round. Hardwood decking and kiln-dried softwood take up moisture outdoors and swell slightly, so they need a gap from the outset — otherwise the boards can press against each other and buckle. The gap width you enter should therefore reflect how damp the material is when you lay it, rather than one fixed figure.
Frequently asked questions
How much gap should I leave between deck boards?
4-6 mm is common to allow the wood to move with moisture, and to let water and debris drain through. Include this gap in the board width you enter in the calculator.
Should I buy extra boards beyond what the calculator shows?
Yes, it's smart to add 5-10% extra for cutting, mistakes, and any future repair, especially if the deck has an irregular shape.