Deck Board Calculator
About Deck Board Calculation
This Deck Board Calculator helps you work out how many decking boards you need, how many rows will fit, and the total board and joist lengths based on your deck area, board size, gaps and pattern. Enter dimensions in millimetres and choose a straight or 45° diagonal layout to see material totals with the built-in waste allowance and optional extra reserve.
The tool also estimates costs (per m², per metre, or per piece) and shows results such as last-row width, rows of joists, and joist length with allowance. Use it as a practical decking calculator or deck size calculator for planning a new build or a refurbishment, all in the metric system.
Tips and Tricks
- Start with clean dimensions. Measure deck length and width in mm. Use Add area or Subtract area for L-shapes, steps, or voids so the net area in m² is correct.
- Choose the pattern wisely. Straight laying minimises cuts; diagonal decking looks great but needs more material. The calculator already includes a base waste for each pattern (see table below). You can add your own extra allowance when matching patterns or buying a reserve.
- Row orientation. Try both “Along length” and “Along width” and check the Last row width. Aim for a last strip that is not too narrow to simplify fixing and appearance.
- Board size and gaps. Common board widths are 90-150 mm; many outdoor decks use around 120-145 mm. Typical gap between boards is 4-7 mm and the recommended end gap at butt joints is 2-5 mm to allow for movement. Keep all inputs in millimetres.
- Joists. A joist spacing of about 400 mm is often used for residential decks; thicker or stiffer boards may allow slightly wider spacing—follow the product guidance. For diagonal layouts, effective spans change; the calculator accounts for this. To reduce offcuts, plan spacing as a multiple of the board length where possible.
- Materials and price modes. Whether you plan timber or composite, the deck boards calculator works the same. Set pricing per m², per metre, or per piece, and enter the joist price per metre to get a quick total.
- Standards. For structural design and detailing refer to European guidance such as EN 1995-1-1 (Eurocode 5) for timber structures and relevant product standards for composites. Always follow the manufacturer's installation instructions.
| Layout pattern | Base allowance already included |
|---|---|
| Straight | 5 % |
| Diagonal 45° | 12 % |
FAQs
How many deck boards do I need?
Enter deck dimensions, board length and width, and set the gaps. The calculator shows Boards required and Boards incl. allowance (which includes the base waste for the chosen pattern plus any extra reserve you add). This replaces manual “how many boards do I need” guesswork.
Does the decking calculator handle diagonal layouts?
Yes. Choose Diagonal 45° in Layout pattern. The tool increases allowance automatically (12 % base) and adjusts both board usage and joist lengths for the angled span, so your totals reflect real-world cutting and trimming.
What joist spacing should I enter?
Many decks use around 400 mm centre-to-centre. Heavier-duty or thicker boards may permit slightly wider spacing—always follow product data and local practice. The deck calculator also factors in the different effective spans that occur with diagonal boarding.
Can this deck calculator estimate cost?
Yes. Pick a price mode: Per m², Per m, or Per piece, then add the joist price per metre. You will see Board cost, Joist cost, and the Total so you can compare options quickly.
I'm used to a “deck square footage calculator”. How do I work in metric?
Measure in metres and millimetres only. Area is simply length × width and is reported in m². If the shape is not a simple rectangle, use Add area and Subtract area to build up the correct metric area before calculating boards and joists.