Deck Joist Spacing Calculator

Calculate the number of deck joists, spacing and total joist length for your decking frame.

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How to Use This Calculator

How to Use

  1. Enter the overall deck length and width.
  2. Choose the joist spacing — 400 mm (16") is standard for timber decking, 600 mm (24") for lighter loads.
  3. Add a waste percentage if needed.

Tips

  • Closer joist spacing gives a stiffer, stronger deck.
  • For boards laid diagonally, reduce joist spacing to around 300 mm (12").
  • Always check span tables for your joist size and timber grade.

Common Questions

What spacing should deck joists be?

400 mm (16 inches) on centre is the most common spacing for timber and composite decking. For diagonal board layouts or heavier loads, reduce to 300 mm (12 inches). 600 mm (24 inches) may be acceptable for lighter-duty decks — always check the board manufacturer and span tables.

How do I work out how many deck joists I need?

Divide the deck length by the joist spacing and add one for the starting joist. This calculator does it automatically and includes outer (rim) joists and blocking.

Do I need blocking between deck joists?

Yes — blocking (noggins) between joists prevents twisting and adds rigidity, especially on longer spans. A row at mid-span is a good minimum.

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