Calculate the number of deck joists, spacing and total joist length for your decking frame.
Enter the overall deck length and width and choose the joist spacing. The tool returns the number of joists and their total length. Closer spacing gives a stiffer, stronger deck; wider spacing uses less timber but needs thicker boards.
| Board layout | Spacing (on-centre) |
|---|---|
| Boards straight across | 16 in / 400 mm |
| Lighter loads / thick boards | 24 in / 600 mm |
| Diagonal / 45° boards | 12 in / 300 mm |
A deck 4 m wide with joists at 400 mm centres needs 4000 ÷ 400 + 1 = 11 joists. If it is 3 m long, that is 33 m of joist timber, plus a rim/edge joist around the perimeter.
Tip: “on-centre” (O.C.) means measured from the middle of one joist to the middle of the next — always set out from a fixed edge so cumulative error does not creep in across the deck.
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.
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.
Yes — blocking (noggins) between joists prevents twisting and adds rigidity, especially on longer spans. A row at mid-span is a good minimum.
It depends on timber size and grade — for example 47 × 150 mm joists at 400 mm centres can span roughly 2.5 m. Always check span tables for your timber.
Yes — use pressure-treated (tanalised) timber for the joists and frame, and brush end-grain preservative onto any cut ends to stop rot where the factory treatment has been broken.
