Decking Calculator

How the decking calculator works

The calculator divides the deck area by the area one board covers, adds waste, and rounds up to whole boards:

Boards = round up( deck area (m²) ÷ coverage per board (m²) × (1 + waste % ÷ 100) )

The key input is the coverage per board: the board's width plus one gap, multiplied by its length.

Worked example: a 4.8 m × 3.6 m deck with 140 mm boards

1. Deck area: 4.8 × 3.6 = 17.28 m²
2. Board 140 mm wide + 5 mm gap = 0.145 m cover width
3. Board length 4.8 m (one board runs the full length of the deck), so coverage per board: 0.145 × 4.8 = 0.696 m²
4. Waste 10%: 17.28 ÷ 0.696 × 1.10 = 27.3, rounded up to 28 boards

If your supplier sells by the lineal metre, divide the area by the cover width: 17.28 ÷ 0.145 = 119.2 m, plus 10% ≈ 131 lineal metres.

Plan the board layout

Where you can, choose board lengths that finish on a joist without too many short offcuts. Here, 4.8 m boards run the full length of the deck with no joins: 3.6 ÷ 0.145 = 25 rows of boards, and the other 3 boards are spares for defects and cuts. Staggering joins across different joists looks better and is stronger.

Joist spacing and fixings

Joist spacing depends on the board. Around 450 mm centres is common for 19 mm hardwood, while many composite boards need 400 mm or closer. Follow the board manufacturer's span table. Allow about two screws per board at each joist. For a deck like this, with joists at 450 mm, that is roughly 12 joists × 25 rows × 2 ≈ 600 screws, plus spares.

Common mistakes

• Forgetting the gap, which undercounts boards by a few per cent.
• Using the nominal board width (e.g., 140 mm) when the actual width is 138 mm.
• Ignoring picture framing or fascia boards around the edge. Add those separately.

How many decking boards per square metre?

With 140 mm boards and 5 mm gaps, about 6.9 lineal metres of board per square metre.

What gap should I leave between decking boards?

Typically 3–5 mm for hardwood, depending on the timber's moisture. Composites have their own spacing rules, so follow the manufacturer.

Do I need a permit for a deck?

Low decks are often exempt, but height, size and closeness to boundaries all matter. Check with your council or a private certifier.

Written by Sam Alex, civil engineer. About the author