Decking Calculator
Boards, joists and fasteners — with the gap counted.
Work out Decking. Boards, joists and fasteners — with the gap counted. Names the misconception directly.
For drainage and expansion. 5-8 mm is typical for timber.
Including 10% waste
31 boards
28 runs of 6 m · 16 joists
The gap between boards is part of the coverage. A 140 mm board with a 5 mm gap covers 145 mm, and ignoring that over-orders by about 3% — which is where the spare bundle in the garage comes from. Gaps are not optional. Timber swells across the grain when wet, and boards laid tight will buckle. 5-8 mm is typical, and boards fitted in dry weather want the wider end of that range. Joist spacing depends on the board thickness and the direction of laying. 400 mm centres suit most decking; boards laid diagonally need closer spacing because the unsupported span between joists is longer.
How the Decking Calculator works
Enter deck dimensions, board width, gap and joist spacing for boards, joists and approximate fasteners. The gap between boards is part of the coverage, and ignoring it over-orders by about 3%.
Also known as: how many deck boards do i need · deck joist spacing calculator · decking material calculator · deck board spacing
Frequently asked questions
How many deck boards do I need?
Divide the deck width by the board width plus the gap. A 140 mm board with a 5 mm gap covers 145 mm, so a 4 m width needs 28 runs — not the 29 an ungapped calculation would suggest.
Why do deck boards need gaps?
For drainage and expansion. Timber swells across the grain when wet, and boards laid tight will buckle. 5-8 mm is typical, and boards fitted in dry weather want the wider end.
What joist spacing should I use?
400 mm centres suit most decking. Boards laid diagonally need closer spacing, often 300 mm, because the unsupported span between joists is longer at an angle.
How much waste should I allow?
Around 10% for a simple rectangle, more for cuts around posts or an irregular shape. Ordering the exact computed quantity is how a job stops halfway on a Sunday.
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