Decking Calculator

Deck boards and joists needed for your deck area.

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About this calculator

Deck boards are bought by the board, not the square metre, and the count depends on board size, expansion gaps and cutting waste. This calculator converts your deck area into boards, estimates the joists beneath at 400mm centres, and even counts the screws.

The formula

Board coverage = Board length × (Board width + 5mm gap) Boards = Deck area ÷ Coverage × (1 + Waste% ÷ 100) Joists = Deck width ÷ 0.4 + 1

Worked example

A 4m × 3m deck with 3.6m × 140mm boards, 10% waste: area 12 m², coverage 0.522 m² per board → 26 boards, 9 joists.

Common mistakes

  • Laying boards parallel to the joists — they must run across them.
  • Skipping the 5mm gap, which traps water and warps boards.
  • Building straight onto grass — joists need supports and a weed membrane below.

Frequently asked questions

400mm centres for standard grooved softwood boards; composite boards often require 300mm — check the manufacturer.

Two per board per joist — it adds up fast, which is why the calculator estimates ~30 per board.

In the UK, usually not if it's under 30cm high and covers less than half the garden — check for conservation areas.

Assumptions & limitations

Rectangular deck, boards laid straight with 5mm gaps; framework timber (beyond joist count) not itemised.

Sources & verification

  • Manufacturer board dimensions and spacing guidance — Board coverage depends on board width and the expansion gap the manufacturer specifies

All sources last accessed 24 July 2026.

Formula last verified: 24 July 2026
Maintained by: Steve Davis, Founder & Editor — see our editorial standards

Spotted an error or an out-of-date figure? Report it — corrections are prioritised over all other work (see our corrections policy).

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