How many decking boards do I need?

Divide the deck width by the board width plus the gap between boards to get the number of rows. Multiply by the deck length for the total lineal metres of decking. The calculator does this and also estimates the joists.

What gap should I leave between deck boards?

A gap of around 3 to 5 mm is common so water drains and the timber can move. Hardwoods and wet timber are laid tighter because they shrink as they dry. Set the gap to match your boards and conditions.

How far apart should joists be?

Joist spacing is usually 400 to 450 mm for timber decking, and sometimes less for thin or composite boards. Always follow the board manufacturer's span tables and the building code. The calculator uses the spacing you enter.

Decking Calculator

Estimate the timber for a deck. Enter the deck length and width, the board width and the gap between boards, the board lengths you will buy, and the joist spacing. The calculator shows the rows of decking, the total lineal metres, the number of boards, and the joists. Always check span tables and the building code.

Deck length (m)

Deck width (m)

Board width (mm)

Gap (mm)

Board length (m)

0 boards

decking boards to buy

Rows of decking0

Lineal metres0 m

Joists0

Estimate for boards running the length of the deck. Add a little for waste and cuts.

How it works

The number of rows is the deck width divided by the board width plus the gap. The total lineal metres is the rows times the deck length. Dividing by the length of the boards you buy gives the number of boards. The joists run across, spaced at the figure you enter, plus one to finish the run.

Worked example

A 5 by 4 metre deck with 140 mm boards and a 4 mm gap needs about 28 rows. That is 140 lineal metres of decking, or about 26 boards at 5.4 metres. With joists at 450 mm, you need around 12 joists.

Related calculators

Square Metre Calculator: deck area.
Fencing Calculator: posts, rails, and palings.
Concrete Calculator: concrete for footings.
Cubic Metre Calculator: work out a volume.

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