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

This calculator works out the timber you need to build a deck, so you can order the right amount of decking boards and joists before you start. Enter your deck length and width in metres, the width of the boards you plan to use in millimetres, the gap you want between boards, the length of the boards you intend to buy, and your joist spacing in millimetres. From these figures it returns the rows of decking needed to cover the width, the total lineal metres of decking required, the number of boards to buy once board length is accounted for, and the number of joists needed to support the deck at your chosen spacing. It works by dividing the deck width by the board width plus the gap to get the rows, multiplying by the deck length for the lineal metres, then dividing by your board length for the boards, while joists come from the deck length divided by the spacing, plus one. Use it before ordering timber so you buy the right quantity rather than guessing and running short. A gap of 3 to 5 mm between boards is typical for drainage and movement, and joist spacing is usually 400 to 450 mm depending on board thickness. This is a materials estimate only, so check span tables and the building code, and add extra for waste and cuts.

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Deck length (m)

Deck width (m)

Board width (mm)

Gap (mm)

Board length (m)

Joist spacing (mm)

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 13 joists.

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