How the decking calculation works

The deck area is divided by the effective board coverage width, which includes the selected gap, to estimate total linear metres. That is converted to full board lengths and a waste allowance is added.

Board spacing

Suitable gaps depend on the decking material, moisture content and manufacturer instructions. Composite and timber products can have different spacing requirements.

This calculator estimates deck-board quantities only. Joists, beams, posts, fixings, structural design and support spacing are not included.

Frequently asked questions

Does board direction matter?

Yes. The area method gives a useful purchasing estimate, but exact board count can change depending on which way boards run and where cuts fall.

Can I use this for composite decking?

Yes, provided you enter the actual board width, board length and the installation gap specified by the manufacturer.

Planning a decking order

The board count is most useful when the board size entered matches the product you actually intend to buy. Measure the deck at finished-board level and decide which direction the boards will run before relying on the result. A 4 m by 3 m deck can produce a different cutting plan depending on whether boards run along the 4 m side or the 3 m side, even though the surface area is identical.

Allow for cuts, joins and full board lengths

The calculator converts the deck area into total linear metres using the board width and the gap between boards, then rounds that requirement into complete board lengths. The waste setting is intended to cover ordinary trimming and unusable offcuts. Diagonal layouts, picture-frame borders, steps and complicated shapes generally need a larger allowance because more boards are cut.

What to check before buying

Use the manufacturer's installed board width and gap rather than a nominal dimension where those differ. Confirm the permitted joist spacing, end-gap requirements and fixing system separately: this tool estimates deck boards only and does not size the supporting structure. For timber, moisture movement can affect the required gap; for composite products, follow the brand's installation instructions and expansion clearances.