How to Use the Decking Calculator

Enter the length and width of the deck, then enter the size of the decking boards you plan to use.

You can also enter the gap between boards, a waste allowance, joist spacing and the number of fasteners used at each board-to-joist connection.

The calculator estimates:

• Total deck area;
• Number of board rows;
• Pieces required per row;
• Total decking boards before waste;
• Total decking boards including waste;
• Linear metres of decking material;
• Approximate number of joists;
• Approximate number of decking screws or fasteners;
• Estimated decking-board cost; and
• Estimated cost per square metre.

How Do I Calculate How Much Decking I Need?

For a simple rectangular deck, the first step is calculating the total area.

However, calculating the number of boards also requires the width of each board, the gap between boards and the length of each board.

Decking Calculator Example

Does the Gap Between Decking Boards Matter?

Yes. The gap affects how much width each row of decking effectively covers.

For example:

A larger gap means fewer board rows may be required across the same width, although the correct gap should be based on the decking product, manufacturer instructions and installation conditions.

How Much Waste Should I Allow for Decking?

A waste allowance helps account for:

• Cutting;
• Board defects;
• End trimming;
• Offcuts;
• Pattern changes;
• Board joins; and
• Installation mistakes.

For a straightforward rectangular deck, a common planning allowance may be around 5% to 10%.

More complex layouts can require a larger allowance.

How Are Decking Boards Calculated?

The calculator determines the number of board rows across the deck using:

It then estimates the number of board pieces required along each row:

Both results are rounded up because partial rows or pieces generally require a full board.

What Are Linear Metres of Decking?

Linear metres describe the total length of decking boards purchased.

For example, 50 boards that are each 5.4 metres long equal:

50 × 5.4 = 270 linear metres

Board Direction and Decking Quantity

The direction in which the decking boards run can affect how many board pieces are required.

This calculator lets you choose:

• Boards running along the deck length; or
• Boards running along the deck width.

For some deck dimensions, changing board direction can reduce joins and waste.

What Is Joist Spacing?

Joists are structural members that support decking boards. Joist spacing is typically measured from centre to centre.

The required spacing depends on factors including:

• Decking material;
• Board thickness;
• Board profile;
• Load requirements;
• Manufacturer specifications; and
• Applicable building requirements.

The calculator uses the entered spacing only to produce an approximate joist count.

How Is the Number of Joists Estimated?

If boards run along the length of the deck, joists typically run across the board direction.

A simplified estimate is:

The final structural design may require additional boundary joists, double joists, blocking, beams or other framing.

How Many Screws Do I Need for Decking?

The number of fasteners depends on the number of boards, joist intersections and fastening system.

For face-fixed decking, two screws at each board-to-joist intersection are commonly used in many installations, but requirements vary.

Timber Decking vs Composite Decking

The same basic quantity calculator can be used for timber and composite decking, but installation requirements can differ.

Timber decking

Timber boards may require specific gaps, coatings, screws and treatment depending on the timber species.

Composite decking

Composite decking may use hidden clips, proprietary fasteners and specific joist-spacing requirements.

Always check the manufacturer’s installation instructions.

Common Decking Board Widths

Decking boards are available in many sizes. Common widths can include approximately:

• 86 mm;
• 90 mm;
• 135 mm;
• 137 mm;
• 140 mm; and
• other manufacturer-specific profiles.

Enter the actual board width you intend to purchase for a more useful estimate.

Common Decking Board Lengths

Available lengths vary by timber species, supplier and decking system. Typical lengths may include:

• 2.4 metres;
• 3.0 metres;
• 3.6 metres;
• 4.2 metres;
• 4.8 metres;
• 5.4 metres; and
• 6.0 metres.

The comparison table above helps show how changing board length can affect the quantity of boards purchased.

Should Decking Boards Run Lengthways or Across the Deck?

There is no single answer for every deck.

Board direction can depend on:

• Appearance;
• Available board lengths;
• Joist direction;
• Drainage;
• Entry direction;
• Number of board joins; and
• Material efficiency.

Try both directions in the calculator to compare board quantities.

How Much Does Decking Cost Per Square Metre?

This calculator estimates board and fastener cost per square metre using the prices entered.

This is not a complete installed-deck cost.

What Costs Are Not Included?

Unless manually accounted for elsewhere, this calculator does not include:

• Bearers;
• Structural joists;
• Posts;
• Concrete footings;
• Post stirrups;
• Brackets;
• Balustrades;
• Stairs;
• Decking oil or coating;
• Delivery;
• Labour;
• Building approval; or
• Site preparation.

Decking Calculator Assumptions

This calculator assumes:

• The deck is rectangular;
• Boards are installed in straight parallel rows;
• The entered board width excludes the gap;
• The entered waste allowance applies to board quantity;
• Board pieces are rounded up to whole boards;
• Joists are evenly spaced;
• The fastener estimate is approximate;
• No picture-frame border is separately calculated;
• No diagonal or herringbone pattern is modelled;
• No structural engineering calculation is performed; and
• Results are material estimates rather than construction specifications.

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