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Generated on: July 20, 2026 at 09:33 PM

Deck Boards Calculator

Inputs

Deck area20 mΒ²
Board width14 cm
Board length3.6 m
Board gap5 mm
Joist spacing40 cm on-center
Waste allowance10 %

This is an estimate based on rectangular coverage and a simple two-screws-per-joist rule. Actual quantities vary with deck shape, board layout (straight, diagonal, or picture-frame), trimming losses, and the fastener system used. Confirm against your cut plan and the fastener manufacturer instructions, and buy a little extra to allow for defects and offcuts.

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Construction & DIY

Deck Boards Calculator

Estimate how many decking boards and fasteners a deck needs from the deck area, board size, board gap, and joist spacing, with a waste allowance.

Inputs

β‰₯ 0 mΒ²
β‰₯ 0.1 cm
β‰₯ 0 m
%
0 – 50 %

Results

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Details

A 20 mΒ² deck built with 3.6 m Γ— 14 cm boards and 5 mm gaps needs about ... boards (... total) and roughly ... fasteners, including 10 % waste.

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Deck Boards

Deck boards are the surface planks of a deck, and the number a project needs is set by the deck area, the width and length of each board, the gap left between boards, and the joist spacing that the boards are screwed to. This calculator estimates the board count, the total lineal length to buy, and the number of fasteners required.

Board count from coverage

Each board covers a strip of the deck equal to its length multiplied by its width plus the gap on one side, because the spacing repeats across every row. Dividing the deck area by that effective strip and rounding up gives the number of boards, with a waste allowance for cuts and defects:

N=⌈A (1+k)L (w+g)βŒ‰\begin{aligned} N &= \left\lceil \frac{A \,(1 + k)}{L \,(w + g)} \right\rceil \end{aligned}

where AA is the deck area, LL and ww are the board length and width, gg is the gap, and kk is the waste factor. Adding the gap to the width is what keeps the count honest: a board claims its own width plus one gap per row, so omitting the gap would overstate each board's coverage. The total lineal length is the board count multiplied by the board length.

Fasteners

The common practice is to drive two screws wherever a board crosses a joist. The number of crossings per board is the board length divided by the joist on-center spacing, plus one for the end joist. Joist spacing is quoted in inches (12, 16, or 24 in on-center), converted to metres with 1Β in=0.0254Β m1\ \text{in} = 0.0254\ \text{m}:

F=⌈Nβ‹…2(Lsβ‹…0.0254+1)βŒ‰\begin{aligned} F &= \left\lceil N \cdot 2 \left( \frac{L}{s \cdot 0.0254} + 1 \right) \right\rceil \end{aligned}

Worked example

Consider a 20 mΒ² deck built with 3.6 m boards that are 14 cm wide, set with 5 mm gaps over joists at 16 in on-center, with a 10% waste allowance:

N=⌈20β‹…1.103.6β‹…(0.14+0.005)βŒ‰=⌈42.15βŒ‰=43F=⌈43β‹…2(3.616β‹…0.0254+1)βŒ‰=848\begin{aligned} N &= \left\lceil \frac{20 \cdot 1.10}{3.6 \cdot (0.14 + 0.005)} \right\rceil = \lceil 42.15 \rceil = 43 \\ F &= \left\lceil 43 \cdot 2 \left( \frac{3.6}{16 \cdot 0.0254} + 1 \right) \right\rceil = 848 \end{aligned}

That is 43 boards totalling 154.8 m of stock, fastened with about 850 screws.

Consider a 200 sq ft deck built with 12 ft boards that are 5.5 in wide, set with 1/4 in gaps over joists at 16 in on-center, with a 10% waste allowance:

N=⌈200β‹…1.1012β‹…5.5+0.2512βŒ‰=⌈38.3βŒ‰=39F=⌈39β‹…2(1216/12+1)βŒ‰=780\begin{aligned} N &= \left\lceil \frac{200 \cdot 1.10}{12 \cdot \frac{5.5 + 0.25}{12}} \right\rceil = \lceil 38.3 \rceil = 39 \\ F &= \left\lceil 39 \cdot 2 \left( \frac{12}{16/12} + 1 \right) \right\rceil = 780 \end{aligned}

That is 39 boards totalling 468 ft of stock, fastened with about 780 screws.

Hidden-clip systems change the fastener count, and diagonal or picture-frame layouts raise the board waste. For working out the deck area in the first place, see Flooring Calculator.

Frequently Asked Questions (FAQ)

How many deck boards do I need?

The board count is the deck area divided by the area one board covers, including its gap, then rounded up and increased for waste. As a worked example, a 20 mΒ² deck built with 3.6 m boards that are 14 cm wide and set with 5 mm gaps needs about 43 boards once a 10% waste allowance is added. Wider boards and longer stock both reduce the count.

The board count is the deck area divided by the area one board covers, including its gap, then rounded up and increased for waste. As a worked example, a 215 sq ft deck built with 12 ft boards that are 5.5 in wide and set with 3/16 in gaps needs about 43 boards once a 10% waste allowance is added. Wider boards and longer stock both reduce the count.

Why is the gap added to the board width?

Each board occupies its own width plus the gap on one side, because the spacing repeats across the deck. A 14 cm board with a 5 mm gap effectively claims 14.5 cm of width per row. Ignoring the gap underestimates the coverage of each board and overestimates how many boards are required.

Each board occupies its own width plus the gap on one side, because the spacing repeats across the deck. A 5.5 in board with a 3/16 in gap effectively claims 5.7 in of width per row. Ignoring the gap underestimates the coverage of each board and overestimates how many boards are required.

How many screws does decking need?

The common standard is two screws at every point where a board crosses a joist. A board crossing eight joists therefore takes sixteen screws. With 0.41 m joist spacing, a 3.6 m board crosses roughly nine joists, so a 43-board deck needs on the order of 850 screws. Hidden-fastener clip systems use a different count set by the clip manufacturer.

The common standard is two screws at every point where a board crosses a joist. A board crossing eight joists therefore takes sixteen screws. With 16 in joist spacing, a 12 ft board crosses roughly nine joists, so a 43-board deck needs on the order of 850 screws. Hidden-fastener clip systems use a different count set by the clip manufacturer.

Disclaimer

This is an estimate based on rectangular coverage and a simple two-screws-per-joist rule. Actual quantities vary with deck shape, board layout (straight, diagonal, or picture-frame), trimming losses, and the fastener system used. Confirm against your cut plan and the fastener manufacturer instructions, and buy a little extra to allow for defects and offcuts.

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