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Decking calculator

How many boards, joists and screws your deck takes, from its size, the board you are buying, the gap and the joist spacing. Waste included, and one tap puts it on a quote.

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The answer

Boards to buy 42 boards
Lineal metres of board, with waste 225.7 m
Rows of board across the deck 38 rows
Joists 13 joists
Screws 1,087 screws
Deck area 19.44 m²
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Three decks, worked out

Rows = (deck width + one gap) / (board width + gap), rounded up. Lineal metres = rows x deck length, plus waste. Boards = lineal metres / board length, rounded up. Joists = deck length / spacing, rounded down, plus one. Screws = rows x joists x 2, plus waste. The boards run along the deck's length.

A standard deck

5.4 by 3.6 m, 90 mm boards, 5 mm gap, joists at 450 mm
Deck length, the way the boards run 5.4 m
Deck width 3.6 m
Board width 90 mm
Gap between boards 5 mm
Board length you are buying 5.4 m
Joist spacing 450 mm
Extra for offcuts 10 %
Lineal metres of board, with waste 225.7 m
Rows of board across the deck 38 rows
Joists 13 joists
Screws 1,087 screws
Deck area 19.44 m²
Boards to buy 42 boards

Wide boards

6 by 4 m, 140 mm boards in 6 m lengths
Deck length, the way the boards run 6 m
Deck width 4 m
Board width 140 mm
Gap between boards 5 mm
Board length you are buying 6 m
Joist spacing 450 mm
Extra for offcuts 10 %
Lineal metres of board, with waste 184.8 m
Rows of board across the deck 28 rows
Joists 14 joists
Screws 863 screws
Deck area 24.00 m²
Boards to buy 31 boards

Wider boards mean fewer rows and fewer screws, at the cost of more cupping on some timbers.

A small landing

2.4 by 1.8 m, 2.4 m lengths, joists at 400 mm
Deck length, the way the boards run 2.4 m
Deck width 1.8 m
Board width 90 mm
Gap between boards 5 mm
Board length you are buying 2.4 m
Joist spacing 400 mm
Extra for offcuts 10 %
Lineal metres of board, with waste 50.2 m
Rows of board across the deck 19 rows
Joists 7 joists
Screws 293 screws
Deck area 4.32 m²
Boards to buy 21 boards

Decking questions#

How many decking boards do I need?
Work out the rows across the deck: the deck width divided by the board width plus the gap, rounded up. Multiply by the deck length for lineal metres, add 10% for offcuts, and divide by the length you are buying. A 5.4 by 3.6 m deck in 90 mm boards with 5 mm gaps is 38 rows and 42 boards of 5.4 m.
How many lineal metres of decking per square metre?
About 10.5 m per square metre for 90 mm boards with a 5 mm gap (1,000 / 95), and about 6.9 m for 140 mm boards. Add your waste on top.
How far apart should joists be?
It depends on the board's thickness and species and on the board maker's or supplier's span tables. 450 mm is a common spacing for 19 mm hardwood; composite boards often want closer. Check the board you are buying.
What gap between decking boards?
Commonly 3 to 5 mm, depending on the timber and how dry it is when it goes down. Set the gap box to suit and the board count follows.
How many screws?
Two at every place a board crosses a joist, plus waste. The calculator counts rows times joists times two.