Free tool · Decks
Decking calculator
Enter the size of the deck and the board you plan to lay. The calculator works out how many boards go across, the lineal metres to order with a cutting allowance, the joists at your centres and the screws.
Your deck
The take-off updates as you type. Boards run along the length.
Deck size
Length is the direction the boards run. Measure the finished deck, not the yard.
The common Australian sizes. Composite boards vary by brand, so check the width on the board you are buying.
Hardwood is usually laid with a 3 mm to 5 mm gap so water drains and the boards can move. Composite uses the gap its maker specifies.
450 mm centres are common for 19 mm hardwood boards. Composite span tables often call for closer joists.
A material take-off for a rectangular deck. It counts boards, metres, joists and screws. It does not size joists, bearers, posts or footings, which come from the span tables or an engineer, and it does not replace a building certifier.
Your take-off
- Board rows
- 34boards 86 mm wide, 4 mm gaps
- Decking to order
- 150m136 m laid, plus 10% for cuts
- Joists
- 10at 450 mm centres, 33 m of joist
- Screws
- 750two per board per joist, plus 10%
- Deck area 12 m², boards running along the 4 m side.
- 34 boards with 4 mm gaps cover 3,056 mm, 56 mm more than the deck is wide. Rip the last board down, or open the gaps slightly to land on the edge.
- Composite boards need the gaps and joist centres from the maker's own span table, which may differ from the figures here.
How the calculator works
It lays the boards along the length of the deck and counts how many rows fit across the width at the board width and gap you set, rounding up so the last board can be ripped to the edge. The lineal metres are the rows multiplied by the length, plus 10% for cuts and the odd bad board. Joists run across the width at the centres you choose, with one at each end of the deck, and screws are counted at two per board per joist.
- Board rows = deck width divided by board width plus gap, rounded up
- Decking to order = rows times length, plus 10%, rounded up to the half metre
- Joists = length divided by the centres, rounded up, plus one
- Screws = rows times joists times two, plus 10%
Before you order
- Decking is sold in set lengths. A deck longer than the boards you can buy will have end joins, and every join needs a double joist or a wide joist under it, so add those to the count.
- Wide boards cup more than narrow ones. Many suppliers lay 135 mm and wider boards on closer joist centres than 86 mm boards.
- Composite boards have their own span tables, end gaps and clips. Follow the brand's figures, because the warranty depends on them.
- Hardwood moves with the seasons. The gap you lay in summer closes in winter, which is why the 3 mm to 5 mm range exists.
What the numbers do not cover
- Joist, bearer, post and footing sizes, which come from the AS 1684 span tables or an engineer
- Decks that are not rectangles: split an L-shape into two rectangles and run each
- Balustrades and stairs, which have their own rules; the stair calculator sets a flight out to the code
- Whether your deck needs building approval, which our approval guide answers job by job
Weighing up the board itself? Our timber versus composite guide sets the two out side by side. If you would rather have the deck measured and built, see our deck building service in Brisbane.
Straight answers
Decking take-off questions.
How many decking boards do I need?
Divide the deck width by the board width plus the gap and round up. A 3 m wide deck in 86 mm boards with 4 mm gaps takes 34 rows. Multiply the rows by the deck length for the lineal metres, then add about 10% for cuts.
What gap should I leave between decking boards?
Hardwood is usually laid with a 3 mm to 5 mm gap so water drains and the boards can swell in wet weather without buckling. Composite boards use the gap the manufacturer specifies, which also allows for movement with heat.
How far apart should deck joists be?
450 mm centres are common for 19 mm hardwood decking. Wider boards and composite boards often need closer centres, and the span table for the board you are buying is the figure to follow. The joists themselves are sized from the AS 1684 span tables for their span and timber.
Which way should the boards run?
Most decks run the boards parallel to the house, across the joists, which run out from the house. Running boards along the long side keeps the number of end joins down. Whichever way they run, each board needs a joist under every join and at both ends.
Get a quote
Rather have it built?
A call or an email books a walk-through. Ben measures the space on site, talks through hardwood or composite and gives you a clear written quote.
- Phone
- 0430 705 916
- Hours
- Mon to Thu 6:30am to 8pm
Fri and Sat 6:30am to 4:30pm
Sun closed - Licence
- QBCC 15357497