Free tool
Decking calculator
Deck size in, lineal metres and joist count out.
Decking is sold by the lineal metre, not the square metre, so the first thing to work out is how many rows of board the deck takes and how long each row is.
The sums
Rows are the deck width in millimetres divided by the board width plus the gap. A 4m wide deck in 90mm boards at 4mm gaps is 4000 divided by 94, which is 43 rows.
Lineal metres are the rows times the deck length. Forty three rows across a 6m deck is 258 lineal metres before waste.
Joists are the deck length divided by the joist spacing, plus one. Same off by one as fence posts, same reason.
Waste
Ten per cent is the usual allowance and it is not generosity. Boards come in fixed lengths, the offcut from a 5.4m board on a 4m deck is not always usable, and you will reject some for splits, wane and bad grain. On a deck with angles or a wrapped edge, allow more.
Joist spacing is set by the board, not by preference
Most 19mm hardwood decking is rated to 450mm joist centres. Thinner boards, softwood and most composite decking want 400mm or sometimes less, and getting this wrong shows up as bounce underfoot that cannot be fixed without lifting the deck.
Check what the board manufacturer specifies before you set out the frame, because the frame is what you cannot change afterwards.
Gaps
Three to five millimetres suits seasoned hardwood, which is roughly stable. Unseasoned timber is laid tight or nearly tight because it shrinks as it dries and the gap opens up on its own. Composite has its own spec and it is usually wider than timber, for expansion rather than shrinkage.
Gaps matter for drainage as much as appearance. A deck laid with no gap holds water and rots from the top down.
What this does not cover
Bearers, posts, footings, stairs, balustrade and fixings. Anything raised more than a nominated height above ground triggers building approval requirements in both states, and a deck attached to the house is structural work. That is a conversation with the council or a private surveyor before it is a materials list.
Post footing concrete is on the concrete calculator.
Common questions
How many lineal metres of decking do I need?
Divide the deck width in millimetres by the board width plus the gap to get the number of rows, then multiply by the deck length. A 6m by 4m deck in 90mm boards at 4mm gaps is about 43 rows, so 258 lineal metres before waste.
How far apart should decking joists be?
It is set by the board, not by preference. Most 19mm hardwood decking is rated to 450mm centres, while thinner boards, softwood and most composite want 400mm or less. Check the manufacturer specification before you build the frame, because the frame is the part you cannot change later.
What gap should I leave between decking boards?
3 to 5mm for seasoned hardwood. Unseasoned timber is laid tight or nearly tight because it shrinks as it dries. Composite has its own specified gap, usually wider, for expansion. Some gap is essential for drainage regardless.
How much waste should I allow on decking?
Ten per cent for a straightforward rectangular deck. Boards come in fixed lengths so offcuts are not always reusable, and you will reject some for splits and bad grain. Allow more for angles, a wrapped edge or a complicated shape.
Do I need approval to build a deck?
Often, yes. Both New South Wales and Victoria trigger building approval requirements above a nominated height above ground, and a deck attached to the house is structural work. Check with the council or a private building surveyor before ordering materials.