Decking Calculator
Board rows, linear feet, joists and screws for a deck. Board widths are the actual ones, not nominal — a "2x6" is 5.5 inches, and working from six leaves you short.
Decking needed
458linear feet
- Board rows
- 26
- Linear feet
- 458
- Deck area
- 192 sq ft
- Joists
- 13
- Joist spacing
- 16 in
- Screws
- 700
- Includes a 10% waste allowance. The bare figure is 416.00 linear feet.
- Board rows assume boards run the length of the deck. Laying them the other way swaps the two dimensions and usually changes the count.
- Nominal board sizes are not actual: a "6 inch" deck board is 5.5 inches wide. The default here is the actual width.
- Composite decking often requires 12 inch joist spacing where timber allows 16. Check the manufacturer's instructions, since getting it wrong voids the warranty.
About the decking calculator
Decking is bought by the linear foot, so the calculation is how many rows of board fit across the deck multiplied by how long each row is. The number that catches people out is board width: timber is sold by nominal size, and a nominal 2x6 deck board is actually 5.5 inches wide. Work from six and you end up about nine per cent short, which on a twelve-foot width is two whole rows missing.
The gap between boards matters too. A quarter inch is normal for drainage and to let timber move as it dries, and once you add it each board occupies 5.75 inches rather than 5.5.
Joists run at right angles to the boards. Sixteen-inch centres is standard for timber, but most composite decking requires twelve, and fitting it at sixteen usually voids the warranty.
What it works out
- Linear feet of decking, with waste
- Board rows and the deck area
- Joist count at 12, 16 or 24 inch centres
- Screws, at two per board per joist
The formula
Rows = Width ÷ (Board width + Gap) Linear feet = Rows × Length
Decking is bought by the linear foot, so the calculation is how many rows of board fit across the deck, multiplied by how long each row is.
The number that catches people out is board width. Timber is sold by nominal size, and a nominal "2x6" deck board is actually 5.5 inches wide, not six. Work from six and you end up about nine per cent short — on a twelve-foot width that is two whole rows missing.
The gap between boards matters too. A quarter inch is normal for drainage and to let the timber move as it dries, and once you add it each board occupies 5.75 inches rather than 5.5. Composite boards often specify their own gap, which can differ at each end of the temperature range.
Joists run at right angles to the boards. Sixteen-inch centres is standard for timber, but most composite decking requires twelve, and fitting it at sixteen usually voids the warranty.
- Length
- The direction the boards run. Swapping length and width changes the board count.
- Width
- The direction across which boards are laid side by side.
- Board width
- Actual, not nominal. A 2x6 board is 5.5 inches.
- Gap
- An eighth to a quarter inch for drainage and movement.
A worked example
A sixteen by twelve foot deck in 5.5-inch boards with a quarter-inch gap, joists at sixteen inches.
That works out to 458 linear feet.
- Board rows
- 26
- Linear feet
- 458
- Deck area
- 192 sq ft
- Joists
- 13
- Joist spacing
- 16 in
- Screws
- 700
Questions
How many deck boards do I need for a 16x12 deck?
About 26 rows of board across the twelve-foot width, using 5.5-inch boards with a quarter-inch gap. At sixteen feet a row that is 416 linear feet, and around 458 once you add ten per cent for waste.
Is a 2x6 deck board really six inches wide?
No — it measures 5.5 inches. Nominal timber sizes refer to the rough-sawn dimension before it is dried and planed. Calculating from six inches leaves you about nine per cent short of boards.
How far apart should deck joists be?
Sixteen inches on centre is standard for timber decking laid square to the joists. Drop to twelve for a diagonal lay, and for most composite decking, which is less stiff than timber and sags at wider spacing.
What gap should I leave between deck boards?
An eighth to a quarter inch, for drainage and to let the boards move. Pressure-treated timber is usually wet when you buy it and shrinks as it dries, so it can be laid tighter; kiln-dried and composite boards need the full gap from the start.
How many screws do I need for a deck?
Two per board at every joist crossing. A 16x12 deck at sixteen-inch joist spacing works out around 700 screws, so buy a five-pound box and expect to open a second.
Should boards run the length or the width?
Either, but it changes the material list, so enter whichever direction they actually run as the length here. Running boards the long way means fewer, longer rows and fewer butt joints, which usually looks better.