Deck Board Calculator
A 2×6 deck board is five and a half inches wide, not six. Divide your deck width by six and you order nine percent short — enough to stop the job with two rows left. Gaps push the other way: an eighth of an inch between twenty boards is two and a half inches, half a board’s worth. This calculator uses actual dimensions and real gaps, and shows both corrections so you can see where the difference came from.
- Deck boards needed
- 26 boards 5/4 × 6 (5.5" actual) across a 12 ft width with 0.125" gaps
- Why not width ÷ 6
- 24 vs 26 A "5/4x6" board is 5.5" wide, not 6". Using the nominal size under-orders by about 9%.
- Gap accumulation
- 3.1" 25 gaps at 0.125" — that is 0.6 board widths. Gaps are why you need fewer boards than bare width suggests.
- Linear feet
- 458 ft 416 ft plus 10% waste
- Pieces to buy
- 26 × 16 ft A 16 ft run fits a 16 ft board with no splice
- Joists
- 13 16 ft at 16" on centre, plus one
- Fasteners
- 676 screws Two per board per joist. Hidden fastener systems use roughly one clip per board per joist instead.
Two errors that pull in opposite directions
Deck board counts go wrong twice, and the two mistakes partly mask each other — which is why people keep making both.
Using nominal width under-orders. A 2×6 or 5/4×6 board is 5.5 inches wide, not 6. Dividing a 144 inch deck width by 6 gives 24 boards; dividing by 5.5 gives 27. That is a nine percent shortfall, and it appears as three missing rows at the end of the job.
Ignoring the gap over-orders. Each board plus its gap covers slightly more than the board alone, so the true count is 144 divided by 5.625, or 26. The gap gave one board back.
Make both errors and you land at 24 against a true 26 — still short, but by less than either error alone would suggest. This is why the mistake survives: the result is wrong in a way that is not obviously wrong until the boards run out.
How much gap, and when
The right gap depends on what the board will do after installation, and the two common materials do opposite things.
Pressure-treated lumber is usually sold wet, straight from the treatment cylinder, and it shrinks as it dries. Installed tight, it opens its own gap over the first few months — typically to about an eighth of an inch. Installing wet treated lumber with a gap produces a deck with quarter-inch gaps a year later, wide enough to lose things through.
Kiln-dried and naturally dry lumber has already shrunk. It needs an installed gap of about an eighth of an inch for drainage and to allow seasonal movement.
Composite moves with temperature rather than moisture, and it expands lengthwise more than most people expect. Manufacturers specify both side gaps and end gaps, and end gaps vary with the temperature at installation — larger if fitted on a cold day. Follow the specific instructions; this is the most common composite installation failure.
Ventilation underneath matters as much as the gap. A deck close to the ground with no airflow will cup and rot regardless of spacing.
Joist spacing is a decking decision
The spacing that carries the boards is set by the boards, not only by the structure.
Wood decking laid perpendicular to the joists is generally fine at 16 inches on centre. Laid diagonally, the effective span between supports increases by roughly forty percent, so 12 inch spacing is normal for diagonal decking.
Composite is less stiff than wood and many products require 16 inches perpendicular and 12 inches diagonal as a minimum, with some requiring 12 even for perpendicular installation. Installing composite on 24 inch joists produces the bouncy, wavy deck that gives the material its reputation, and it is an installation error rather than a product fault.
Stair treads are stricter again — many manufacturers require 12 inch spacing under composite treads regardless of the field spacing.
Check the decking manufacturer’s instructions before framing, not after. Adding joists to an existing frame is far more work than spacing them correctly at the start.
What this is based on
- Nominal versus actual dimensions for standard decking lumber
- Common gap practice for wet-treated, dry and composite decking
A material takeoff estimate. It does not size joists, beams or posts, which depend on span and load under the IRC deck provisions. Follow the decking manufacturer’s spacing and fastening instructions.
Frequently asked questions
Why is a 2×6 only 5.5 inches wide?
Nominal sizes describe the rough-sawn dimension before drying and planing. A 2×6 leaves the mill at roughly two by six inches and finishes at one and a half by five and a half. Every calculation that matters uses the actual size.
How big should the gap be?
For pressure-treated lumber bought wet, install tight — it shrinks as it dries and opens its own gap. For dry or kiln-dried wood, an eighth of an inch is standard. Composite decking moves with temperature rather than moisture, so follow the manufacturer’s figure, often three sixteenths or more.
Which direction should boards run?
Perpendicular to the joists, always. Beyond that, running them the long way looks better but needs splices on decks over 20 feet; running them the short way avoids splices at the cost of more boards. Diagonal decking adds roughly 15% waste and requires closer joist spacing.
Do I need closer joists for composite?
Often yes. Many composite products require 16 inches on centre for perpendicular installation and 12 inches for diagonal, because they are less stiff than wood. Check the specific product — this is a common cause of bouncy composite decks.
How many screws will I need?
Two per board at every joist crossing, which is what this calculates. Hidden fastener systems use about one clip per board per joist and cost considerably more, but leave no visible heads and let boards move.