How many deck boards do I need?
Rows of decking = deck width in inches ÷ (board width + gap), rounded up. With 5.5 in boards and a ¼ in gap, each row covers 5.75 in, so a 12 ft deck needs 144 ÷ 5.75 = 25 rows.
Boards per row = deck length ÷ board length, rounded up. A 16 ft deck using 16 ft boards is one board per row, which is ideal — no butt joints anywhere in the field.
Worked example: a 16 × 12 ft deck with 5.5 in boards at ¼ in gaps needs 25 rows. With 16 ft boards that is 25 boards, or 400 linear feet.
Buying by linear foot instead? Multiply deck area by 2.1 for 5.5 in boards with ¼ in gaps. A 192 sq ft deck needs about 403 linear feet.
How the calculation works
Joist count = (deck length in inches ÷ spacing) + 1. The "+1" is the joist at the far end — omit it and the last board has nothing to land on.
Joist spacing depends on decking, not on the joists. Wood decking laid perpendicular spans 16 in comfortably. Most composite decking also allows 16 in perpendicular but drops to 12 in when laid diagonally, because the diagonal increases the effective span by about 40%.
Span limits matter: a PT southern pine 2×8 joist spans about 12 ft at 16 in on centre, a 2×10 about 15 ft. Exceeding span produces a bouncy deck that still passes a visual inspection.
Screws are estimated at two per board-joist crossing plus 5% for waste and pilot mistakes. Hidden fastener systems use a different count — check the manufacturer coverage per box.
Typical deck material cost, mid-2026
| Material | Unit | Typical range |
|---|---|---|
| PT pine decking 5/4×6 | per linear ft | $1.80 – $3.50 |
| Cedar decking | per linear ft | $3.00 – $6.00 |
| Composite decking | per linear ft | $4.00 – $9.00 |
| PVC decking | per linear ft | $6.00 – $13.00 |
| PT 2×8 joist | per linear ft | $2.20 – $4.00 |
| PT 6×6 post | per linear ft | $4.00 – $9.00 |
| Joist hangers | each | $1.50 – $4.00 |
| Composite deck screws | per 350 ct box | $35.00 – $70.00 |
| Installed deckLabour and materials | per sq ft | $30.00 – $90.00 |
What goes wrong on this job
- Order boards long enough to span the full length. A 16 ft deck built with 12 ft boards creates a butt-joint seam down the middle that collects water, needs doubled blocking underneath, and is the first thing anyone notices.
- Gap wet-treated lumber tighter than composite. Green PT shrinks as it dries — an ⅛ in gap opens to ¼ in within a year, while composite barely moves and needs its full ¼ in from day one.
- Use hot-dipped galvanised or stainless fasteners with treated lumber. Modern ACQ treatment is highly corrosive and will eat standard zinc screws in a couple of seasons.
- Flash the ledger board properly. Ledger failures cause most deck collapses, and almost all of them start with water getting behind an unflashed ledger and rotting the rim joist.
- Anything over 30 in above grade needs a guardrail, and decks generally need a permit. Check before you build — an unpermitted deck surfaces at resale, at the worst possible moment.
Frequently asked questions
How many deck boards do I need for a 12x16 deck?
About 25 rows using 5.5 in boards with ¼ in gaps across the 12 ft dimension. With 16 ft boards that is 25 boards, or roughly 400 linear feet.
What spacing should deck joists be?
Sixteen inches on centre suits most decking laid perpendicular to the joists. Drop to 12 in for composite laid diagonally, or where the manufacturer specifies it.
How much gap should I leave between deck boards?
A quarter inch for composite and kiln-dried wood. An eighth of an inch for wet pressure-treated lumber, which shrinks to about ¼ in as it dries.
How far can a deck joist span?
A PT southern pine 2×8 spans roughly 12 ft at 16 in on centre, and a 2×10 about 15 ft. Species and grade change these figures, so check your local span table.