How to calculate how many deck boards you need
Deck boards come down to deck area ÷ what one board covers. Each board covers its own width plus the gap between boards, times its length. Figure that coverage, divide the deck area by it, and add waste.
- Deck area = length (ft) × width (ft)
- Board coverage = ((board width + gap) ÷ 12) × board length
- Boards = (deck area ÷ board coverage) × (1 + waste%), rounded up
- Lineal feet = boards × board length
Including the gap in the coverage matters — a 5.5-inch board with a 1/8-inch gap actually claims about 5.625 inches of deck width, so ignoring the gap overcounts your boards.
The deck mistake that traps water and rot
Butt your deck boards tight together and water has nowhere to go — it pools, the boards cup and warp, and rot sets in early. Leave a consistent gap between boards (about an eighth of an inch, a bit more for wet pressure-treated lumber) so water drains straight through.
That gap also changes your board count, so figure it in — the calculator spaces the boards for you.
Pro move: use a consistent spacer — even a nail or the edge of a speed square — so every gap is identical down the whole deck.
What gap should you leave between deck boards?
| Material | Typical gap |
|---|---|
| Composite decking | 1/8 – 3/16 in |
| Kiln-dried lumber | 1/8 in |
| Wet / green pressure-treated | Tight (it shrinks as it dries) |
| Tropical hardwood | 3/16 in |
Gaps let water drain and air circulate. Wet lumber shrinks as it dries, so install it tighter; dry boards get a deliberate spacer gap.
How many deck boards in a square foot?
A 5/4×6 board (5.5 in wide) covers about half a square foot per running foot of length. So a 16-ft board covers roughly 7–8 sq ft after gaps. Narrower 2×4 decking takes more boards for the same area. The calculator handles either.
Deck calculator FAQ
- Does this include joists and framing?
- No — this counts surface decking only. Joists, beams, posts and fasteners are figured separately based on your span and spacing.
- Why does board length matter?
- It sets the lineal feet and how the boards lay out across the deck. Matching board length to deck length cuts down on butt joints and waste.
- How many screws per board?
- Two screws at every joist crossing is standard. For a 16-ft board over joists at 16-inch spacing, that's roughly 24 screws per board.