Deck Calculator — Boards, Joists and Screws
Enter your deck dimensions and board width. The gap between boards is included, because leaving it out is how people end up a row short.
About the Deck Calculator
Deck board counting has one subtlety: each board covers its own width plus the gap beside it. Ignore the gap and the count comes out short — by a row or two on a big deck, which is exactly the kind of shortfall that shows up at 4pm on a Sunday.
How to use it
- Enter the deck length and width in feet.
- Enter board width — 5.5 inches for a nominal 6-inch board.
- Use a quarter-inch gap for wet-treated lumber, an eighth for kiln-dried or composite.
- Boards run the length; rows are counted across the width.
The formula
Coverage per board = (width + gap) ÷ 12
Rows = deck width ÷ coverage per board
Joists = (length × 12) ÷ 16 + 1
A 5.5-inch board with a ¼-inch gap covers 5.75 inches, not 5.5. Over a 12 ft width that difference is a full extra row.
Worked examples
| Deck | Board | Gap | Rows | Linear feet | Joists |
|---|---|---|---|---|---|
| 16 ft × 12 ft | 5.5" | ¼" | 26 | 416 ft | 13 |
| 20 ft × 16 ft | 5.5" | ¼" | 34 | 680 ft | 16 |
| 12 ft × 12 ft | 3.5" | ⅛" | 40 | 480 ft | 10 |
| 10 ft × 8 ft | 5.5" | ¼" | 17 | 170 ft | 8 |
Compare rows one and three. Narrower boards mean far more rows, more linear feet, and dramatically more screwing — 5×4 boards look good but they roughly double the fastening work.
Common mistakes
- Ignoring the gap. Each board covers its width plus the gap. Over a wide deck that is a whole extra row you did not buy.
- Butting wet lumber tight. Pressure-treated lumber is usually wet and shrinks as it dries. Boards butted tight open into wide, uneven gaps.
- Spacing joists too far apart. Sixteen inches on centre is standard; 12 inches for diagonal decking or composite boards, which sag more.
- Forgetting the fasteners. Roughly 2.2 screws per linear foot of decking. A modest deck needs well over a thousand.
Terms explained
- On centre (OC)
- Spacing measured from the middle of one joist to the middle of the next. 16" OC is the residential standard.
- Nominal vs actual
- A "2×6" is actually 1.5" × 5.5". Always calculate with actual dimensions.
- Linear feet
- Total length of board needed, regardless of width. This is how decking is often priced.
- Ledger
- The board fixing the deck to the house. The single most safety-critical connection on the whole structure.
Common questions
- How many deck boards do I need for a 16x12 deck?
- Twenty-six rows of 16 ft boards — about 416 linear feet — using 5.5-inch boards with a quarter-inch gap.
- What gap should I leave between deck boards?
- A quarter-inch for wet pressure-treated lumber, which shrinks as it dries. An eighth for kiln-dried or composite, which barely moves.
- How far apart should deck joists be?
- Sixteen inches on centre for standard decking. Twelve inches for diagonal patterns or composite boards, which need more support.
- How many screws per deck board?
- Two per joist crossing, so about 2.2 screws per linear foot of decking overall.
- What is the actual width of a 2x6 deck board?
- Five and a half inches. Nominal lumber sizes are always larger than actual, and calculating with the nominal figure gives the wrong count.
- Should deck boards run lengthwise or across?
- Across the joists, always. Boards need support underneath at regular intervals or they flex.
- How much does a deck cost per square foot?
- Materials alone typically run $15 to $30 per square foot for pressure-treated, considerably more for composite or hardwood.