Enter your deck’s length and width to get the materials — deck boards, joists and a fastener estimate.
What Is a Deck Calculator?
A deck calculator estimates the framing and surface for a rectangular deck: deck boards run the length of the deck, spaced across the width — a 12 ft width with 5.5-inch boards needs ceil(144 ÷ 5.5) = 27 boards of 16 ft. Joists run under them every 16 inches: ceil(192 ÷ 16) + 1 = 13 joists. Screws are estimated at 2 per board per joist crossing (27 × 13 × 2 = 702). It is a materials estimate for planning and budgeting, not a structural design — spans and loads need an engineer or code table.
How to Use the Deck Calculator
- Enter the deck’s length and width in feet.
- Enter the deck board width (5.5 in is standard for 6-inch boards).
- Enter the joist spacing (16 in standard; 12 in for diagonal decking).
- Click “Calculate Deck”.
Deck Formula
Boards = ceil(width in inches ÷ board width). Joists = ceil(length in inches ÷ joist spacing) + 1. Screws ≈ boards × joists × 2.
Worked Example
A 16 × 12 ft deck: 192 sq ft; 27 boards (16 ft long); 13 joists; about 702 screws.
How It Works, In Plain Words
Think of the deck as two layers counted separately. The surface layer is the deck boards: they run along the length of the deck, so how many you need depends on the width they have to cover. The tool converts the width to inches — 12 ft becomes 144 — and divides by the board width, 5.5 inches for a standard 6-inch board. 144 ÷ 5.5 is about 26.2, and because you cannot lay a fraction of a board, the count rounds up: 27 boards, each as long as the deck itself, 16 ft. The frame layer is the joists underneath, which run across the length at the spacing you choose. 16 ft is 192 inches; 192 ÷ 16 gives 12 spaces, and the formula adds one more joist to close the far end: 13 joists. Finally the fasteners: every board crosses every joist, and the estimate allows 2 screws at each crossing, so 27 × 13 × 2 = 702 screws. The area line is simply length × width, 192 sq ft — the figure you would use for stain or oil coverage.
Real-World Uses
First scenario: a homeowner sketching a weekend project. Before visiting the timber yard they enter 16 and 12 and walk away with a shopping list — 27 boards, 13 joists and, as the screw FAQ suggests, a box of 1,000 screws — instead of guessing at the counter. Second: comparing two deck sizes for the same garden. Running the tool twice shows how the counts move with the footprint, and the 192 sq ft area figure makes the two options easy to weigh against each other and against the budget. Third: planning a diagonal layout. The How to Use steps say diagonal decking takes 12-inch spacing, so the builder changes one input and watches the joist count climb — and remembers the mistakes list’s warning that a diagonal pattern needs roughly 15% more boards. Each case starts from the same four boxes; what changes is the decision the numbers feed.
Benefits of the Deck Calculator
- Boards, joists and screws in one estimate
- Standard 5.5-in board and 16-in joist defaults
- Screw estimate prevents the hardware-store trip
- Budget-ready material counts
Features of Our Tool
- Instant results in your browser
- No sign-up, no download, 100% free
- Works on mobile, tablet and desktop
- Clear step-by-step explanation below the tool
- Private — your entries never leave your device.
Common Mistakes
- Treating this as structural design — spans, beams and footings need code tables or an engineer
- Forgetting the ledger, flashing and concrete for footings (surface framing is not the whole deck)
- Diagonal decking needs 12-inch joist spacing and ~15% more boards
Common Mistakes to Avoid
Two quiet traps sit in the defaults. Clear the board-width or spacing box and the tool silently falls back to 5.5 inches and 16 inches — sensible standards, but if your boards are a different width the count will be wrong while looking perfectly precise. Swapping length and width also changes the answer even though the area stays 192 sq ft: boards are counted across the width and joists along the length, so the orientation you enter matters. Buy to the estimate, but treat it as a floor rather than a ceiling — the worked example’s 702 screws assume every crossing takes its two screws cleanly, with none dropped, bent or spoiled. Finally, keep the estimate in its lane: it counts boards, joists and screws for a rectangular deck, and the page is explicit that spans, beams and footings belong to code tables or an engineer, not to this page.
FAQ
How many boards for a 16 × 12 ft deck?
27 boards of 16 ft (5.5-inch boards across the 12 ft width).
How many joists do I need?
13 joists at 16-inch spacing for a 16 ft length.
How many screws for a deck?
About 702 for the example (2 per board per joist crossing) — buy a box of 1,000.
What joist spacing should I use?
16 inches on centre is standard; 12 inches for diagonal decking or heavy loads.
Do I need a permit?
Often yes — most jurisdictions require permits for decks above a certain height or size; check local code before building.
What happens if I leave the board width or joist spacing blank?
The tool substitutes the standards named on this page: 5.5 inches for the board width and 16 inches for the joist spacing, and calculates as normal. That is handy for a quick estimate, but enter your real board width if it differs, because the board count is worked out from that figure.
Why does the joist count add one at the end?
Because joists close both ends of the deck. The 16 ft length at 16-inch spacing gives 12 spaces, and a run of 12 spaces needs a joist at the start of the first space and at the end of the last one — 13 in total, which is why the formula is spaces plus one.