Deck Board Calculator

How the deck board calculator works

A 16 × 12-ft deck takes 39 twelve-foot 5/4×6 boards and about 676 deck screws. At a 5½-inch face plus a ⅛-inch gap, 12 feet of width is 26 rows of board; 26 rows × 16 ft = 416 linear feet, or 458 linear feet with a 10% waste factor, which is 39 boards at 12 ft each. Screws assume two per board at each of the 13 joist crossings on 16-inch centers.

Decking runs perpendicular to the joists. The number of rows = deck width ÷ (board face width + gap). Linear feet = rows × deck length. Convert to board count by dividing by your board length, with a 10% waste factor for cuts. Screws: typically 4 per board face per joist crossing (2 per joist), so deck length × 2 × rows × 4 screws.

Choosing a decking material

The board you pick drives everything else on this page — the joist spacing you can get away with, the fastener you have to use, the gap you leave, and how many weekends a year you spend maintaining the thing. Board count barely changes between materials because almost all of them share the same 5½-inch face; what changes is cost, lifespan, and how much work the deck asks of you afterwards.

Two honest notes on that table. First, composite and PVC are not maintenance-free, they are refinish-free — you still wash them, and a badly drained deck still grows mould on the surface. Second, the cheapest board is rarely the cheapest deck: the substructure, footings, railing, and labour are usually 60–70% of the job, so upgrading only the surface material changes the total by far less than the per-foot prices suggest.

Joist spacing is set by the board, not by habit

The framing under a deck is designed around the decking that will sit on it, which is why the joist-spacing input lives in Advanced options — get it wrong and you either waste money on extra joists or build a deck that sags between them. Joist spacing does not change your board count on this page, but it changes your screw count directly, and it changes whether the deck passes inspection.

The single most expensive mistake on this page is framing at 16" OC and then buying composite that requires 12". You cannot fix it from above — the fix is sistering an extra joist into every bay, which means pulling the boards you just laid. Download the spec sheet for the exact brand and colour line before you order a single joist hanger; two products from the same manufacturer can carry different spans.

Fastener selection

The calculator assumes two screws per board at every joist crossing, which is the top-screwed standard. Modern pressure treatment uses copper-based chemistry (ACQ, MCA, CA-C) that corrodes plain steel quickly, so the fastener has to be rated for the wood as well as for the weather.

If you are using a hidden fastener system, the calculator's screw figure becomes your clip count instead — the geometry is identical, one fastener per board per joist rather than two. Add roughly 10% for the perimeter boards, stair treads, and the last board in every run, which almost always get face-screwed regardless of the system.

Pick the right board length

For a 16 ft long deck, buy 16-footers if you can — single board run, no end-to-end seams. For a 14 ft deck, 12-footers are cheaper than 16-footers but require an end seam mid-run (looks fine if staggered). Mid-deck butt joints should land on a joist — never floating.

There is a real cost decision hiding here. Lumber is priced per linear foot but sold in fixed lengths, so a 13-ft deck run built from 16-footers throws away three feet on every single row. On a 26-row deck that is 78 feet of board in the offcut pile. Two 12-footers with a staggered butt joint cost less and waste less, at the price of a visible seam. Where the seam falls matters more than whether it exists: stagger joints so no two adjacent rows break over the same joist, and double the joist (or add a scab block) under any run of butt joints so each board end gets its own full bearing surface.

The other option is a picture-frame border — a band of decking mitred around the perimeter over a doubled rim joist — which hides every cut end at the edges and makes an awkward deck size look deliberate. It adds roughly the deck perimeter in linear feet to your order, so add it to the deck length in the calculator rather than trying to absorb it in the waste factor.

Worked example: a 20 × 10-ft composite deck

Set deck length 20 ft, width 10 ft, composite board (5½" face), a ¼-inch gap for kiln-dry composite, 16-ft boards, and 12" OC joists in Advanced options. Each row occupies 5.5 + 0.25 = 5.75 inches, or 0.479 ft, so 10 ÷ 0.479 = 20.9 → 21 rows. Linear feet = 21 × 20 = 420, and 10% waste takes that to 462. At 16 ft per board that is 462 ÷ 16 = 28.9 → 29 boards.

Screws: at 12" OC there are 20 ÷ 1 = 20 joist bays plus the starting joist, so 21 crossings per row. 21 rows × 21 crossings × 2 screws = 882 screws — a 1,000-count bucket, not a small box. Deck area comes out at 200 sq ft, which is the number to carry over to the stain calculator later.

Worked example: a small 12 × 8-ft PT deck

Deck length 12 ft, width 8 ft, 5/4×6 pressure-treated, ¼-inch gap (this one is being built from a kiln-dried, already-seasoned stack), 12-ft boards, 16" OC joists. Rows = 8 ÷ 0.479 = 16.7 → 17 rows. Linear feet = 17 × 12 = 204, plus 10% waste = 224.4, at 12 ft per board = 18.7 → 19 boards.

Screws: 12 ÷ 1.333 = 9 bays plus one = 10 crossings per row, so 17 × 10 × 2 = 340 screws. Note how favourably this one orders: the deck is exactly 12 ft long and the boards are 12 ft, so every row is a single uncut piece and the 10% waste covers nothing but breakage and the two rip-cuts at the outside edges. Sizing a deck to land on standard lumber lengths is the cheapest design decision available.

Failure modes worth avoiding

• Framing before checking the decking spec. Covered above, and it is the one that costs a full teardown. Confirm required joist spacing in writing first.
• No gap, or the wrong gap. Wet pressure-treated lumber shrinks across its width as it dries; tight-butted wet boards end up with a ⅜-inch gap and a wavy surface. Kiln-dried and composite boards barely shrink, so they need their gap built in at install.
• Bark side down. Look at the end grain: boards laid with the growth rings arcing downward tend to cup upward and hold water in a trough. Lay them the other way, or better, use a board thick and dry enough that it does not care.
• Butt joints floating between joists. The ends drop, the screws pull, and you get a soft spot that turns into a trip hazard. Every board end needs bearing.
• Untreated cut ends. Every cut through pressure-treated lumber exposes untreated wood. Brush the end with a copper naphthenate end-cut preservative, especially on anything within six inches of the ground.
• Ignoring board expansion at the ends. Composite and PVC move lengthwise with temperature. Leave the manufacturer's specified end gap — often 1/16" to ⅛" per board end at 70°F, more if installing on a cold day — where boards butt to a wall or a picture frame.

When to bring in a pro

The board surface is genuinely DIY-friendly. What sits underneath it is not always. Call a licensed contractor or engineer if the deck attaches to the house more than a storey above grade, if you are replacing decking on an existing frame whose ledger flashing you cannot inspect, if a hot tub, outdoor kitchen, or planter wall will sit on the deck, or if the deck is cantilevered, roofed, or carries a roof load. Ledger failure is the single most common cause of catastrophic deck collapse, and it is invisible from above — a deck can look perfect right up until the moment the ledger board's rotted fasteners let go. If you are only replacing boards on an old frame, pay a home inspector for an hour to look at the ledger, posts, and beam connections before you spend a dollar on decking.

Common questions

For deck stairs, see the deck stairs calculator. To stain or seal your finished deck, see the stain calculator.