Deck board calculator: exactly how many boards you need

What this deck board calculator does

A deck board calculator answers the question every deck project starts with: how many boards do I need, and in which lengths? You enter the size of your deck, the board you plan to use and the gap between boards. The calculator works out how many rows of boards fit across the deck, how long each row is, where butt joints need to fall, and finally how many boards of each length to buy.

Many online tools stop at a square-foot estimate: multiply length by width, divide by what one board covers, add 10%. That gets you close, but it can't tell you whether 16-ft or 20-ft boards suit your deck, where the seams go, or whether your last row needs to be ripped narrow. This calculator lays the deck out row by row, the way you would on site, and builds a real cut plan from it. The square footage is still shown, so you can compare with a quote or a quick estimate.

Everything runs in your browser and the calculator is free to use with no account. In particular, it gives you:

• A shopping list by length — for example “26 × 16 ft + 6 × 12 ft” — instead of a single total.
• Spacing built in. The side gap is part of the row calculation, so the deck board count with spacing is exact rather than an afterthought.
• Square and linear footage side by side, plus the split between installed boards, offcuts and your waste allowance.
• A live plan and 3D preview showing every board, seam and joist, with your chosen board colour.
• Wood and composite presets that load typical widths, gaps and stock lengths for treated pine, cedar, redwood, composite and PVC boards.
• Feet and inches or metres and millimetres, so it works for US, Canadian and UK decks alike.
• Extras on the same order: picture-frame borders, a breaker board and stair treads.
• Share, save and print. Your inputs live in the page link, plans can be saved on your device, and the print sheet fits on one page. On a phone you can add the site to your home screen and use it like an app, even offline.

Deck boards by square feet (and square metres)

If you already know your deck's area, you can turn square feet into a rough board count by dividing by the area one board covers. A board covers its length times its width — plus its share of the gap, because the gap is part of the deck surface too. For standard 5-1/2" boards:

So a 12 ft deck board covers about 5.7 sq ft once laid with a 3/16" gap. A 192 sq ft deck using 16-ft boards would need roughly 192 ÷ 7.58 = 25.3 boards before any waste. Some manufacturers publish the same shortcut: TimberTech's installation guide divides deck square footage by 5.5, 7.3 and 9.1 for 12, 16 and 20 ft boards, then adds 10% (TimberTech installation guide).

Use this as a sanity check, not a shopping list. It assumes every offcut gets used, which only happens when your deck length is an exact multiple of the board length. That's why the deck board calculator above starts from the dimensions instead and reports the square footage alongside the board count.

Deck boards per square metre

For metric decks, think in linear metres. A common 140 mm board with a 5 mm gap covers 145 mm of width, so one square metre of deck needs about 6.9 linear metres of board. That's about 1.92 boards per square metre in 3.6 m lengths, or 1.44 in 4.8 m lengths. Switch the calculator to “m / mm” to get an exact count with metric stock lengths.

The deck board formula, worked by hand

If you'd like to check the calculator's numbers yourself, three steps get you a reliable count for a rectangular deck with boards running one way:

1. Count the rows. rows = (deck width + gap) ÷ (board width + gap), rounded up. Use the board's actual width (5-1/2" for a “6-inch” board) and the same units throughout.
2. Find the linear footage. linear feet = rows × deck length. This is what ends up on the deck.
3. Turn it into boards. If the deck length is no longer than your boards, each row is one board: boards = rows. If it's longer, each row needs two or more pieces, and offcuts from one row can sometimes finish another.

Then add an allowance for defects and mistakes — usually 5–10% for a straight layout.

Example: how many deck boards for 10 feet?

Take a 10 × 10 ft deck with 5-1/2" composite boards and a 3/16" gap. Across the 10 ft (120") width: (120 + 0.1875) ÷ (5.5 + 0.1875) = 21.13, which rounds up to 22 rows. Each row is 10 ft long, so the deck takes 220 linear feet of board. With 12-ft stock, every row is a single board with a 2 ft offcut, and a 10% allowance brings the purchase to 25 boards.

Notice the .13 left over: the last row would be a sliver only about 5/8" wide, which is too thin to install. In practice you would widen the gap to about 7/32" so 21 full boards fill the width, or adjust the overhang at the edge. The calculator flags thin rips like this, and the deck board spacing calculator finds the gap that fits whole boards.

The 10×10 deck page and the boards by deck size index show the same calculation for other common sizes.

Hand maths gets harder once rows need seams, you have several board lengths to choose from, or you add a border. That's where the calculator's cut plan earns its keep: it staggers the joints, reuses offcuts and compares lengths for you.

Wood, 5/4 and composite deck boards

The board you choose changes three inputs: width, gap and the lengths you can buy. The material presets fill in typical values, but it's worth checking each against the boards you'll actually use.

• 5/4 wood deck boards (“five-quarter”) are about 1" thick and 5-1/2" wide, and are the most common choice for pressure-treated pine and cedar decks. They usually come in 8 to 16 ft lengths. Treated boards can arrive wet and shrink as they dry, so many builders butt them tight or leave only a small gap.
• 2×6 wood boards are thicker (about 1-1/2") but the same 5-1/2" width, so the board count matches 5/4×6. The extra thickness lets them span wider joist spacing.
• Composite and PVC boards are typically around 5-1/2" wide and sold in 12, 16 and 20 ft lengths. Each brand publishes its own gap and joist spacing rules.

If you're using a particular composite brand such as Trex or TimberTech, open that product's installation guide and enter its actual board width, side gap and available lengths. The board count will then match that product line. The composite deck board calculator adds hidden-clip and composite-specific notes, and deck board sizes explained covers nominal versus actual sizes in more detail.

Diagonal and 45-degree deck boards

This calculator plans boards running straight across the joists, which is how most decks are built. If you're laying boards at 45 degrees, you can still use it for a starting figure, but the maths changes in a few ways:

• The area is the same, but the rows aren't. Diagonal rows vary in length, from long boards through the middle to short triangles in the corners, so far more boards end in an angled cut.
• Waste is higher. Every row ends with a 45° cut at both edges. A common planning allowance for diagonal layouts is about 15%, compared with 5–10% for straight boards.
• Joists may need to be closer. A board crossing joists at 45° spans further between them. Composite manufacturers commonly call for 12" joist spacing on diagonal installs rather than 16". Check your board's guide before you frame.

A practical method: run the deck through the calculator as a straight layout, note the linear footage, then raise the extra allowance to around 15%. Board lengths matter even more on a diagonal deck, because long boards can be cut into two shorter diagonal pieces. When you're comparing options, 16- and 20-ft boards usually waste less than 12s. The deck planning guide covers layout choices before you commit.

How the calculator works, step by step

Behind the results, the calculator follows the same order a deck builder would:

1. Rows. It divides the deck width by the board width plus the gap — rows = (width + gap) ÷ (board + gap), rounded up. If the last row doesn't fit, it's shown as a ripped board.
2. Seams. When a row is longer than your longest board, it splits the row at a joist, staggering joints between neighbouring rows and avoiding short stubs at the ends.
3. Cut plan. It fits every piece onto the board lengths you can buy, reusing offcuts where they're long enough and allowing 1/8" for each saw cut.
4. Best mix. It compares buying a single length against a mix and picks the option with the lowest cost (or least material if you haven't entered prices). Each seam counts as roughly a foot of extra board, because it means a doubled joist and more cutting — so a seam-free layout wins unless it's clearly more wasteful.
5. Extras. Picture-frame borders, a breaker board and stair treads are laid out and added to the same cut plan.
6. Allowance. Finally it adds your extra allowance for defects and mistakes, giving the number to buy.

The preview draws exactly what was calculated: every board, every seam on its joist, and the ripped final row.

How to measure your deck

Good numbers start with good measurements. Measure the finished deck surface — the area you'll walk on — not the hole in the lawn or the old deck you're replacing.

Length and width

For an attached deck, “length” is usually the distance along the house and “width” (sometimes called projection) is how far the deck comes out. The calculator doesn't mind which you call which; it asks for board direction separately. Measure to the outside edge of the framing, and if you plan to let the boards overhang the rim or fascia, add that overhang to both measurements so the outer rows aren't short.

Feet, inches or metres

Type measurements the way you took them: 16.5, 16' 6" and 16ft 6in all work. Switch to metres and millimetres with the unit toggle; values convert as you switch.

L-shaped decks

Choose “L-shape” and enter the overall length and width, then the size of the corner that's cut away. Rows that cross the step are laid full length, the way you'd actually board them, so the count stays honest. For more complicated outlines, the square footage calculator lets you add rectangles, triangles and circles and subtract cut-outs.

Worked example: a 20 × 12 ft deck

Say you're building a 20 × 12 ft pressure-treated deck with boards running along the 20 ft side. Your lumberyard stocks 8, 10, 12 and 16 ft boards, the boards are 5-1/2" wide, and you'll leave a 1/8" gap.

• Each row takes up 5-1/2" + 1/8" = 5.625" of the 144" width, so you need 26 rows, with the last board ripped to about 3-3/8".
• The rows add up to 520 linear feet of installed decking.
• Because 20 ft is longer than your longest board, every row needs a seam. The best plan uses 26 × 12-ft + 26 × 8-ft boards with 26 staggered seams and 0.0% offcut waste.
• Adding a 10% allowance gives a recommended purchase of 32 × 12-ft + 26 × 8-ft boards (58 in total).

The square-foot shortcut would have told you to buy 240 sq ft × 1.1 = 264 sq ft of decking — a number you can't take to the lumber counter.

Reading your results

The headline — for example 32 × 12-ft + 26 × 8-ft boards — is the shopping list: how many boards of each length to buy, including your allowance. Underneath it:

• Board rows is how many courses of boards cross the deck. If the last row doesn't fit a full board, you'll see the width it has to be ripped to. Very thin rips are flagged, with a link to find a gap that avoids them.
• Linear footage to buy is the total length of all boards purchased; “installed” is what actually ends up on the deck. The difference is offcuts plus allowance.
• Total waste splits into offcut waste from the cut plan and your extra allowance, so you can see whether waste comes from the layout or from the buffer.
• Butt seams counts the places where two boards meet end to end. Each lands on a joist, and many manufacturers ask for a doubled joist there (TimberTech's guide says board ends must fall on a joist and joists should be doubled at butt seams — TimberTech installation guide).

Below the results, the planning panels go deeper: a board-length table comparing single lengths with the best mix, a direction comparison, fastener and framing counts, a cut plan showing exactly how each stock board is cut, and a material comparison that runs your deck through treated pine, cedar, redwood, composite and PVC at once. “Copy share link” saves every input in the web address, so you can send the plan to a partner or a supplier, and “Print estimate” gives you a one-page sheet for the lumber counter.

Choosing board lengths

The length you buy matters as much as the count. Boards at least as long as the run give clean, seam-free rows. Shorter boards are cheaper to handle and easier to find, but every extra seam means another doubled joist, more cuts and another visible joint.

Here's how the options compare for a 24 × 12 ft composite deck with boards running the long way, before any allowance:

A 24 ft run is longer than any composite board, so every option needs at least one seam per row — and 12-ft boards need extra ones, because two 12s would put every row's seam on the same joist. The other difference is waste: some lengths leave offcuts too short to reuse, while the recommended option puts them to work in other rows. If longer boards cost more per foot, enter per-board prices under “Cost estimate” and the recommendation switches to the cheapest plan instead. The board length guide covers this in more depth.

Which way should the boards run?

Boards normally run across the joists, so direction is usually set by the framing: if joists run out from the house, boards run parallel to it. When you have a choice, direction can change how much you buy.

Take an 18 × 12 ft treated-pine deck with lumber in 8 to 16 ft lengths. Running boards along the 18 ft side, every row is longer than the longest board, so rows need seams: 45 × 10-ft + 13 × 8-ft boards (554 linear ft, 26 seams). Turned to run across the 12 ft side, each row is a single 12-ft board: 43 × 12-ft boards (516 linear ft, 0 seams).

Leave “Board direction” on “Best fit” and the calculator compares both directions and picks the one that uses less material (or money, if you've entered prices). The direction table under the calculator shows both options side by side, so you can make the call with your framing in mind.

Deck board sizes: nominal vs. actual

Boards are sold by nominal size, but the calculator needs the actual width. Measure a board if you can — treated lumber in particular varies.

Thickness doesn't change the board count, but it matters for joist spacing. The building code (IRC Table R507.7) allows 5/4 wood decking at 16" on center when each board spans at least three joists, and 2× decking at up to 24". Composite boards follow the manufacturer's span rules, usually 16" on center for square layouts and 12" for diagonals.

How spacing changes the board count

The gap is small, but it repeats across every row. On a 12 ft wide deck with 5-1/2" boards, a 3/16" gap adds about 4-3/4" of width in total — almost a whole board. That's why the calculator asks for it, and why the count can drop by one when you widen the gap slightly.

Side gaps let water drain and debris fall through. End gaps at butt joints allow composite boards to expand and contract, so manufacturers tie them to the temperature when you install. If the last board would be a thin rip, the deck board spacing calculator finds a gap that fits whole boards.

Picture frames and breaker boards

A picture-frame border is a ring of boards around the edge, mitred at the corners, that hides the cut ends of the field boards. It looks finished, but it adds material and framing. On a 16 × 12 ft composite deck, a one-board frame changes the purchase from 29 × 16-ft boards to 29 × 16-ft + 2 × 12-ft boards, with 56 ft of border. Borders need extra support underneath — TimberTech's guide calls for additional support for picture-frame structure (TimberTech installation guide) — and a slightly larger allowance for the mitre cuts.

A breaker board runs across the middle of a long deck and splits every row in two. Instead of staggered seams scattered across the surface, you get one deliberate line. On a 30 × 12 ft composite deck with 16-ft boards, rows need 26 butt seams without a breaker; with one, that drops to 0, and the breaker itself adds 12 ft of board. The breaker sits on doubled joists or blocking, so plan the framing to suit. Both options are in the calculator, they show in the plan and 3D views, and the picture-frame and breaker board guides explain the framing.

Screws or hidden clips

The code minimum for wood decking is two fasteners per board at every joist it crosses (IRC R507.7, reproduced in the Anne Arundel County deck guide), and most composite instructions follow the same pattern. Hidden clips usually go one per board at each joist, sitting in the gap between boards.

For the 20 × 12 ft treated deck in the worked example, that's about 884 face screws or 442 hidden clips, counted from the actual layout — including the extra fastening at each seam. The fasteners panel under the calculator gives the number for your deck, and the screw and clip calculator turns it into boxes. Edge boards are usually face-screwed even on a clip-fastened deck.

Why you need a waste allowance

Waste comes from a few places: offcuts from cutting boards to length, boards with splits, warps or knots you won't install, damage in delivery or storage, extra cuts for patterns or borders, and the occasional mistake.

The calculator handles these separately. Offcut waste comes from the cut plan, so it's specific to your deck and the lengths you buy. The extra allowance is your buffer for everything else. The results show both:

base requirement + allowance = recommended purchase

Use 5–10% for a straight layout with quality boards, and more for diagonal, herringbone or picture-frame designs.

Estimating cost

Open “Cost estimate” and enter prices in whichever form you have them — per board, per linear foot or per square foot. Prices are yours; nothing is looked up or guessed. The breakdown shows the cost of boards, the share that goes to waste, and any other materials you add.

Per-board pricing is often the most accurate, because longer boards can cost more per foot. With per-board prices entered, the length optimizer picks the cheapest combination rather than just the least material. For a full price breakdown and material comparison, use the deck cost calculator.

Material considerations

Choosing a material in the calculator loads typical board sizes, gaps and lengths. A few practical differences that affect the estimate:

• Pressure-treated pine is widely stocked in short lengths, so rows on bigger decks usually need seams. Widths vary, especially when the wood is wet.
• Cedar and redwood are sold in similar sizes, but long lengths can be harder to find.
• Composite and PVC come in 12, 16 and 20 ft lengths, which often means seam-free rows. They need specific gaps, often doubled joists at butt seams, and many use hidden clips — see the composite decking calculator.

The material comparison table under the calculator runs your deck through every material at once.

Common estimating mistakes

Most short orders and overflowing scrap piles come from a handful of avoidable errors:

1. Using the nominal width. A “6-inch” board is about 5-1/2" wide. Divide by 6 and you'll come up short on every deck bigger than a landing.
2. Forgetting the gap. Even 1/8" per row adds up to inches across a deck, and changes the row count.
3. Trusting one board's width. Treated lumber can vary noticeably from board to board, especially while it's still wet. Measure several and use a typical value.
4. Ignoring board length. A square-foot estimate can't tell you that 16-ft boards on a 20 ft run leave 4 ft offcuts you may never use.
5. Letting seams fall anywhere. Butt joints must land on a joist, and seams lined up in neighbouring rows look poor and concentrate movement. The cut plan prevents both.
6. Forgetting the extras. Stair treads, a border, a breaker board and fascia all come out of the same order. The first three are in the calculator; fascia isn't.
7. Getting the allowance wrong. Zero allowance means a trip back to the store for one cracked board; 25% on a simple rectangle means paying for boards you'll store for years. 5–10% suits a straight layout.
8. Assuming lengths are in stock. Check which lengths your supplier actually carries and untick the rest under “Board lengths you can buy” before trusting the shopping list.

Taking your estimate to the store

Once the numbers look right, a little preparation makes ordering painless:

• Print the estimate. The one-page sheet lists the deck size, board, gap, direction, joist spacing, the purchase by length and the fastener count — everything a counter clerk or supplier needs.
• Order by length, not just quantity. “32 boards” is ambiguous; “26 × 16 ft and 6 × 12 ft” isn't.
• Ask about returns. If unopened boards can go back, a slightly larger allowance costs little; if not, stay nearer 5–10%.
• Price it two ways. Enter per-board prices from your supplier — longer boards are sometimes cheaper per foot, sometimes not — and let the length table pick.
• Check boards on delivery. Set aside split, badly bowed or damaged boards before you start cutting; that's what the allowance is for.

Save the plan on your device or keep the share link, so you can come back and adjust if the supplier is out of a length.