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Deck Material Calculator

Enter your deck's size and boards to get the board count with waste, joists, screws and a shopping list you can copy or print — with the joist spacing checked against the code and the board makers' guides.

Units
Boards run this way.Enter a number.
Joists run this way.Enter a number.
Board layout
A nominal 6 in deck board is 5½ in wide.Enter a number.
3/16 in is the Trex and Fiberon minimum; about 1/8 in for dry wood (AWC DCA 6).Enter a number.
Blank = 10% straight, 15% diagonal. Trex suggests 5%; TimberTech 10–15% straight and 30% diagonal.Enter a number.
For a cost estimate of the decking.Enter a number.
Deck boards to buy
29
Linear length bought
464 ft
Joists
13
Deck screws
676
Decking cost
—
Plan view. House side at top. Dashed = joists, solid = board joints.

Show the working

  1. Board coverage = width + gap = 5.5 + 0.188 = 5.688 in
  2. Rows = (deck width + gap) ÷ coverage = (144 + 0.188) ÷ 5.688 → 26 rows
  3. Net decking = 26 rows × 16 ft = 416 linear ft
  4. Boards = 416 × 1.1 (10% waste) ÷ 16 ft = 28.6 → 29
  5. Joists = deck length ÷ spacing + 1 = 192 ÷ 16 + 1 → 13
  6. Screws = 26 rows × 13 joists × 2 = 676

Shopping list

QtyItemNote
29Deck boards, 16 ft464 ft incl. 10% waste
676Deck screws2 per board at every joist
13Joists, 12 ft16 in on centre, incl. both end joists
1Ledger board, 16 ftAlong the house; size and fixing per local code
1Rim joist, 16 ftAcross the joist ends
13Joist hangersOne per joist at the ledger

Not included: beams, posts, footings, stairs and railings. Size those from your local code's span tables.

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What this deck calculator covers

This calculator handles the decking surface and the joists under it: how many boards to buy, how many joists, how many screws, plus the ledger, rim joist and hangers to go with them. It assumes a rectangular deck attached to the house, with joists running from the house out to a rim, boards running along the house, and the deck's length measured along the house wall.

It does not size the structure. Joist size, beam size, posts and footings depend on span, species and load, and come from your local code. The American Wood Council's DCA 6 guide covers them for typical attached decks. Our guide to planning deck framing walks through them in order.

How the numbers are worked out

  • Rows of boards = (deck width + gap) ÷ (board width + gap), rounded up. Each board covers its own width plus one gap, and the last board needs no gap after it.
  • Net decking = rows × deck length, in linear feet. For a 45° layout it's area ÷ (board width + gap).
  • Boards = net linear feet × (1 + waste) ÷ board length, rounded up. Off-cuts are assumed to be reused in other rows, with butt joints staggered over joists.
  • Joists = deck length ÷ spacing, rounded up, + 1, so both ends get a joist.
  • Screws = 2 at every place a board crosses a joist.

Worked example: a 12 × 16 ft deck

A deck 16 ft along the house and 12 ft out, with 5½ in composite boards, a 3/16 in gap, 16 ft boards and joists at 16 in on centre. These are the calculator's defaults.

  1. Coverage per row = 5.5 + 0.1875 = 5.6875 in.
  2. Rows = (144 + 0.1875) ÷ 5.6875 = 25.35 → 26 rows.
  3. Net decking = 26 × 16 ft = 416 linear ft.
  4. Boards = 416 × 1.10 ÷ 16 = 28.6 → 29 boards (464 linear ft). With 12 ft boards it's 39.
  5. Joists = 192 ÷ 16 + 1 = 13 joists, each 12 ft.
  6. Screws = 26 rows × 13 joists × 2 = 676.

As a cross-check, TimberTech's install guide says to divide deck area by 7.3 for 16 ft boards and add about 10%: 192 ÷ 7.3 × 1.1 = 28.9, so 29 boards.

Load this example in the calculator

Boards, joists and screws for common deck sizes

Computed with the defaults: 5½ in boards, 3/16 in gap, 16 ft boards, 16 in joists (12 in for diagonal), 10% / 15% waste.

Deck (along house × out)Boards, straightBoards, 45°JoistsScrews
10 × 10 ft16169396
12 × 12 ft222210520
16 × 12 ft293013676
20 × 12 ft363716832
20 × 16 ft4749161088
24 × 16 ft5759191292

Choosing the inputs

Board width: use the actual width, not the name. A 5/4×6 or 2×6 board is 5½ in wide when dry, and most composite boards are too. Gap: composite makers set minimums (3/16 in for Trex and Fiberon). For dry wood about 1/8 in is typical. Joist spacing: 16 in on centre suits most 1¼ in wood and composite boards laid square to the joists. Boards at 45° need 12 in under the 2021 IRC for wood and under the Trex, TimberTech and Fiberon guides. More on gaps in deck board spacing.

Common mistakes

  • Using the nominal board width. A "6 inch" board is 5½ in. Counting rows at 6 in under-orders by about 8%.
  • Forgetting the gap. Gaps add up across a deck: 26 rows with 3/16 in gaps is nearly 5 in of extra width.
  • Laying diagonal boards on 16 in joists. At 45° a board spans about 41% farther between joists. Plan 12 in spacing before you frame.
  • Ending boards between joists. Every butt joint must land on a joist (or doubled joists), and the AWC and Trex both say each board segment must bear on at least three joists.
  • Gapping wet treated lumber. Simpson Strong-Tie notes that wet-treated boards are installed butted tight and the gaps appear as they dry. Kiln-dried after treatment (KDAT) boards get about 1/8 in.

If you're also building a roof over the deck, the roof pitch calculator and roofing squares calculator use the same imperial/metric switch.

Frequently asked questions

How many deck boards do I need for a 12×16 deck?

With 5½ in boards, a 3/16 in gap and boards running the 16 ft way, a 12 × 16 ft deck needs 26 rows of boards: 29 boards of 16 ft including 10% waste, or 39 boards if you buy 12 ft lengths. The worked example above shows each step.

How far apart should deck boards be?

For composite boards, follow the maker: Trex and Fiberon require at least 3/16 in side to side, and TimberTech 1/8–3/16 in. For dry wood, the American Wood Council's DCA 6 says about 1/8 in. Pressure-treated boards that are still wet are usually butted tight and open up as they dry. See deck board spacing for end gaps and temperature charts.

What joist spacing do I need for composite decking?

Trex, TimberTech and Fiberon all allow 16 in on centre for standard residential boards laid square to the joists, and 12 in on centre for boards at 45°. Some boards are rated for 24 in. Check your product's install guide. The calculator warns if your spacing is wider than these limits.

How many screws do I need per deck board?

Two at every joist. That is what the 2021 IRC (R507.7), the AWC's DCA 6 and the composite makers' guides all specify. With 5½ in boards on 16 in joists it works out to about 3.5 screws per square foot of deck.

How much extra decking should I buy for waste?

Sources differ. Trex suggests 5% extra, and TimberTech at least 10–15% for straight boards and 30% for diagonal or picture-frame layouts. The calculator defaults to 10% straight and 15% diagonal, and you can type your own figure in the waste box.

Sources & last checked

Last checked .