Deck Calculator
Deck boards, joists, fasteners and linear footage from deck size, board width, gap and joist spacing — with waste, cost and the layout that wastes the least.
Deck Calculator: with the default inputs, deck boards to buy is 29.
The direction the boards run. Joists run across it.
The direction you cross when walking the boards — this is what the board rows have to cover.
Actual, not nominal: a 5/4×6 or 2×6 deck board measures 5½ in.
⅛ in for wet pressure-treated lumber that will shrink, ¼ in for dry or composite boards.
Boards come in 8, 10, 12, 16 and 20 ft. A length that divides your deck evenly avoids butt joints.
16 in for most decking; 12 in for composite laid diagonally or where the manufacturer requires it.
Cuts, culls and the board you split. 10% for a rectangle, 15% for angles or a picture frame.
Whole boards at your chosen length, including waste.
- Rows of decking
- 26How many board widths it takes to cross the deck.
- Linear footage of decking
- 416 ftBefore waste. Some suppliers price decking by the linear foot.
- Joists
- 13Including one at each end.
- Linear footage of joists
- 156 ft
- Fasteners
- 676
- Deck area
- 192 ft²
- Material cost
- $0.00
Assumptions
- Boards run parallel to the deck length; joists run across it at the spacing you set, with one at each end.
- Board face width is the actual dimension (5½ in for a nominal 6-in board), not the nominal one.
- Only decking, joists and fasteners are counted. Beams, posts, footings, ledger, flashing, hardware, stairs and railing are separate.
- Waste covers cuts, culls and rips; angled or picture-framed decks need more.
| Board length | Boards needed | Joint per row? |
|---|---|---|
| 8 ft | 58 | No — 2 boards butt exactly |
| 10 ft | 46 | Yes — 6 ft offcut |
| 12 ft | 39 | Yes — 4 ft offcut |
| 16 ft | 29 | No — one board spans it |
| 20 ft | 23 | No — one board spans it |
Boards that divide the run evenly cost less and look better. Every butt joint has to land on a joist.
How this is worked out
The formula
Rows = deck width ÷ (board face width + gap), rounded up Linear footage = rows × deck length Boards = linear footage × (1 + waste) ÷ board length, rounded up Joists = ⌊deck length ÷ joist spacing⌋ + 1 Fasteners = rows × joists × screws per crossing
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Deck length
- The direction the boards run. Joists run across it.in ft, in, m, cm · 0 or more · defaults to 16
- Deck width
- The direction you cross when walking the boards — this is what the board rows have to cover.in ft, in, m, cm · 0 or more · defaults to 12
- Board face width
- Actual, not nominal: a 5/4×6 or 2×6 deck board measures 5½ in.in in, mm, cm · 0.5 or more · defaults to 5.5
- Gap between boards
- ⅛ in for wet pressure-treated lumber that will shrink, ¼ in for dry or composite boards.in in, mm · from 0 to 1 · defaults to 0.25
- Board length
- Boards come in 8, 10, 12, 16 and 20 ft. A length that divides your deck evenly avoids butt joints.in ft, m · 1 or more · defaults to 16
- Joist spacing on centre
- 16 in for most decking; 12 in for composite laid diagonally or where the manufacturer requires it.in in, cm, mm · 4 or more · defaults to 16
- Waste allowance
- Cuts, culls and the board you split. 10% for a rectangle, 15% for angles or a picture frame.a percentage · from 0 to 100 · defaults to 10
- Fasteners per board per joist(under More options)
- Two screws per board at every joist is standard face-screwing; hidden clips use one per crossing.from 1 to 4 · whole numbers only · defaults to 2
- Price per deck board(under More options)
- A number.in dollars · 0 or more · defaults to 0
- Price per joist(under More options)
- A number.in dollars · 0 or more · defaults to 0
What you get back
- Deck boards to buymain answer
- Whole boards at your chosen length, including waste.
- Rows of decking
- How many board widths it takes to cross the deck.
- Linear footage of decking
- Before waste. Some suppliers price decking by the linear foot.
- Joists
- Including one at each end.
- Linear footage of joists
- Fasteners
- Deck area
- Material cost
What this assumes
- Boards run parallel to the deck length; joists run across it at the spacing you set, with one at each end.
- Board face width is the actual dimension (5½ in for a nominal 6-in board), not the nominal one.
- Only decking, joists and fasteners are counted. Beams, posts, footings, ledger, flashing, hardware, stairs and railing are separate.
- Waste covers cuts, culls and rips; angled or picture-framed decks need more.
About this calculator
A deck takeoff is two counts that people get backwards. The boards are counted across the deck — how many 5½-inch widths plus gaps it takes to cross it — and the joists are counted along it, one every 16 inches plus one at the far end. Get those two directions straight and the rest is arithmetic.
How to use it
Deck length is the direction the boards run. Deck width is what the rows have to cover. Use the actual board face width, not the nominal one: a "2×6" or "5/4×6" deck board is 5½ inches wide, so with a ¼-inch gap each row eats 5¾ inches of width.
The gap matters. Wet pressure-treated lumber straight off the pile will shrink as it dries, so it can go down at ⅛ inch or even tight. Kiln-dried wood and composite boards are already at their final width and need the full ¼ inch for drainage and thermal movement.
Butt joints and board length
If the deck is longer than your boards, every row gets a joint, and every joint has to land on a joist — ideally a doubled joist, because a single 1½-inch joist gives each board ¾ inch of bearing and one of them will always be the one that cups. The table under the results shows which stock lengths divide your run evenly. A 16-foot deck out of 16-foot boards has no joints at all; the same deck out of 12-footers has a joint in every row.
If joints are unavoidable, stagger them across the deck rather than lining them up, or run a deliberate breaker board perpendicular to the field and butt into it from both sides. Straight-lined joints look like a mistake; a breaker board looks like a decision.
Joists and fasteners
Sixteen inches on centre is the default and what most decking is rated for. Twelve inches is required for many composites laid diagonally, and for anything spanning further than its rating. The joist count includes one at each end, which is the "+1" people forget.
Two screws per board per joist is standard face-screwing. Hidden fastener clips use one per crossing but cost several times as much, so price both — the calculator's fastener count follows whatever you set under More options.
Reading the results
Deck boards to buy is whole boards at your chosen length with waste included. Linear footage is before waste, because some suppliers price decking by the linear foot rather than the board. Framing beyond the joists — beams, posts, footings, ledger, hardware, railing — isn't included; footings and posts are a concrete question, and stairs are their own calculation.
Frequently asked questions
▸How many deck boards do I need for a 12 × 16 deck?
Crossing 12 ft (144 in) with 5½-in boards and ¼-in gaps takes 26 rows. At 16 ft per row that's 416 linear feet, or 29 sixteen-foot boards with 10% waste — and no butt joints, because the boards span the whole run.
▸What gap should I leave between deck boards?
¼ in for dry lumber and composite. ⅛ in — or even tight — for wet pressure-treated boards, which shrink about ¼ in across a 5½-in face as they dry. Never zero: the gap is drainage.
▸How far apart should deck joists be?
16 in on centre suits most decking laid perpendicular to the joists. Drop to 12 in for boards laid diagonally, for many composites, and wherever the decking manufacturer's span table says so.
▸How many screws do I need for a deck?
Two per board at every joist crossing. For a 12 × 16 deck that's 26 rows × 13 joists × 2 = 676 screws, so buy a 5 lb box (about 350) times two, plus extras.
▸Should the boards run the long way or the short way?
Whichever direction lets a single board span without a butt joint. Boards running the short way of a long deck need fewer joints but more rows; boards running the long way look better if your stock length matches the run.
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