Deck Calculator
Your details
Advanced options
Result
Deck boards to buy
- Deck area
- 192 sq ft
- Joists
- 13 at 16 in centres
- Footings and posts
- 6 footings, 6 posts
- Rows of boards
- 26 rows
- Linear feet of decking
- 416 linear ft
- Screws needed
- 676 screws (2 per joist crossing)
- This covers the deck surface and its framing only. Footings, ledger attachment, railings, stairs and joist hangers are all code-governed — a deck is one of the few DIY projects where a failure injures people, and most jurisdictions require a permit and inspection.
Board count by stock length
The same deck in each available board length, with waste applied.
| Stock length | Boards per row | Boards to buy | Butt joints |
|---|---|---|---|
| 8 ft | 2 | 58 | 1 per row |
| 10 ft | 2 | 58 | 1 per row |
| 12 ft | 2 | 58 | 1 per row |
| 16 ft | 1 | 29 | none |
| 20 ft | 1 | 29 | none |
Joist count by spacing
| On-centre spacing | Joists needed | Typical use |
|---|---|---|
| 12 in | 17 | Composite, diagonal layouts |
| 16 in | 13 | Standard for timber decking |
| 24 in | 9 | Rarely adequate — check span tables |
How many deck boards do I need?
To calculate deck boards, divide the deck width in inches by the board width plus the gap between boards to get the number of rows, then multiply by the deck length. A 16 by 12 ft deck in 5.5 inch boards with quarter-inch gaps needs 26 rows.
rows = ⌈(width_in) ÷ (board_w + gap)⌉ boards = rows × ⌈length ÷ stock⌉ × (1 + waste)- width_in
- Deck width in inches, across the boards
- board_w
- Actual board width — 5.5 in for a nominal 6 in board
- gap
- Spacing between boards, typically 1/4 in
- stock
- The board length you can buy
The board count comes down to row pitch: each row occupies its own width plus one gap, so 5.5 inch boards at a quarter-inch gap repeat every 5.75 inches. A 12-foot deck width is 144 inches, which is 25.04 rows — so 26, because you cannot lay a part row. The detail that catches people is nominal versus actual lumber dimensions. A "2×6" is 1.5 by 5.5 inches and a 5/4×6 deck board is also 5.5 inches wide; calculating with 6 inches undercounts the rows by about 9%. Joist count is a fencepost problem rather than a division: joists at 16 inch centres along a 16-foot deck is 192 ÷ 16 = 12 spaces, which needs 13 joists. And board length matters beyond quantity — a stock length shorter than the deck forces a butt joint in every row, which must land on a joist and should be staggered between rows.
- Convert the deck width to inches and add the gap to the board width.
- Divide to get the number of board rows, rounding up.
- Divide the deck length by the stock board length for boards per row.
- Multiply rows by boards per row, then add ten percent waste.
- Count joists as deck length divided by spacing, plus one.
Worked example: A 16 × 12 ft deck in 5.5 in boards with 1/4 in gaps, 16 in joist centres, 16 ft stock
A 16 × 12 ft deck in 5.5 in boards with 1/4 in gaps, 16 in joist centres, 16 ft stock
The deck is 192 square feet. Across the 12-foot width — 144 inches — each board row occupies 5.5 + 0.25 = 5.75 inches, so 144 ÷ 5.75 = 25.04, which rounds up to 26 rows. Because the 16-foot stock length matches the deck length exactly, each row is one board, so 26 boards before waste. Adding 10% gives 29 boards. That is 416 linear feet of decking. Joists run across the 12-foot width, spaced along the 16-foot length: 192 inches ÷ 16 = 12 spaces, needing 13 joists. Footings sit at roughly 8-foot centres along two beam lines, so 3 posts per beam and 6 footings. At two screws per board per joist crossing, that is 26 × 13 × 2 = 676 screws. Choosing 12-foot boards instead would force a butt joint in every row and raise the count to 58 boards.
What each deck input means
- Deck length
- The dimension the deck boards run along, in feet.
- Deck width
- The dimension perpendicular to the boards, in feet. Joists run this way.
- Board width
- The actual board width, not the nominal. A 2×6 is 5.5 in and a 5/4×6 deck board is also 5.5 in — nominal sizes are always larger than real ones.
- Gap between boards
- Spacing for drainage and movement. A quarter inch is standard for dry lumber; wet pressure-treated boards shrink, so they are laid tighter.
- Joist spacing
- On-centre spacing. 16 in is standard; composite decking and diagonal board layouts normally require 12 in.
- Board length to buy
- Stock length. Matching it to your deck length avoids butt joints, which are both a weak point and a visual one.
- Waste allowance
- Covers cuts, defects and the occasional split board. Ten percent is standard on decking.
What this deck does not account for
- Nominal lumber sizes are not actual. A 2×6 is 1.5 × 5.5 inches. Entering 6 inches for board width undercounts the rows by roughly 9% and leaves you a board short.
- Board gaps depend on moisture. Wet pressure-treated lumber shrinks as it dries, so it is laid tighter — sometimes butted — while dry or composite boards need the full gap for movement.
- Composite decking almost always requires 12 or 16 inch joist centres and has its own span table. 24 inch centres are rarely adequate for anything.
- This covers the surface and joists only. Beams, ledger boards, joist hangers, flashing, railings and stairs are all separate and all code-governed.
- Footing depth and diameter depend on frost line and soil bearing, which vary by jurisdiction. The count here assumes a simple rectangular deck on two beam lines.
- Most jurisdictions require a permit and inspection for a deck. Ledger attachment failure is the most common cause of deck collapse, and it is not a calculation problem.
Frequently asked questions
How many deck boards do I need for a 16x12 deck?
About 29 sixteen-foot boards, including 10% waste. The 12-foot width gives 26 rows at 5.75 inch pitch for 5.5 inch boards with quarter-inch gaps, and matching board length to deck length avoids joints.
What gap should I leave between deck boards?
A quarter inch for dry or composite boards, for drainage and movement. Wet pressure-treated lumber shrinks as it dries, so it is laid tighter — sometimes butted — and opens up on its own.
How far apart should deck joists be?
Sixteen inches on centre is standard for timber decking. Composite boards and diagonal layouts usually need 12 inches. Check the decking manufacturer's span table — it governs, not a general rule.
How many footings does a deck need?
Posts at roughly 8-foot centres along each beam line, so a 16-foot deck needs 3 posts per beam and 6 footings for two beams. Depth and diameter depend on your frost line and soil.
Is a 2x6 actually 6 inches wide?
No, it is 5.5 inches. Nominal lumber sizes are always larger than actual, which is why calculating deck rows with 6 inches leaves you short by about one row in twelve.
Do I need a permit to build a deck?
In most jurisdictions, yes, above a modest height or size. Ledger attachment is the most common point of deck failure, and inspection exists because a collapse injures people rather than just costing money.
Sources and review
Last reviewed .