Deck Calculator
Find how many deck boards you need for any deck. Enter the deck size and board type, and get the boards, screws or hidden fasteners, joists, and cost, with a waste allowance built in.
A deck calculator turns your deck dimensions into a board count, the fasteners to install them, and a framing reference, so you can buy decking in one trip instead of guessing. The math hinges on a detail people miss: each board covers its own width plus the expansion gap beside it, so the boards needed depend as much on the gap and board width as on the deck size.
This calculator converts your deck area into linear feet of decking, divides that into whole boards of your chosen length, and counts the screws or hidden clips from the joist crossings. It covers the common board sizes (the 5/4 by 6 and 2 by 6 profiles most decks use, plus composite) and reports the joist count as a planning reference. Board width, gap, joist spacing, and fastener type are all adjustable in the advanced section.
A deck calculator turns your deck dimensions into a board count, the fasteners to install them, and a framing reference, so you can buy decking in one trip instead of guessing. The math hinges on a detail people miss: each board covers its own width plus the expansion gap beside it, so the boards needed depend as much on the gap and board width as on the deck size.
This calculator converts your deck area into linear feet of decking, divides that into whole boards of your chosen length, and counts the screws or hidden clips from the joist crossings. It covers the common board sizes (the 5/4 by 6 and 2 by 6 profiles most decks use, plus composite) and reports the joist count as a planning reference. Board width, gap, joist spacing, and fastener type are all adjustable in the advanced section.
How to Use This Calculator
Step 1: Measure the deck surface
Enter the finished length and width of the deck in feet or meters. Boards are assumed to run along the length, with joists spanning the width; if you plan to lay boards the other direction, simply swap the two numbers. For an L-shaped or wrapped deck, calculate each rectangle separately and add the results.
Step 2: Choose the board size and length
Pick your board profile. The 5/4 by 6 (actual 5.5 inches wide) is the most popular residential deck board; 2 by 6 is the same width in a thicker board, and composite comes in 5.5 and true 6 inch faces. Then choose the stock length you will buy, ideally one that divides into your deck dimension to minimize cutoffs.
Step 3: Set the gap and framing (advanced)
Open advanced to set the gap between boards (1/8 inch for dry wood, around 1/4 inch for many composites, per the manufacturer), the joist spacing (16 inches on center is standard; 12 inches for heavy loads or some composite, 24 inches only if the board is rated for it), and the fastener type.
Step 4: Pick the fastener type (advanced)
Face screws drive through the board top into each joist (2 per crossing) and are the simplest for DIY. Hidden clips sit in grooved board edges for a screw-free surface (1 per crossing) but require grooved-edge boards. The calculator adjusts the fastener count to match.
Step 5: Set waste and cost
Keep about 10 percent waste for a straight layout, more for diagonal or patterned decking. Enter a price per board for a material cost estimate.
Reading the results
The cards show the boards and fasteners to buy, with deck area, joist count, and total linear feet listed below. The joist figure is a planning reference only, so confirm structural sizing against your board’s span table and local code. Use the copy button to take the list to the lumberyard.
How the Calculation Works
The estimate works in four steps: area, linear feet, boards, then fasteners.
Step 1: Deck area
Area = Length × Width
A 16 by 12 foot deck is 192 square feet. For an L-shaped deck, split it into rectangles, calculate each, and add them.
Step 2: Linear feet of decking
Linear feet = Area × 12 / (board width + gap)
Each board covers a strip equal to its face width plus the gap. With 5.5 inch boards and a 1/8 inch gap, each strip is 5.625 inches, so 192 square feet needs 192 × 12 / 5.625 = about 410 linear feet of decking.
Step 3: Boards to buy
Boards = round up (linear feet × (1 + waste) / board length)
At 16 foot boards with 10 percent waste: 410 × 1.10 / 16 = 28.2, rounded up to 29 boards. Matching board length to the deck dimension reduces offcut waste.
Step 4: Fasteners
Fasteners come from how many times the boards cross the joists. The boards form rows across the deck width, and each row crosses every joist:
Rows = round up (deck width in inches / (board width + gap))
Joists = round up (deck length in inches / joist spacing) + 1
Crossings = Rows × Joists
Face screws use 2 per crossing; hidden clips use 1. For our deck, 26 rows × 13 joists = 338 crossings, so 676 face screws or 338 clips.
Cost (optional): Cost = Boards × price per board, the way decking is usually purchased.
On gaps: 1/8 inch is standard for dry lumber; green pressure-treated boards are often set tighter (about 3/16 inch) because they shrink as they dry, and composite brands frequently specify 1/4 inch. The gap matters: widening it reduces the boards needed.
Worked Example
Scenario: A 20 by 14 foot composite deck with hidden fasteners
You are building a 20 by 14 foot deck with composite 5.5 inch boards in 16 foot lengths, the manufacturer’s 1/4 inch gap, 16 inch joist spacing, hidden clips, a 10 percent waste allowance, and boards at $35 each.
Step 1: Deck area
20 × 14 = 280 square feet
Step 2: Linear feet of decking
Strip width = 5.5 + 0.25 = 5.75 inches
280 × 12 / 5.75 = about 584 linear feet
Step 3: Boards
584 × 1.10 / 16 = 40.2, rounded up to 41 boards
Step 4: Fasteners
Rows = 14 × 12 / 5.75 = 29.2, rounded up to 30 rows
Joists = 20 × 12 / 16 + 1 = 16 joists
Crossings = 30 × 16 = 480, and hidden clips use 1 per crossing, so 480 clips
Step 5: Cost
41 × $35 = about $1,435 in boards
Final list: 41 composite boards, about 480 hidden clips, across roughly 16 joists, about $1,435 in decking. The clips need grooved-edge boards, so confirm your product has them. This figure is decking only: the framing lumber (joists, beams, posts), footings, railings, and stairs are separate, and their sizes come from the composite board’s span table and your local deck code, not from the joist count above. Switching to face screws would roughly double the fastener count to about 960 screws.
Understanding the Variables
Deck Length and Width
The finished surface dimensions. Boards run along the length and joists span the width, so the orientation follows these inputs; swap them to run boards the other way. Measure to the nearest inch, and split irregular decks into rectangles.
Board Size
The board profile, which sets the face width that drives the count. The 5/4 by 6 (5.5 inches) is the standard residential deck board; 2 by 6 is the same 5.5 inch width in a thicker, stiffer board often used where joists are spaced wider; composite comes in 5.5 and true 6 inch faces. Selecting a profile fills the actual width in advanced, and editing that width switches to Custom.
Board Length
The stock length you buy, from 8 to 20 feet. Choosing a length that divides evenly into your deck dimension reduces offcut waste and the number of butt joints. Sixteen feet is a common, widely stocked length.
Board Width and Gap (advanced)
The actual board width (not the nominal name) and the expansion gap beside each board. Use 1/8 inch for dry lumber, about 3/16 inch for green pressure-treated boards that will shrink, and the manufacturer’s figure (often 1/4 inch) for composite. A wider gap reduces the linear feet of board needed and therefore the count.
Joist Spacing (advanced)
The on-center distance between joists, which sets the fastener count and the joist estimate. Sixteen inches is standard for residential decks. Twelve inches suits heavy loads, diagonal layouts, and some composite boards; twenty-four inches is allowed only where the decking is rated for that span. Always confirm against the board’s span table and code.
Fastener Type (advanced)
Face screws drive through the board into the joist (2 per crossing) and are the easiest method; hidden clips fit grooved board edges for a clean, screw-free top (1 per crossing) but need grooved boards and add cost. Whichever you choose, use corrosion-resistant fasteners (stainless or coated for treated lumber), since modern treated wood corrodes standard steel.
Waste Factor (advanced)
The allowance for cuts, offcuts, and the occasional bad board. About 10 percent for a straight layout, 15 percent or more for diagonal and patterned decking, where angled cuts at the perimeter waste more material.
Board Cost
Price per board, the way decking is usually sold. As rough ranges, pressure-treated boards are the least expensive, cedar more, and composite the most per board, varying widely by brand and length. Optional input, affects only the cost figure.
Industry Standards & References
IRC Section R507 (Decks)
The International Residential Code devotes a full section to decks, covering joist and beam spans, ledger attachment to the house, lateral load connections, guards, and footings. It is the baseline that local building departments enforce, and deck permits are checked against it. Attached decks in particular must meet its ledger and flashing requirements, a common failure point.
AWC DCA 6 (Prescriptive Residential Deck Construction Guide)
The American Wood Council’s DCA 6 translates the code into ready-to-use span tables and details for wood decks: how far a joist of a given size can span at 12, 16, or 24 inch spacing, how to size beams and posts, and how to connect everything. It is the document most builders and inspectors reach for when sizing the structure.
Decking span ratings and ASTM D7032 (composite)
Composite and PVC decking is manufactured to ASTM D7032 and carries a maximum joist spacing rating from the manufacturer (commonly 16 inches for straight installs, 12 inches for diagonal). The board’s own installation guide governs gap, spacing, and fastener type, and following it is required to keep the warranty valid.
Guards and railings (IRC R312)
Decks more than 30 inches above grade require a guard at least 36 inches high, with balusters spaced so a 4 inch sphere cannot pass through. These are safety requirements, not options, and they affect the post and baluster materials beyond the decking itself.
Frost-depth footings (IRC R403)
Deck footings must bear below the local frost line so freeze-thaw cannot heave the structure, the same rule that governs fence posts. Frost depth ranges from near zero in the far south to 4 feet or more up north.
Corrosion-resistant fasteners: modern pressure-treated lumber is more corrosive to metal than older formulations, so code and manufacturers call for hot-dipped galvanized or stainless steel fasteners and connectors in contact with it. Stainless is the safest choice in coastal and high-moisture settings.
Frequently Asked Questions
Convert your deck area to linear feet of decking (area times 12, divided by board width plus gap), then divide by your board length and add about 10 percent for waste. A 16 by 12 foot deck with 5.5 inch boards in 16 foot lengths needs about 29 boards. The calculator above does the full calculation, including the fasteners.
The 5/4 by 6 board (actual 5.5 inches wide) is the most popular residential decking, light enough to handle and stiff enough at 16 inch joist spacing. A 2 by 6 is the same width but thicker, used where joists are spaced wider or for a more solid feel. Composite comes in 5.5 and true 6 inch faces. Always calculate with the actual width, not the nominal name.
About 1/8 inch for dry, seasoned lumber. Green pressure-treated boards are set a little tighter (around 3/16 inch) because they shrink as they dry to roughly a 1/4 inch gap. Composite usually specifies 1/4 inch; follow the manufacturer. Some gap is essential for drainage and expansion, so boards are never butted tight.
Sixteen inches on center is the residential standard and works for most wood and composite decking. Twelve inches is used for heavy loads, tile decks, and diagonal or some composite boards; twenty-four inches is allowed only when the decking is specifically rated for that span. Always confirm against your board's span table and local code.
Plan on 2 screws per board at every joist it crosses. The calculator counts the board-to-joist crossings for your deck and multiplies by two for face screws, or by one for hidden clips. A typical 16 by 12 foot deck works out to around 676 face screws or 338 hidden clips.
Face screws are cheaper, faster, and the easiest for DIY, but they leave visible screw heads. Hidden clips give a clean, screw-free surface and are popular with composite, but they require grooved-edge boards, cost more, and install a bit slower. Both are structurally sound when installed correctly.
Wood is cheaper to build; composite costs more upfront but needs almost no maintenance and lasts longer, so its lifetime cost is competitive. The right answer depends on your budget and how long you will keep the deck. Our wood vs composite decking guide breaks down cost, lifespan, and upkeep in detail.
Usually yes, especially for attached decks and any deck more than a foot or two off the ground. Building departments check decks against the code's deck provisions (ledger attachment, footings, joist spans, and guards), so confirm requirements and get the permit before building. This calculator estimates decking and fasteners; size the structure to code.