About the Decking Calculator
The Decking Calculator turns two dimensions (length and width) plus a handful of design choices — board size (5/4×6 vs 2×6), joist spacing (12″, 16″, or 24″ on-center), board orientation, footing depth, stair count — into a complete bill of materials: deck boards, joists, beams, posts, footing concrete, fasteners, and optional railing. A configurable waste factor (10% rectangular / 15% angled or picture-framed) is baked in.
It is built for first-time DIY deck builders pricing a project before the first lumberyard run, contractors building a quick takeoff for a bid, homeowners gut-checking a quote from a deck pro, and shoppers weighing pressure-treated vs composite over the 20-year ownership horizon (composite breaks even around year 10–15 once you price in repeated stains).
Every quantity runs locally in your browser. Deck dimensions, material choices, and exported shopping lists never leave your device — the page makes no network call after first load. Span tables, joist-spacing rules, and frost-line defaults are bundled as static JSON.
Deck building is code-regulated almost everywhere. Footings must extend below your local frost line (12″ Deep South, 36–48″ northern U.S., 60″+ Minnesota / Maine / Alaska), ledger attachment to the house has IRC-specific fastener and flashing requirements, and most jurisdictions require a permit for decks >30″ above grade or attached to a dwelling. Treat this calculator as a planning takeoff; confirm with your local building department and the IRC chapter that governs your install before pouring concrete.
Compare pressure-treated, cedar, and composite over 20 years including maintenance, staining, and replacement costs.
| Material | Initial Cost | Maintenance (20yr) | Replacement | 20-Year Total |
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Posts, rails, and balusters for your full deck perimeter minus one entry side.
How to Calculate Deck Materials
Every deck material estimate starts with two numbers: length and width. Multiply them for total square footage, which drives nearly every other calculation. To figure out how many decking boards you need, divide the deck length by the board face width (typically 5.5 inches for a 2×6) plus the gap between boards. That gives you the number of rows of boards running across the width. Multiply the row count by the deck width to get total linear feet of decking, then divide by the board length you plan to buy to get a board count. Always round up and add 10–15 percent for waste — end cuts, knots, warped boards, and starter-course trimming add up fast, especially on larger decks. Joists, beams, and posts follow from span tables published by the American Wood Council and adopted into the International Residential Code.
Pressure-Treated vs Composite: The Real Cost Comparison
Pressure-treated lumber costs roughly $0.75–$1.50 per linear foot for standard 2×6 decking, making it the cheapest upfront option by a wide margin. Cedar runs about $2.00–$3.50 per linear foot depending on grade. Mid-range composite boards from manufacturers like Trex or TimberTech typically fall between $3.00–$5.00 per linear foot, while premium capped composites and PVC can exceed $6.00. But upfront cost tells only half the story. Pressure-treated decks need sanding, staining, and sealing every two to three years at roughly $1.50–$2.50 per square foot per treatment. Over twenty years that adds $2,000–$4,000 for a typical 240-square-foot deck. Cedar requires similar maintenance though it resists rot better without treatment. Composite decks need only occasional soap-and-water cleaning with no staining or sealing. When you factor in seven or eight rounds of refinishing versus zero, composite often costs less over a twenty-year lifespan despite the higher initial price.
Joist Spacing and Span Tables
Joist spacing determines how many joists you need and how stiff the deck surface feels underfoot. The two standard spacings are 12 inches on center and 16 inches on center. Twelve-inch spacing uses more lumber but allows thinner decking and eliminates the “trampoline” feel some people notice on wider-spaced joists. Sixteen-inch spacing is the default for most residential decks with 2×6 or 5/4×6 decking. For composite boards, always check the manufacturer’s span rating — many mid-grade composites require 12-inch spacing for perpendicular installation and 16-inch maximum for diagonal. Joist sizing depends on the unsupported span: 2×6 joists span up to about 8 feet, 2×8 joists reach 10–12 feet, and 2×10 joists can span up to 14–16 feet when using No. 2 or better southern pine or Douglas fir. If your deck width exceeds the single-joist span, you need an intermediate beam and additional posts.
Deck Footing Requirements
Footings anchor your deck to the ground and transfer loads to stable soil below the frost line. Most building codes require footings to extend at least 42 inches below grade in cold climates (below the local frost depth) with a minimum diameter of 12 inches for decks and 16–18 inches for heavy or multi-story structures. A standard 12-inch diameter Sonotube filled 42 inches deep takes about 2.9 cubic feet of concrete — roughly one and a half 80-pound bags of pre-mix. The number of footings equals the number of posts, which depends on beam span limits: for a typical 2-ply 2×10 beam, code limits the span between posts to about 8 feet, so a 20-foot-long deck needs at least three posts per beam. Always check local soil conditions — expansive clay or sandy soils may require wider footings or engineered solutions like helical piers.
IRC Code Requirements for Deck Stairs
The International Residential Code (IRC Section R311.7) sets clear rules for deck stairs. Maximum riser height is 7-3/4 inches, with a minimum tread depth of 10 inches. The largest riser and the smallest riser within a flight cannot differ by more than 3/8 inch — consistency is critical for safety. Stair width must be at least 36 inches clear between handrails. For a deck 30 inches or higher above grade, a graspable handrail on at least one side is required, with a diameter between 1-1/4 and 2 inches. Stringers should be cut from 2×12 lumber, and most codes require a minimum of three stringers for stair widths of 36 inches. The total rise (deck height to ground) divided by 7 to 7.5 inches gives you the ideal number of risers, with the total run calculated by multiplying the number of treads by 10 to 11 inches each.
Estimating Fastener Quantities
Deck screws are typically spaced every 16 inches along each joist, with two screws per board per joist crossing. For a 240-square-foot deck at 16-inch joist spacing, expect roughly 650–800 deck screws depending on board width. A five-pound box of 3-inch deck screws contains about 75–100 screws, so budget 8–10 boxes. If using hidden fastener clips for composite decking, you need one clip per board-joist intersection plus starter clips at the first board. Joist hangers require 10 joist-hanger nails (10d x 1.5 inches) per hanger — never use standard framing nails or deck screws in hangers, as they lack the required shear strength. Hurricane ties, post-beam connectors, and ledger bolts add hardware costs that many first-time builders overlook but that inspectors always check.
Looking for related tools? Try our Concrete Calculator for your deck footings, or explore all Home & Real Estate tools.
Frequently Asked Questions
How much does it cost to build a deck?
Pressure-treated decks run 15 to 25 dollars per square foot installed, or 8 to 12 dollars per square foot DIY. Composite decks cost 30 to 60 dollars per square foot installed. A 300 sq ft PT deck typically costs 4,500 to 7,500 dollars DIY; the same deck in composite runs 9,000 to 18,000 dollars.
What joist spacing should I use?
For 5/4-inch deck boards, use 16-inch on-center joists. For 2x6 decking, 16 or 24 inches works. Composite decking manufacturers usually require 16-inch max for perpendicular installations and 12-inch for diagonal patterns. Check the specific board's span chart.
Is composite decking worth the extra cost?
Composite costs 2 to 3 times more upfront but needs no staining or sealing. Over 20 years, pressure-treated requires roughly 4 to 6 re-stains at 200 to 500 dollars each. Composite usually breaks even around year 10 to 15 depending on labor rates and maintenance intensity.
Do I need a permit for a deck?
Most jurisdictions require a permit for decks over 30 inches off the ground, attached to a dwelling, or exceeding 200 square feet. Unpermitted decks can cause problems at resale and may void homeowners insurance after a collapse. Check with your local building department before starting.
How deep should deck footings be?
Footings must extend below the frost line in your area. That's 12 inches in the Deep South, 36 to 48 inches in the northern US, and 60 inches or more in Minnesota, Maine, and Alaska. Diameter is typically 10 to 12 inches for 6x6 posts. The IRC and local code govern.
