Deck Calculator

How to Estimate Deck Materials

Building a deck is one of the most rewarding home improvement projects — it adds usable living space and significant resale value. Accurate material estimates prevent costly over-ordering and frustrating mid-build shortages.

Deck Boards

Deck boards run perpendicular to joists. Calculate how many boards span the deck width: deck width ÷ (board width + gap). For a 12-ft wide deck with 5.5-in boards and 0.25-in gaps: 144 in ÷ 5.75 in = ~25 boards. Each board needs to be at least as long as the deck length (16 ft in our example). Linear footage = boards × deck length. Add 10–15% waste.

Joists

Joists span the deck length, spaced 16 inches on-center for standard lumber (12 in for composite decking or heavy loads). Number of joists = (deck length × 12 ÷ spacing) + 1. Add rim joists (2) and a doubled ledger if attached to the house. Joist length = deck width + a few inches for the rim joist attachment.

Decking Material Comparison

Pressure-treated pine is the most affordable choice at $1–$2.50 per linear foot for 5/4×6. Cedar costs $2.50–$5/lf and naturally resists rot and insects. Composite decking (Trex, Fiberon) runs $4–$10/lf but requires almost no maintenance for 25+ years. Hardwood decking (ipe, cumaru) costs $8–$15/lf but is extremely durable.

Structural Members

Beyond deck boards and joists, you'll need posts (6×6 for elevated decks), beams (doubled 2×10 or LVL), ledger board (attached to house), post bases, joist hangers, and structural screws. Hardware costs can add $300–$800 to a mid-size deck project.

Stain and Sealer

Stain or sealer extends the life of pressure-treated and cedar decks; composite decking only needs occasional cleaning. How far a gallon goes depends mostly on how thirsty the wood is. New, smooth boards absorb slowly and a gallon covers roughly 200–300 sq ft. Wood that has weathered for a few seasons is more porous, about 150–200 sq ft per gallon, and rough, gray, or freshly stripped wood can drink it at 100–150 sq ft per gallon. The product label has the final word, since brands vary.

Stain gallons = (deck area + railing area) × coats ÷ coverage. Railings are the hard part because every baluster has four faces; a typical rail with top rail, bottom rail, and balusters works out to about 2.5 sq ft of stainable surface per linear foot. For example, a 16×12 deck (192 sq ft) with 40 feet of railing (100 sq ft) in weathered condition, two coats: (192 + 100) × 2 ÷ 175 = 3.3, so buy 4 gallons. Buy it all at once so every can comes from the same batch and the color matches.

Choosing a finish. Clear and transparent sealers show the most grain but need reapplying every 1–2 years. Semi-transparent stain adds color with the grain still visible and lasts about 2–3 years, which makes it the usual choice. Semi-solid stain hides more of the grain and covers weathered wood better, lasting 3–4 years. Solid stain is opaque like paint and lasts 4–6 years, but when it wears it peels, and you cannot go back to a semi-transparent look without stripping the deck.

Prep and application. Stain on dirty or damp wood will not soak in and tends to peel within a season. Scrub the deck with a deck cleaner, rinse, and give it 48–72 hours of dry weather. Sand rough spots, replace soft boards, and set any proud fasteners. Stain when it is 50–90°F with no rain forecast for 24 hours. Spraying or rolling is fast, but back-brush right away to work the stain into the grain and board edges, and let the first coat dry (usually 4–8 hours) before the second.