How much decking do I need?

Calculate deck area (L × W), then add 10–15% for waste from end cuts and boards that run perpendicular. Factor in your board width plus gap: a 5.5-inch board with 1/4-inch gap covers about 5.75 inches of width. Divide total deck width by 5.75 and multiply by deck length to get linear feet of boards needed.

How many joists do I need for a deck?

Divide deck length by joist spacing (usually 16 inches = 1.33 ft), then add 1 for the end joist. A 16-ft deck with 16-in OC spacing: (16 ÷ 1.33) + 1 = 13 joists. For a doubled ledger board and rim joist, add 2 more. Always add a header and rim joist at the perimeter.

What is the best wood for decking?

Pressure-treated lumber (ACQ treated): most affordable, widely available, lasts 15–30 years. Cedar: naturally rot-resistant, attractive, splinter-resistant, more expensive. Redwood: premium appearance and durability. Composite decking (Trex, TimberTech): no maintenance needed, 25+ year warranty, but higher upfront cost.

Do I need a permit to build a deck?

Most jurisdictions require a permit for decks over 200 sq ft or elevated more than 30 inches above grade. Attached decks typically always require permits. Check your local building department before starting — unpermitted decks can cause issues when selling your home.

How much does it cost to build a deck?

DIY materials for a pressure-treated wood deck run $15–$25 per sq ft. Composite decking materials: $30–$45 per sq ft. Professional installation adds $20–$45 per sq ft for labor. A 200 sq ft deck: $7,000–$14,000 professionally built, or $3,000–$5,000 DIY materials.

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Deck Calculator

Estimate deck boards, joists, and materials for your deck project.

Deck Length (ft)

Deck Width (ft)

Decking Board Width

Board Gap (in)

Joist Spacing

Waste Allowance

Results

Deck Area—

Deck Boards (linear ft)—

Joists Needed—

Est. Board Count (12-ft)—

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.

Finishing

Stain or sealer extends the life of pressure-treated and cedar decks. Apply sealer within the first year and reapply every 2–3 years. Composite decking only needs occasional cleaning. Budget 1 gallon of deck stain per 200–300 sq ft.

Frequently Asked Questions

Calculate deck width ÷ (board width + gap) for the number of rows. Multiply by deck length for linear feet needed. Add 10–15% waste. Our calculator does this automatically.

16 inches on-center for 5/4×6 pressure-treated or cedar decking. 12 inches OC for composite decking (most manufacturers require this). 24 inches OC only for very stiff decking materials or light-use platforms.

Usually yes for attached decks and elevated decks over 30 inches. Most jurisdictions require permits for decks over 200 sq ft. Check with your local building department.

Pressure-treated pine is most affordable and durable. Cedar and redwood are naturally rot-resistant and look great. Composite decking has the lowest maintenance and longest warranty but costs more upfront.

Pressure-treated pine with regular maintenance: 15–30 years. Cedar/redwood: 20–30 years. Composite decking: 25–50+ years. The biggest factors are quality of construction, drainage, and maintenance frequency.

Formula sources & accuracy standards: Calculator Methodology · Editorial Policy