Deck Area & Board Calculator

Building a deck requires calculating the total area, estimating decking board quantities, determining framing materials, and estimating total cost. Getting the math right before purchasing prevents expensive waste and mid-project shortages.

Key formulas:

Deck Area = Length × Width (for rectangular; add sections for irregular shapes) Number of Decking Boards = (Deck Area ÷ Board Coverage) × Waste Factor Board Coverage = Board Width × Board Length (subtract the expansion gap for a tighter count) Number of Joists = (Deck Length ÷ Joist Spacing) + 1 Estimated Cost = Area × Cost per Square Foot

What each variable means:

• Net Board Coverage: actual width used per board after accounting for the expansion gap.
• Gap Width: typically 1/8 inch for pressure-treated wood; ¼ inch for composite.
• Waste Factor: add 10% for straight runs, 15% for diagonal patterns, 20% for complex angles.
• Joist Spacing: typically 16 inches on center for perpendicular decking; 12 inches for diagonal.

Decking board coverage (5/4×6 = actual 5.5 inches wide): This calculator uses the full board width for a quick estimate, so a 5.5-inch board is 5.5 ÷ 12 = 0.458 feet wide. For a 12-foot-long board: covers 12 × 0.458 = 5.5 sq ft per board. For a gap-precise board count, use the Deck Board Spacing calculator, which subtracts the expansion gap.

Cost benchmarks per square foot (installed):

• Pressure-treated pine: $15–$25/sq ft
• Cedar / redwood: $25–$40/sq ft
• Composite (Trex, TimberTech): $35–$60/sq ft
• Hardwood (ipe, mahogany): $40–$75/sq ft

Worked example: Deck: 16 ft × 20 ft = 320 sq ft. Pressure-treated 5/4×6 boards in 12-ft lengths.

Each board covers 5.5 sq ft, so boards = 320 ÷ 5.5 = 58.2 → 59 boards, or 65 with the 10% waste allowance. Estimated cost at $22/sq ft installed = 320 × $22 = $7,040