How do I calculate how many deck boards I need?

First, find the total square footage of your deck by multiplying length by width. Next, calculate the effective width of one board including the expansion gap (e.g., a 5.5-inch board with a 0.25-inch gap is 5.75 inches, or 0.479 feet). Divide the total deck area by the effective board width to get total linear feet needed. Finally, add your waste factor (typically 10%) and divide by your chosen board length (e.g., 12, 16, or 20 ft).

How many deck boards are in a square foot?

For standard 5/4×6 decking (actual width 5.5 inches) installed with a 1/4-inch expansion gap, each linear foot covers approximately 0.479 square feet. This means you need approximately 2.086 linear feet of decking per square foot of deck surface before factoring in the 10% cutting waste.

What joist spacing should I use: 12", 16", or 24" On-Center?

16 inches On-Center (OC) is the standard residential code requirement for standard wood decking laid perpendicular to joists. If you are installing composite decking or laying wood boards diagonally at a 45-degree angle, IRC building codes typically require 12 inches OC to prevent sagging. 24 inches OC is generally reserved only for heavy commercial 2x6 timber decking.

How many deck screws do I need per square foot?

On average, plan for 3.5 to 4 screws per square foot of deck surface. For standard installations, 2 screws or hidden fasteners are required at every board-joist intersection. For a 320 sq ft deck (20×16 ft) on 16" OC joists, you will need approximately 1,100 to 1,250 screws (roughly 3 to 4 standard 350-count buckets).

Why should I include a 10% to 15% waste factor?

A 10% waste buffer accounts for end-squaring off-cuts, knots, split board ends, and staggered joints across joists. For complex designs like diagonal herringbone layouts, multi-level stairs, or picture-frame perimeter borders, contractors advise bumping the waste factor to 15% or 20%.

Deck Material & Cost Calculator (2026) – Board Count & Joist Takeoff

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