dopecalc

Deck Calculator

Plan your deck build with estimates for decking boards, joists, screws, and concrete footings. Choose joist spacing and board width to match your design.

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About this Calculator

Plan your deck build with estimates for decking boards, joists, screws, and concrete footings. Choose joist spacing and board width to match your design.

Formula & Calculations

Formula

Decking Boards = (Deck Length × 12) / Board Width (for perpendicular layout); Joists = (Deck Length / Joist Spacing) + 1; Screws = Decking Boards × Joists × 2; Concrete = Posts × Bags per post
Where:
  • DL=Deck length in feet
  • DW=Deck width in feet
  • Joist Space=Spacing between joists (12, 16, or 24 inches on center)
  • Board W=Width of decking boards in inches (typically 4 or 6 inches)

Assumptions

  • Decking boards run perpendicular to joists (across the width).
  • Standard deck board widths are 4 inches (actual ~3.5") or 6 inches (actual ~5.5").
  • Two screws per board intersection with each joist.
  • One concrete footing bag per post, assuming posts at each corner plus every 6-8 feet along the perimeter.
  • Includes a rim joist around the perimeter.

Calculation Examples

Example 1

Inputs:Deck: 12 ft × 10 ft, 16 in joist spacing, 6 in boards
Result:~22 decking boards, ~10 joists, ~440 screws, ~10 concrete bags

Boards running 12 ft direction: 10 ft × 12 / 5.5 ≈ 21.8 → 22 boards. Joists: 12/1.33 + 1 ≈ 10. Screws: 22 boards × 10 joists × 2 = 440.

Example 2

Inputs:Deck: 16 ft × 12 ft, 12 in joist spacing, 4 in boards
Result:~42 boards, ~17 joists, ~1,428 screws, ~12 concrete bags

Boards: 12 ft × 12 / 3.5 ≈ 41.1 → 42. Joists: 16/1 + 1 = 17. Screws: 42 × 17 × 2 ≈ 1,428.

Frequently Asked Questions

What joist spacing should I use?

16-inch on-center spacing is standard for residential decks. 12-inch spacing provides a sturdier feel. 24-inch spacing is acceptable for 2×6 or thicker decking boards but may feel bouncy.

Should I use 4-inch or 6-inch decking boards?

5/4×6 (6-inch nominal) is the most common and cost-effective. 5/4×4 boards create a more formal appearance but cost more per square foot due to more boards and fasteners needed.