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Deck Calculator

Planning a deck? This calculator estimates the decking boards, joists, and support posts you'll need based on your deck dimensions. It covers standard residential deck construction with 16-inch joist spacing.

Enter Your Measurements

Results

Enter your measurements to see results

How to Use This Calculator

  1. Enter the overall deck length and width in feet.
  2. Enter the width of your deck boards in inches (5.5" is standard for 2×6 or composite).
  3. Enter the length of deck boards you plan to buy.
  4. Review the material list.

How Deck Calculation Works

This calculator uses standard residential deck construction practices. Joists are spaced 16 inches on center, which is code for most deck boards. Support posts are calculated at 8-foot maximum spacing per IRC code requirements. The calculator adds 10% waste for decking boards to account for cuts and defects. For composite decking, the board width is typically 5.5 inches; for 2×6 lumber, actual width is also 5.5 inches.

Formula

Deck area = Length × Width. Decking boards = (Width ÷ Board Width in feet) × (Length ÷ Board Length), rounded up, +10% waste. Joists = (Length ÷ 1.33) + 1 for 16" spacing. Posts = based on 8-foot max spacing per code.

Frequently Asked Questions

How many deck boards for a 12×16 deck?
A 12×16 foot deck using 5.5-inch wide boards (standard 2×6 or composite) needs approximately 28 boards if using 16-foot lengths with about 10% extra for waste, totaling around 31 boards.
Wood or composite decking?
Pressure-treated wood costs $2–$4 per linear foot but needs annual maintenance. Composite costs $4–$10 per linear foot but is virtually maintenance-free and lasts 25+ years. Composite pays for itself in about 7–10 years when you factor in stain, seal, and replacement costs.
How far apart should deck joists be?
16 inches on center is standard and required by most building codes. Some composite decking manufacturers require 12-inch spacing — check your product specifications.

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