Deck Calculator
Estimate deck boards, joists, and support posts needed for a deck based on dimensions, board size, and joist spacing.
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ft
in
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ft
Deck Boards
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Joists —
Support Posts —
Deck Area —
How to Use This Calculator
- Enter the deck length and width in feet.
- Enter the deck board width and gap in inches.
- Enter joist spacing and post spacing in inches.
- Results show boards, joists, and posts needed.
Formula
Boards = ceil(Deck width × 12 ÷ (board width + gap)) × (1 + waste%)
Joists = floor(Deck length × 12 ÷ joist spacing) + 1
Posts = ceil(Deck length ÷ post spacing) × ceil(Deck width ÷ post spacing)
Example
Example: 16 × 20 ft deck, 5.5 in boards, 0.25 in gap, 16 in joist spacing → 37 boards, 16 joists, 12 posts.
Frequently Asked Questions
- Divide the deck area by the coverage of each board (width minus gap × length). Add 10–15% for waste and end cuts.
- Standard joist spacing is 12 or 16 inches on center for composite decking, and 16 or 24 inches for pressure-treated wood.
- Divide the deck length by joist spacing (in feet) and add 1. For example, 20 ft deck at 16 in spacing = 16 joists.
- Typical decks use 4×4 or 6×6 posts. Post spacing of 6–8 feet is standard for residential decks.
- Add 10% for straight decks, 15% for diagonal or herringbone patterns, to cover end cuts and defects.