Deck Board Calculator
How many deck boards and joists for your deck — from its size, the board width and gap, and joist spacing.
Your project
Material estimate only — confirm quantities against your own measurements and the product packaging before you buy. Round up and keep a little extra for cuts, waste, and mistakes.
What you need
What to buy
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Deck boards
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Joists
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Linear feet
— ft
Shopping list
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How we calculate it
rows = round up( deck width / ((board width + gap)/12) ) • boards = round up( rows × deck length / board length ) • joists = floor( deck length×12 / spacing ) + 1
Assumptions & sources
- Board coverage includes the gap: rows use (board width + gap). Defaults are a 5/4×6 board (5.5 in actual) with a 1/8-in gap — source: Trex / TimberTech board specs
- Joists default to 16-in on-center (use 12-in for diagonal decking or heavier loads) — source: decks.com joist spacing
- This gives board COUNT and joist COUNT only — joist span and beam sizing are structural (load tables) and must follow code, not this tool
Formulas reviewed against manufacturer spec sheets and industry standards. Last verified 2026.
How to estimate deck boards
Deck boards run across the joists, so the number of rows depends on your deck’s width divided by each board’s coverage (its width plus the gap between boards). Multiply rows by the deck length for total linear feet, then divide by your board length. Joists are spaced along the deck at 16 inches on-center.
Tips
- Mind the gap: A 1/8-inch gap lets wood swell and drain. Composite boards often specify their own gap — use the manufacturer’s number.
- Board direction matters: This assumes boards run across the width. If you change direction or add a picture-frame border, add a few extra boards.
- Structure needs an engineer: This counts boards and joists. Joist span, beam size, and footing depth are structural — follow your local building code.
Common questions
How many deck boards for a 12×12 deck?
With 5/4×6 boards (5.5 in wide, 1/8-in gap) run across the width and 12-ft boards, about 26 boards and 10 joists at 16-in on-center.
What gap should I leave between deck boards?
About 1/8 inch for pressure-treated wood, to allow for drainage and movement. Composite decking often specifies its own gap — follow the manufacturer.
Does this size my joists and beams?
No. It counts boards and joists at your chosen spacing. Joist span, beam size, and footings are structural and must follow your local building code.