How this is calculated
This calculator assumes the common layout where boards run the full length of the deck, laid side-by-side across its width — the number of boards is the deck's width divided by a single board's face width (plus its gap), rounded up.
A waste allowance is added on top for trimming board ends and any cutting around posts or stairs — composite and hardwood boards both lose some usable length to end-cuts.
Formula & methodology
Boards across width = Deck width ÷ (Board face width + gap), rounded up. Total boards = Boards across width × (1 + waste %), rounded up.
Worked example
A 16 ft × 12 ft deck, 5.5 in boards with a 0.25 in gap, running the 16 ft length
- 1.Board footprint = 5.5 in + 0.25 in = 5.75 in = 0.479 ft
- 2.Boards across the 12 ft width = 12 ÷ 0.479 = 25.05 → 26 boards
- 3.With a 10% waste allowance: 26 × 1.10 = 28.6 → 29 boards
- 4.Assumes boards are available in lengths that span or can be joined across the 16 ft run.
Frequently asked questions
What gap should I leave between deck boards?
1/4 inch (6 mm) is standard for pressure-treated wood to allow for expansion; composite decking typically calls for 3/16–1/4 inch (5–6 mm) — check your specific product's installation guide.
Does this calculator account for joist spacing or fasteners?
No — it estimates deck boards only. Joists, hangers, and fasteners depend on your framing plan and local building code and should be calculated separately.
What if my deck has an unusual shape, like an L-shape or angled corner?
Split it into rectangular sections, run the calculator for each, and add the board counts together — the same approach used for irregular flooring layouts.
Should boards run the length or width of the deck?
Most decks run boards along the longer dimension for fewer seams and a cleaner look, which is the assumption this calculator uses. Boards perpendicular to the house are also common for drainage — swap length and width when entering your measurements if that's your layout.
Last updated 2026-05-28. Formulas are planning methods — confirm with a supplier before you order.