Deck Material Calculator
Estimate decking board count and material cost from your deck size, board size and spacing.
How to Use
- Choose Imperial or Metric — deck dimensions use feet/meters, board width and gap use inches/centimeters.
- Enter your deck's length and width, assuming boards run the full length in rows across the width (swap Length and Width if your boards run the other direction).
- Enter your board's width, its purchasable length, and the gap you'll leave between boards (0 for none).
- Optionally add a waste allowance and a price per board to get a material cost estimate — leave the price blank to skip cost.
Formula
- Rows across the width
- rows = ceil(deck_width ÷ (board_width + gap))
- Boards per row
- boards_per_row = ceil(deck_length ÷ board_purchase_length)
- Total boards
- total = ceil(rows × boards_per_row × (1 + waste% ÷ 100))
Worked Example
A 20 ft by 12 ft deck, 5.5" boards with a 0.25" gap, 12 ft board lengths, 10% waste.
- Effective board width: 5.5 + 0.25 = 5.75 inches ≈ 0.479 ft
- Rows across the 12 ft width: ceil(12 ÷ 0.479) = 26
- Boards per row across the 20 ft length (12 ft boards): ceil(20 ÷ 12) = 2
- Total before waste: 26 × 2 = 52 boards
- With 10% waste: ceil(52 × 1.10) = 58 boards
Result: Deck area: 240 ft² (22.3 m²) / Boards needed: 58 (including 10% waste) / Board layout: 26 rows × 2 per row
About This Tool
What this tool does
This calculator estimates how many decking boards a deck needs, based on the deck's footprint, the board's width and purchase length, the gap left between boards, and an optional waste allowance — plus a material cost estimate if you provide a price per board.
When to use it
Use it while planning a deck build to get an early materials estimate, or to compare how board width or spacing choices change the board count.
What the result means
The board count assumes boards run the full "length" dimension you entered, laid in rows across the "width" dimension — if your boards actually run the other way, swap which measurement you enter as length vs. width. Board count is always rounded up (a partial board still needs a full board purchased).
Assumptions & limitations
This tool estimates decking board quantity and material cost only. It does NOT calculate joist spacing, beam sizing, footing requirements, or any structural or building-code requirement — those depend on span tables, local code, and load requirements that need a qualified professional, not a fixed formula. All dimensions in one calculation must use the unit system you selected (feet/inches for Imperial, meters/centimeters for Metric) — the calculator converts between them internally so you don't have to.