How to estimate deck boards
- Enter the finished rectangular deck length and width in feet or metres.
- Choose the actual board width and the stock length you will buy. Confirm both with your supplier.
- Enter the gap required by your specific product and choose whether boards run along the deck length or width.
- Set an allowance for cuts and waste. Read the conservative board estimate and underlying row count.
Imperial presets such as 5.5 inches and 12 ft, the 0.125 inch gap, and the 10% waste allowance are editable examples, not universal recommendations. Metric dimensions are product-specific. Check actual dimensions and spacing instructions for your material and installation.
How the board count is calculated
This is a straight-row layout for a rectangular surface. The gap goes between boards, with no gap after the last row. Whole stock boards are counted independently for every row. Reusable cutoffs are not credited; short stock can therefore produce a higher count than an optimized cutting plan. The waste percentage is added only after each row is rounded to whole stock boards.
- Area: length × width, in square feet or square metres.
- Rows: ceiling((perpendicular deck span + gap) ÷ (actual board width + gap)), using inches or millimetres throughout.
- Whole boards per row: ceiling(board running span ÷ purchased board length).
- Boards before waste: rows × whole boards per row.
- Conservative board estimate: ceiling(boards before waste × (1 + waste percentage ÷ 100)).
- Deck coverage: rows × running span. Purchased stock: conservative board estimate × stock length. These are different linear-foot measures.
Worked example: 16 × 12 ft deck with 12 ft stock
With 5.5 inch actual-width boards, 0.25 inch gaps, boards running along the 16 ft length, and 10% waste: the 12 ft layout span is 144 inches. Ceiling((144 + 0.25) ÷ (5.5 + 0.25)) gives 26 rows. Each 16 ft row requires ceiling(16 ÷ 12) = 2 whole stock boards, so the raw count is 52 boards. Ceiling(52 × 1.10) gives a conservative estimate of 58 boards, or 696 purchased linear feet. The board coverage is 26 × 16 = 416 linear feet. Actual end-joint placement and cut reuse need a separate layout plan.
Assumptions and limits
The conservative estimate covers only straight surface-board rows on one rectangular deck. It does not model border patterns, diagonal boards, stairs, posts, curved edges, offcut reuse, fastener types, joists, beams, footings, loads, or local code compliance. A short stock board requires supported end joints; check the layout and structural details with a qualified professional before building. Your chosen waste percentage is a planning allowance, not a guarantee of the final order.
Board spacing and fastening depend on the material, product, temperature, and installation method. Follow the current instructions for your exact product. Trex’s product-specific spacing guidance advises checking the manufacturer’s installation guide. The American Wood Council explains U.S. nominal versus actual dressed lumber dimensions. Sources reviewed 26 September 2026.
Explore the layout and related tools
For a footing base, try the Gravel Calculator; for more project tools, browse Roofing & Construction.
Calculations run in your browser. This tool does not send your entered dimensions to a calculation server or save them in an account. Found a problem with this method or a result? Report it to FivoTools.