Deck Board Calculator

How the deck board calculator works

This deck board calculator counts the boards you need by working across the deck row by row, then along each row, and finally adds an allowance for waste. It treats the deck as a simple rectangle, so set the length as the direction your boards run and the width as the distance across the boards.

The method has four steps:

• Rows = ⌈ (deck width in × 12) ÷ (board width + gap) ⌉ — how many board-widths, plus their gaps, fit across the deck.
• Linear feet = rows × deck length — total length of decking laid end to end.
• Boards = ⌈ linear feet ÷ board length ⌉ — how many stock boards that covers.
• With waste = ⌈ boards × 1.10 ⌉ — a 10% cushion for off-cuts and mistakes.

Worked example: a 12 × 12 ft deck

For a 12 ft × 12 ft deck with standard 5.5-inch boards and a 1/8-inch (0.125 in) gap, the pitch per row is 5.625 inches. Across the 12-foot (144-inch) width that is 144 ÷ 5.625 = 25.6, rounded up to 26 rows. Each row is 12 ft long, so 26 × 12 = 312 linear feet. Using 12-ft boards, 312 ÷ 12 = 26 boards, and adding 10% waste gives 29 boards to buy.

Common deck board widths

Gaps let decking drain and expand; composite boards usually need a wider gap than wood, so check the manufacturer's spec. Choosing board lengths that match your deck dimension reduces off-cuts and the waste you have to buy.

Wood or composite

The board count is the same either way, but the material changes the gap, the fixings and the cost per board considerably.

• Pressure-treated softwood — the cheapest option and the most common. Needs cleaning and re-sealing every year or two, and will cup or split if neglected. Expect 10–15 years.
• Hardwood (ipe, balau, cumaru) — dense, very durable, handsome. Expensive, hard on tools, and often needs pre-drilling for every fixing.
• Composite — wood fibre and plastic. No sealing, no splinters, 25-year warranties are common. Two to three times the material cost, gets hot in direct sun, and expands more with temperature — which is exactly why it needs a wider gap.

If you are pricing a deck, note that the boards are usually less than half the total: joists, posts, footings, fixings and railing make up the rest.

Joist spacing and board direction

The calculator counts boards across the deck, but the frame underneath sets the rules. Joists at 16 inches on centre suit most timber decking laid square. Drop to 12 inches for boards run diagonally, or for most composites — composite has less stiffness than timber and will feel bouncy on 16-inch centres.

Board direction is worth deciding before ordering. Running boards the long way across the deck means fewer, longer boards and fewer end joints, but it also means every board is visible along its full length, so any bow shows. A diagonal layout looks good and stiffens the structure, but adds around 15% waste and needs the tighter joist spacing.

Gapping and fastening

Gap width is not cosmetic. Timber decking is usually laid with a 1/8-inch gap, but if the boards are wet when installed — as pressure-treated stock often is — they will shrink as they dry, so butting them nearly tight gives roughly the right gap in a few months. Dry or kiln-dried boards should be gapped from the start. Composite gaps are set by the manufacturer, typically 3/16 inch, because thermal movement is greater.

For fixings, budget roughly 350 screws per 100 square feet with two per board per joist. Hidden-fastener clip systems give a cleaner surface with no visible screw heads, cost more, and are slower to install — and their clips set the gap for you, which removes one variable. Whatever you use, it must be rated for exterior use and compatible with treated timber, or the fixings will corrode long before the boards fail.

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