How many deck boards do I need?

Work out the linear feet of decking (deck area divided by the board width plus gap), then divide by your board length and add waste. A 12 × 16 ft deck with 5.5 in boards needs roughly 27 sixteen-foot boards.

How many joists do I need?

Divide the deck width (in inches) by the joist spacing and add one for the closing joist. A 12 ft deck at 16 in on-center needs 10 joists.

What gap should I leave between deck boards?

About 1/8 inch for kiln-dried boards, or a slightly larger gap for wet pressure-treated lumber that will shrink as it dries.

Deck Calculator

Work out the decking materials for your deck. Enter the deck size and board details to get the number of deck boards and joists, plus the linear feet of decking. For an installed-price estimate, see the deck cost calculator.

Deck material calculator

Deck-board count is the linear feet divided by your board length, plus waste. Joists run across the deck width at the spacing you set, plus the closing joist. Framing and railing are separate.

How it's calculated

Linear feet of decking is the deck area divided by the board's coverage width (board width plus the gap between boards). Deck boards are the linear feet divided by your board length, plus waste. Joists run across the deck width at your on-center spacing, plus the closing joist.

Frequently asked questions

How many deck boards do I need?: Work out the linear feet of decking (deck area divided by the board width plus gap), then divide by your board length and add waste. A 12 × 16 ft deck with 5.5 in boards needs roughly 27 sixteen-foot boards.
How many joists do I need?: Divide the deck width (in inches) by the joist spacing and add one for the closing joist. A 12 ft deck at 16 in on-center needs 10 joists.
What gap should I leave between deck boards?: About 1/8 inch for kiln-dried boards, or a slightly larger gap for wet pressure-treated lumber that will shrink as it dries.

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<p style="font:12px sans-serif">Deck calculator by <a href="https://howmanycalc.com/deck-calculator/">HowManyCalc</a></p>
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Last updated June 2026.